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CHAPTER 6 Dating methods, and establishing timing in the landscape
It is of great use to the sailor to know the length of his line, though he cannot with it fathom all the depths of the ocean.
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Within the last two decades, a major revolution has occurred in our ability to establish timing in the landscape. The presentation of dating methods in this chapter will be significantly skewed toward the new methods that have driven this revolution, the new lines with which we plumb time. While the exercise of exploring the processes involved in a landscape is largely one of physics and chemistry, the documentation of the rates at which these processes are acting in the landscape requires obtaining dates of surfaces and of horizons within deposits. New dating techniques have often triggered surges of research activity. This is especially the case if the new technique allows use of materials thought to be barren of timing information, or if the timescale over which such techniques will yield dates is a new one. While we will very briefly review older or classical relative dating techniques, in which we can tell that one surface is older or younger than another surface, we will focus on the newer techniques that yield absolute ages. See more comprehensive reviews of dating methods in Pierce (1986), and in Burbank and Anderson (2000). Several of these methods are quite young at present. One must therefore be aware of the pitfalls of each method, the sources of uncertainty. We will once again encounter the utility of writing out a balance equation. In addition, we describe briefly methods used to establish timing and process rates at both very short and very long scales. The first entails the use of cosmogenic radionuclides, with a focus on 10Be and 26Al. The latter employs proxies for how long a rock parcel has spent below a particular temperature. These thermochronometric methods include the counting of fission tracks and documentation of the quantity of trace gases in a mineral that reflect the decay of radioactive elements in the mineral lattice.
Jason Briner and Aaron Bini sampling granitic bedrock outcrop in Sam Ford Fjord, eastern Baffin Island, for cosmogenic exposure dating of the retreat of the Laurentide Ice Sheet from the fjord (photo by R. S. Anderson).
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roughness ought to decline as the inverse square Relative dating methods root of time. Given this, quantifying the roughness of a surface became the challenge. This can be done using The classic tools available to the geomorphologist topographic profiles collected either on the ground include an array of relative dating methods. Perhaps using classical survey methods, albeit at small spacing, the most rudimentary of these are associated with or from airborne methods. Tom Farr at JPL in basic stratigraphic methods (the oldest layers are at Pasadena employed this latter method on the fans of the bottom of a pile), and structural geologic principles the eastern California desert, using radar (Farr, 1992; such as cross-cutting relationships. The peat at the Evans et al., 1992). While this is an expensive and bottom of a moraine-ponded lake is the oldest peat rather blunt tool to quantify the many surfaces in these in the stratigraphic column in the lake deposit. The fans, the effort served as both a method development moraine with continuous margins that cross-cuts or opportunity and an inspiration for the important truncates another moraine is the younger of the two. shuttle radar topography mission (SRTM) that in Long before radiometric dating tools were the late 1990s collected the topography of the Earth developed, the geomorphologist was employing the from 60 Nto60 S (Farr et al., 2007). degree of weathering of a surface as an indicator of its age (see Pierce, 1986). Among the various means by which degree of weathering has been quantified are Absolute dating methods the carbonate and/or clay content of an arid-region soil (see Birkeland, 1999), the heights of weathering The most easily understood absolute dating methods posts or depths of weathering pits on surfaces of involve materials that are annually layered. These boulders, and the thicknesses of weathering rinds on include tree rings and varves (laminated muds). While boulders, and of hydration rinds on obsidian. Where no individual tree lives more than a few thousand quantified in settings where the age of the surface is years, longer chronologies can be constructed by known independently, for example on lava flows, patching together the living tree record with that from these weathering rinds appear to increase in thickness older now-dead logs. This has been done in part by as the square root of time (e.g., Pierce et al., 1976; matching sections of tree ring width time series in Colman and Pierce, 1986). Such behavior smacks of a two logs, the pattern of which is dictated by climatic diffusive system, here involving the diffusion of conditions that are at least regional. These chrono- weathering products through the outer skin of the logies, which contain important paleoclimate infor- rock. As long as the reaction rates involved depend mation, now extend through and beyond the upon the gradient of the concentrations, then as the Holocene. As we will see below, a principal use of rind thickens the growth rate of the rind will decline. these chronologies is now in the calibration of other This is another example of a growing boundary layer. methods, such as radiocarbon dating, which can be As geomorphology has evolved to demand more performed on the same materials. quantitative ages, some have turned to the roughness Varves are annually laminated sediments. The of a surface as a proxy for the age of the surface. This layers are usually one to a few millimeters in thickness has been most successfully applied in desert alluvial and like tree rings vary in thickness through time. or debris flow fan dating. The idea is that such sur- In this case, the thickness of a varve reflects a compli- faces begin with high local roughness due to channels cated function of distance from the shoreline of the and levees, and due to the large grain sizes involved in water body, and the sediment supplied to it in that these flows, and that these features decay through year. In using varves, one must demonstrate that they time. As we see in the hillslope chapter (Chapter 10), are indeed annual layers. This can be done either by many processes tend to smooth or diffuse topography. independently dating material in a particular varve In addition, boulders break down to smaller clasts as (say a radiocarbon date on a piece of plant material they break apart by weathering. If these processes are or charcoal), or by documenting a pattern of some diffusive (dependent on the local topographic slope), other nuclide whose history is independently known. then the surface ought to decline in roughness rapidly Two candidates for this are 210Pb and bomb-derived at first and more slowly thereafter; theory suggests that 137Cs (Figure 6.1). The peak of 137Cs concentration
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1998 The most recent reversal, from the Matayama reversed to the Bruhnes normal polarity epoch, occurred at roughly 700 ka. Happily for geomorpho- 1800 logists interested in the latest Cenozoic, reversals have occurred roughly every million years in this time. This 1600 contrasts greatly with the extended normal epoch called the Cretaceous Quiet Zone during which the magnetic field appears not to have reversed for some 1400 20 million years. The Earth's magnetic field not only performs these dramatic flips: over shorter timescales, 1200 the field wanders (polar wander), and occasionally 2000 undergoes short-lived reversals. Age (yr A.D.) 1990 1000 1980
1970 Optically stimulated luminescence (OSL) 800 1960
Varve year Two luminescence techniques, thermal luminescence 1950 (TL) and optically stimulated luminescence (OSL), rely 1940 600 on a solid state property of common minerals quartz 1930 0 .010 .020 .030 and feldspar that allows them to record the time they 137Cs concentration (bq/g) 442 have been sitting in a deposit (e.g., see Berger, 1995; 0 5 10 15 20 25 30 Varve thickness (mm) Aitken, 1998). The property is that ionizing radiation, most of it from decay of radioactive elements (U, Th, Figure 6.1 Time series of varve thickness from K, Rb) in nearby sedimentary grains, can create free sedimentary section in freshly drained Iceberg Lake, Alaska. electrons that become trapped in defects in the mineral Record extends more than 1500 years. Inset: photograph of varves with 1 cm scale bar. Lower inset: profile of 137Cs, lattice. They are then released as luminescence upon which is expected to peak in the year of maximum atmospheric exposure to radiation of sufficient intensity. The atomic bomb testing (1963; horizontal band). That the amount released is proportional to the duration of peak corresponds to the year that layer counting suggests is 1963 strongly supports the interpretation of the layers the exposure to radiation (age of the deposit) and as being annual layers, i.e., varves (after the local intensity of the radiation, as depicted in Loso et al., 2004, Figures 3, 5, and 7). Figure 6.3. This means that one must measure both the luminescence in the lab, called the equivalent dose
or paleodose, De, and local radiation in the field should occur in a varve that corresponds to 1963, the setting, called the dose rate, Dr; the age is then simply spike in atmospheric testing of nuclear weapons. D T ¼ e ð6:1Þ Dr Paleomagnetic dating Operationally, one must sample the deposit in the On much longer timescales, one may employ the dark, or beneath a cover that limits the sunlight, so reversals of the Earth's magnetic field as a means of that the sample is not zeroed in the sampling process. dating stratigraphic sequences. Various compilations Stored in a light-tight container, it is then measured in of the reversal chronology have been published. In the lab. The local radiation is either measured in the general they are limited by the age of the oldest ocean field, or a sample of the nearby sediment is collected floor from which the magnetic field can be assessed. to be analyzed for the concentrations of radioactive Cande and Kent (1992, revised in 1995) have elements in the lab. OSL has become the preferred produced the most complete tables of the reversal method, as the measurement of the optically stimu- dates. Most recently, unmanned airborne surveys lated luminescence can be done in small pulses, have generated magnetic profiles across ocean basins allowing multiple measurements on a single sample, (Gee et al., 2008) that may serve to increase our and the time needed to extract the luminescence knowledge of the timescale, shown in Figure 6.2. signal is short. The radiation used to stimulate
Figure 6.2 Paleomagnetic timescale through mid-Mesozoic. Dark bands ¼ normal polarity, white ¼ reversed. Note the long Cretaceous quiet zone (KQZ) from 121–83 Ma (after Gee et al., 2008, Supplementary Figure 1, with permission from the American Geophysical Union).
release from the most sensitive sites is commonly blue, green, or infrared. This method has been applied to both fine-grained sediments (e.g., silt in loess), and more coarse sedi- a b c ments (e.g., fluvial packages, see Wallinga, 2002). The range of reliable ages extends to 200–300 ka, although some ages up to almost 1 Ma have been reported. One ate must assume that the grains being dated have been lower dose r Luminescence signal intensity "reset" or zeroed during transport to the site of the Time deposit. In bright sunlight, the exposure time is of the age of order of 100–10 000 seconds, or much less than a day. deposit The silt in loess, which travels to the depositional site in suspension in the air, over distances that can be up Figure 6.3 Hypothetical history of luminescence signal in a to thousands of kilometers, will certainly be reset. representative grain in a sedimentary deposit whose age we wish to determine. Three transport events (a–c, shaded) expose the grain Coarser sediment that travels in bedload, bouncing to light, releasing the electrons trapped in crystal defects. The time along the bed at the base of a flow of water, will take represented by these transport events can be as short as a few longer to be zeroed. hours, while the time between events can be thousands to tens of thousands of years. In the case shown, only event b fully "zeros" the signal. The luminescence measured Amino acid racemization in the lab will therefore yield an age estimate that is too large. The radiation dose rate provided by the nearby sediment in While 14C dating is the better known method for deter- the deposit in the interval between transport events a and b is smaller than those during its other times mining the age of biological specimens, we will see that of repose. it is restricted in its age range to about 40–50 ka.
Another method employs amino acids in biological temperatures. The mean annual temperatures during specimens and has a greater range, perhaps viable the LGM in the interior of western North America back to several hundred thousand years. Amino acids must have been at least 10 C below present tempera- are the building blocks of life. In live tissues and bones tures, confirming inferences made from periglacial and shells, the amino acids are twisted or coiled in a features such as frost wedge casts from nearby certain direction and are said to be racemized. Upon Wyoming. death, the molecules begin to flip back toward being 0.6 randomly racemized, half to the left, half to the right. 20 °C The degree to which the amino acids in a specimen have departed from purely left-coiling is a clock. The 0.4 10 °C rate at which this transformation takes place, and
hence the rate at which this clock ticks, depends upon 0 °C temperature in an orderly fashion, as in any chemical 0.2 D/L isoleucine reaction (Figure 6.4). It is an Arrhenius process with a definable activation energy. The different amino acids 0 have different activation energies, so that in a single 0 200 400 600 800 1000 specimen there may be several amino acid clocks Time (ka) ticking. Given the thermal dependence of the process, one Figure 6.4 Evolution of the D/L ratio in the amino acid isoleucine at three steady temperatures. Soon after can turn this dating method on its head and use it as a death of the organism, the L–D reaction dominates and the thermometer if we know the age independently. This D/L ratio climbs rapidly from zero. At later times, the forward has been done on the shorelines of Lake Bonneville, L–D reaction and backward D–L reaction rates are roughly 14 equal, and the D/L ratio ceases to evolve. The warmer the which are independently dated using C on shells (see temperature, the greater the rate of racemization, such Kauffman, 2003; see also Miller et al., 1997, for a that at 125 ka, for example, the D/L ratios are 0.1, 0.2, southern hemisphere story). The reaction progress in and 0.4, for the average temperatures of 0, 10, and 20 C (after Kaufman and Miller, 1992, Figure 1 and the shells is well below what we would expect if the temperature dependence after Hearty shells had been maintained at the present day et al., 1986, Figure 2).
Box 6.1 Extinctions in Australia The amino acid racemization dating of shells of two large flightless birds in Australia (the emu and a now-extinct variety known as Genyornis) has allowed the telling of an amazing tale of ecological change (Miller et al., 1999, 2005; Magee et al., 2004)(Figure 6.5). The ostrich-sized Genyornis, as well as 85% of the Australian megafauna, disappeared about 55 000 years ago. The emu and Genyornis co-existed until then. This timing coincides with the arrival of humans on the Australian continent, raising the question of the mechanism of extinction. The stable isotopes of C and O in the shells tell some part of this story. Because "you are what you eat" (meaning the stable isotopes of what you eat are recorded in your tissues and bones), we can deduce the diets of animals from their bones, or in this case the eggs that they laid. It appears that while the Genyornis continued to eat the same plants, the emu switched its diet from C3 to the C4 plants that now dominate (Miller et al., 2005). This in turn raises the question of why the change in plant types appears to coincide with the arrival of humans. The use of fire is hypothesized to have driven this change in vegetation. The switch to spinifex plants in northern Australia, which are far less effective at evapotranspiration, is now thought to alter the ability of moisture to penetrate into the interior of the continent; the shorelines of the large interior Lake Eyre in monsoon cycles that post-date the arrival of humans are lower than those during monsoon intervals prior to human arrival.
small Holocene rise 10 (a)
Lake level (m) Lake level –20
20°S 20°S ) 200 2 (b) 220
260 at 50 °N (W/m January insolation (c) 1050
30°S ) 2
950 35°S 850 130 °E 150°E at 20 °S (W/m 020 406080100120140 January insolation Time (ka)
Figure 6.5 Map of Australia highlighting the closed interior Lake Eyre basin. The history of Lake Eyre levels has been documented using 14C, U/Th, and amino acid racemization of shells from huge birds (a). January insolation histories at 50 N (b) and at 20 S (c) suggest control of monsoonal delivery of precipitation into the Australian interior at 125 ka and 9 ka (dashed vertical lines) by northern hemisphere insolation. The small size of the Phase I lake compared to the Phase IV and V lakes has been interpreted to reflect a significant change in vegetation in northern Australia that began at 50 ka associated with the arrival of humans on the continent (after Magee et al., 2004, Figures 1, 2 and 3).
Oxygen isotopes and the marine where the subscript "standard" refers to an accepted isotope stages value of this ratio in an aliquot of a world-accepted standard. For water samples this is commonly Any student of geomorphology should be keenly SMOW, standard mean ocean water, and in carbonate aware of the record of oxygen isotopes derived from samples this is commonly PDB, Peedee Belemnite, sediment cores from the deep sea floor. Our ability to a marine carbonate shell. As the departures of the measure stable isotopes and to drill deep cores ratios from the standards are generally small, we mul- evolved in the aftermath of World War II. The chief tiply the numbers by 1000, yielding units of permil (%). stable isotopes of concern are those of hydrogen and of By the early 1970s, records of the depth series of oxygen found in water: H, 2Hand3H, 16O, 17O, 18O. d18O from foraminifera (forams) in cores were being These lead to different masses of water molecules, produced. These revealed complicated patterns that which in turn influence their behavior in the physics were interpreted to represent some combination of of phase changes, and in biological processes. These temperatures of the water in which the forams grew, isotopes are now routinely measured in small mass and the isotopic concentration in the ocean above the spectrometers, and are reported as the ratio of the core site. A breakthrough came when the picking of isotopes in a sample relative to the ratio in a standard, forams (the selection of individual tests from the known as delta values. For example, d18O: 2 3 sediment sample) became more selective, in particular 18 =16 18 =16 isolating those forams that were known to grow in the 6 O O O O 7 d 18O 10004 sample standard5 deep-water column or in the sediments themselves – 16 18O= O benthic forams. (As these are much rarer than the standard planktic forams, picking enough tests to run a sample ð6:2Þ on the mass spectrometers of the time was quite
painstaking.) The signal and the interpretation of it variations in the Earth's orbit. They defined what have were simplified because surface water temperature was become known as marine isotope stages (MIS). effectively eliminated as a variable. In a series of now Such records from benthic forams have now been classic papers, Shackleton and Opdyke (1973, 1977) extracted from many sites around the global oceans. showed that the d18O signal varies strongly on 100 ka, The story is indeed a global one. The spectra of the time 40 ka, and 20 ka cycles over the last million years, and series consistently show distinct peaks at 100 ka, 40 ka, that the signal was found in numerous cores. This is and 19–23 ka, as expected from control of climate by shown in Figure 6.6. They argued that this signal ellipticity of the orbit, tilt of the spin axis (also called revealed a global story that supported the Milankovitch obliquity), and precession of the equinox, respectively theory that global climate varied with a beat set by the (Figure 6.7). At least in the Quaternary, the strongest component of the benthic foram signal is global ice volume. The argument is as follows. Water containing lighter isotopes of H and of O is easier to evaporate warmer 5e 1 than that with heavier isotopes. Their smaller mass high SL 11 9 7 low ice translates into higher speeds for the same temperature 5c 5a volume (see Problem 1 in Chapter 1). If (and only if ) the water O 18
δ 3 that evaporates is sequestered on land as ice (in the great ice sheets, for example), the water remaining in 8 4 colder 10 low SL the ocean will become enriched in the heavy isotopes. 6 2 high ice The degree of enrichment is proportional to the 800700 600 500400 300 200 100 0 volume Age (ka) amount of water sequestered as ice sheets, and hence is a proxy for global ice volume. Forams grown in Figure 6.6 Marine oxygen isotopic signal over last the ocean will take on the isotopic concentration of 800 ka, based upon benthic forams. The dominant 100 ka period ice volume signal is clear, with 40 ka and 20 ka the water, and will therefore faithfully record this proxy. period smaller amplitude fluctuations. The last few marine While forams are ubiquitous, and a continuous isotope stages (MIS) are numbered. Of greatest importance depth series can be obtained by closely sampling the to the geomorphic story written on the landscape are the last glacial maximum (LGM) at 20 ka, and last major deep sea cores, the absolute timing of these records is interglacial (MIS 5e). less well known than one might think. Absolute dates
2.5 100 kyr world 41 kyr world
O (‰) 3.5 18 δ Early Pliocene 4.5 warm period (b) 5 10
4 δ 18 ) O(‰) –2 Onset of NHG 3 6 2 1 (Watts m Solar forcing 2 0
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Age (Myr ago)
Figure 6.7 (a) Oxygen isotope record covering the majority of Pliocene–Pleistocene time showing early warm period of the Pliocene, onset of northern hemispheric glaciation at roughly 2.7 Ma, and transition from 41 ka world to 100 ka world at 1 Ma. (b) Solar radiative heat flux (after after Ravelo et al., 2004, Figure 1, reproduced with permission of Nature Publishing Group).
come largely from magnetic stratigraphy. Cores Paleocene–Eocene boundary (the PE thermal max- extracted from the deep sea are sampled densely and imum, or PETM). One can also pick out the onset of assessed in sensitive magnetometers. Flips in the mag- major Northern hemisphere ice sheets at around netic field are documented, and counted backwards 2.4 Ma. All of these signals, so remarkably recorded down through the core. The last time the magnetic field in deep sea cores, not only pose challenges to the was reversed was roughly 700 ka (recall Figure 6.2). paleoclimate community for understanding of the cli- This Bruhnes-Matayama transition from reversed mate system. Many of the explanations put forth for (Matayama) to normal (Bruhnes) is therefore often the major events in the record involve geomorphic the first major tie-point in the isotopic record. One systems, including variations in ice sheets or can imagine that if such tie points to absolute time are weathering. In turn, the variation in climate that is so rare (roughly every million years), there is a lot of implied by these and other proxies derived from wiggle room in the true dates of events between them. deep-sea cores must be acknowledged in the study of The isotopic records have now been measured back the Earth's surface, as they are the best continuous through the entire Cenozoic – basically as deep in proxy we have of how the climate that intimately influ- time as the oldest ocean floor (see review in Zachos ences all geomorphic systems has evolved over the et al., 2001; and Figures 1.6 and 6.6). As we hinted in timescales of landscape evolution. the introductory chapter, several features of this record are worth becoming familiar with. Times of light water, low in 18O, correspond to low ice volume, Radiometric dating methods high sea level interglacials. These are labeled with odd The most common absolute dating method is the marine isotope stages (MIS). We are presently in 14C or radiocarbon method. In this method we take 18 MIS 1. Times of heavy ocean water, high in O, advantage of the decay of 14C atoms to 14N, which correspond to high ice volume, low sea level glacials, occurs with a half-life of roughly 5730 years. and are even isotope stages. The last glacial maximum 14C atoms are produced in the atmosphere by the (LGM) is MIS 2, and is centered around 22 ka. collision of cosmic ray particles (see next section) with Inspection of the marine isotopic record reveals the gases in the atmosphere. These atoms then become
complicated beat of the climate first documented by incorporated in the trace gases CO2 and CO, the Shackleton and Opdyke. The ice volume history former of which is incorporated in plants upon
leading to the LGM was punctuated by smaller scale photosynthesis. Some small fraction of the CO2 reversals in ice volume that have been subdivided (about 10–10 percent) used by the plant has this using letters. The last major glacial was in MIS 6, 14C atom, as long as the plant is alive, it continues and was followed by MIS 5e, the last time sea level to incorporate these anomalous atoms in the ratio in exceeded present sea level (by about 6 m). Letters a, c, which they occur in the atmosphere. Upon death, the and e correspond to low ice volume, b and d to high 14C atoms in the tissues of the plant will begin to ice volume. MIS 5e, 5c, and 5a, for example, all have decay to their daughter products. We can measure corresponding sea level highstands recorded in coral the ratio of the 14Cto12C in the organic material of terraces in Papua New Guinea. the plant as a clock. While the decay of any one atom Taking a little broader view, we can see in Figure 6.6 of 14C is a stochastic or probabilistic event, we do 18 that the oscillations in O in the last million years have know that the probability of the decay of any one of been dominated by the 100 ka cycle. The larger glacials the 14C atoms to its daughter in any unit of time (say are separated by 100 thousand years. This was not the a year) is a constant, set by the decay constant, l: case in the early Pleistocene (Figure 6.7), when the dN signal is dominated by 40 ka cycles; variations in ¼ lN ð6:3Þ the tilt of the Earth's axis ruled the climate then. The dt transition around 1 Ma remains a target of study, with where N is the number of parent atoms. The rate of many hypotheses about its cause. At an even larger decay of the concentration of parent atoms, N, scale (Figure 1.6), we can see the trend toward a depends simply upon the concentration. The resulting cooler Earth through the Cenozoic, a trend that is equation for the total concentration of parent punctuated with abrupt events such as that at the 14C through time is then
lt N ¼ Noe ð6:4Þ The first step is simply algebra, and the second step requires taking the natural logarithm of both sides ¼ where No is the initial concentration at t 0, the time and rearranging. The half-life is therefore related to of death of the plant. Let us make sure we understand the inverse of the decay constant (as the decay this most fundamental equation for radiometric constant describing the likelihood of decay at any dating. You are probably familiar with the notion of moment goes up, the half-life goes down). As ln (0.5) ¼ a half-life. This is the time, t1/2, that it takes for the –0.693, this relationship simplifies to t1=2 ¼ 0:693ð1=lÞ. concentration to fall to one half, or 50%, of the Just to be complete about this discussion, note original concentration, as seen in Figure 6.8. Given that the units of l must be the inverse of time, or 1/T. a graph of the concentration history, you could read Its inverse is then a timescale, which is called off this time on the time-axis. At two half-lives, the the "mean life" of the radioactive system. This is the concentration will have fallen to half of a half, or a average lifetime for a 14C atom, given the probability quarter, of its original concentration, and so on. How of its decay at any moment, l. It may also be inter- do we connect this to the equation shown above? We preted as the time it takes for the concentration of must find the relationship between the decay constant 14C to fall to (1/e) of its original value. We now see ¼ and the half-life. N No/2 should occur at the half- that the half-life is 69% of the mean life. Returning to life. Inserting this into the equation we find that Figure 6.8, we can graph both the half-life and the
No l mean life. ¼ N e t1=2 2 o Now consider an application. We have found a 1 piece of wood in a landslide deposit (or a river ter- lt1=2 ¼ e ð6:5Þ 14 2 race, etc.), and wish to use the C method to date the 1 1 wood and hence establish the age of the landslide. t = ¼ ln 1 2 l 2 How do we measure this? There are two methods, one old and less expensive, the other new and more expensive. In the first method we count the number of (a) decays of 14C atoms per unit mass of carbon in what 2.5 amounts to a large Geiger counter. In essence, we are N = 1010 2 o using the equation for the decay rate, dN/dt ¼ –lN,to 1.5 estimate N, the concentration of parent atoms. Here 1 the sample is placed in a sealed container surrounded 0.5 by scintillation counters that detect and count decays.
Activity, decay/min 0 0 5 10 15 20 25 30 35 40 45 50 The higher the decay rate, or "activity," the faster we Time (ka) get an answer. You can also see that the larger the (b) 1 sample is, the more atoms will decay within it in a 0.8 given time. However, if the sample is very old, and
o 0.6 now has very low N, the decay rate will be similarly 0.5
N/N 0.4 1/e = 0.368 low and we will have to wait a long time to detect 0.2 enough decays for us to have much confidence in the 0 answer. In addition, the lower the decay rate, the 0 5 10 15 20 25 30 35 40 45 50 λ more likely the signal will be contaminated by t1/2 1/ Time (ka) counting of events that are not from the sample itself
Figure 6.8 (a) Rate of decay of 14C concentration in a but from other energetic particles such as cosmic rays. sample (measured by its "activity"), and (b) concentration Such laboratories are typically sited below ground 14 N of C relative to its initial concentration, o. Note that the half-life in order to minimize counting of stray events. The corresponds to N/N ¼ 0.5, while the mean life (1/l) corresponds o facility at the Quaternary Research Center at the to N/No ¼ 1/e ¼ 0.368. By 30–40 ka, the concentration and hence the activity of the 14C system is so low that the signal becomes University of Washington was also lined with lead difficult to measure relative to the noise (gray bar). Gray bricks constructed of pre-WWII materials. These horizontal bars representing measurement uncertainty correspond to increasing uncertainty in sample age issues place an effective limit on the ages that can be as the age increases. documented using this method (see Figure 6.8)of
about 40–50 ka. Sample ages that are beyond this age 0 are said to have yielded an infinite age, which merely 20 U/Th means that the reported age cannot be distinguished 14C from twice or ten times this age, or for that matter 40 from an infinite age. An alternative method has revolutionized 14C dating 60 within the last couple of decades. In this method one 80 counts not only those atoms that decay within some Depth (m) 14 12 reasonable time, but the ratio of Cto C in the whole 100 sample. Because this uses all of the C atoms in the sample, and not that tiny fraction of them that decay, 120 it requires a much smaller sample. We can now date 140 individual foraminifera, for example, or individual 0 5000 10 000 15 000 20 000 25 000 14 seeds, or a small thread from the Shroud of Turin. C (years) The measurement uses a mass spectrometer, but a fancy Figure 6.9 Discrepancy between 14C and U/Th dates on a set one. The method is dubbed the accelerator mass of coral samples from Barbados. The U/Th ages are consistently spectrometric, or AMS, method because it requires older, the discrepancy being greatest for older samples (after an accelerator to generate high speeds in the isotopes Fairbanks, 1989, Figure 2, reproduced with permission of Nature Publishing Group). so that their small mass differences can be measured.
Non-steady production Given the importance of the 14C method in geology As with any dating method, there are several prob- and archeology, significant effort has been expended to 14 lems with which the user of the method must be calibrate the C method against others. We have seen aware. The radiocarbon method is not perfect, even that when a tree ring chronology is available, this has within its 40–50 ka range of application. This is been fruitful in demonstrating the unsteadiness of the primarily because the production rate of 14C in the production. But what about that part of the timescale atmosphere is not steady. We have known this for that is beyond available tree ring chronologies? The some time simply by comparing the date derived longest such chronology is roughly 10 ka. In a now- using 14C with ring-counts on trees. Ideally, these famous paper in Nature in 1989, Fairbanks compared 14 methods should yield the same results. That they the C clock with the U/Th clock on corals. Happily, 14 do not documents the variability of the production the C in corals can be used for C, while U substitutes rate through time. But why does it vary? There are for Ca in coral skeletons. The two methods could be two culprits. While the cosmic ray flux to our pos- used on the same samples. Fairbanks showed that the 14 ition in the solar system should be steady through C clock was off from the U/Th clock by at least 2 to time, the magnetic fields of both the Earth and Sun 20 ka (Figure 6.9). Given that the U/Th clock is a vary with time. Both influence the deflection of the purely atomic one, and is immune from the production 14 incoming cosmic ray beam. The strength of the Sun's rate variability to which C is subjected, the U/Th magnetic field is reflected in the number of sunspots results are considered the more reliable. The radiocar- on its surface. This varies with an 11-year period, bon clock has subsequently been calibrated to that although there are longer timescale variations in derived from the U/Th results, or more accurately, which this 11-year cycle is embedded. The Earth's the production rate history for cosmogenic radio- 14 magnetic field varies both in its strength and orien- nuclides, of which C is a member, has been back- tation. This directly affects the steering of charged calculated from these and other results. particles (protons, hydrogen nuclei, which constitute most of the cosmic rays impacting the Earth's The reservoir effect upper atmosphere): times of low magnetic field strength result in less effective shielding and higher Recall that a living organism will incorporate production of 14C. 14C from the atmosphere or the ocean from which it
draws its carbon. If the residence time of 14C in that Cosmogenic radionuclides fluid is long, some of the 14C will have decayed by the These rare isotopes are born of interaction of cosmic time the organism incorporates it in its shell or plant rays with atmospheric and Earth surface materials. tissue. The concentration of 14C will therefore be 10Be, 26Al, 36Cl, 14C are radioactive isotopes, and lower than expected, causing a shift in the calculated therefore decay with a characteristic timescale (see age. This is called the reservoir age. While the mean Table 6.1). It is this timescale that determines the residence time for C in the atmosphere is short, it is a temporal range of application of the nuclide. We have few hundred years in the ocean. This shift is therefore just discussed 14C, which is produced by interaction applied to the calculation of the age of ocean-dwelling of cosmic rays with N in the atmosphere. Two other organisms, and can be several hundred years. It has commonly used cosmically produced nuclides, 3He also been shown by careful comparison of the and 21Ne, are stable, meaning they do not decay 14C ages of marine organisms in varves (annual layers) that can be counted from some specially sited oceanic sediments that this reservoir effect is not 50 steady over time. The shift shifts. This reflects vari- ation in the water balance of the ocean, which is seriously tweaked in times of large rates of change 40 in ice sheets. This effect may be quantified if we know independ- 30 ently the age of a deposit. This is the case in the C age (Ka BP)
wonderfully varved deposits of the Cariaco basin in 14 20 South America. Varves have now been counted back to roughly 50 ka (Hughen et al., 1998, 2000, 2004). The 14C ages of the sediments can then be compared 10 with the ages derived from independently dated 10 20 30 40 50 records such as the Greenland ice core from GISP2. Calendar age (Ka BP) If the production of 14C in the atmosphere has been Figure 6.10 Reduction of 14C age below calendar age steady, and the reservoir effect has not changed for interval including the LGM. Equivalent ages would lie through time, these chronologies should line up on on 1:1 line (dashed). Departure reflects the effective reservoir a 1:1 line. As seen in Figure 6.10, they do not, and age of the oceans, which varies greatly through the LGM (redrawn from Hughen et al., 2004, Figure 2, with the departures can be used to deduce how the reser- permission from the American Association for voir effect has evolved through the last glacial cycle. the Advancement of Science).
Box 6.2 Use as a tracer for documentation of fossil fuel burning An interesting twist on this reservoir effect is that associated with the burning of fossil fuel. As all fossil fuel (coal, oil, natural gas) is extracted from deposits that are very old (tens of millions of years) 14 relative to the half-life of C (thousands of years), the CO2 emitted by fossil fuel-burning power plants 14 14 will all be " C dead." The C concentration of the CO2 will be zero. This gas is then mixed efficiently with the rest of the atmosphere as the wind blows the emissions from power plants downwind. The air downwind 14 14 will therefore display a lower than expected C concentration, reported as Á CO2. That the degree of lowering is directly proportional to the emission rate means that the 14C concentration of this air may be used to deduce the emission rate from power plants (e.g., Figure 6.11). This method has been proposed as a quantitative means of monitoring emission rates, replacing present methods that involve estimation based upon economic metrics such as gross national product.
Table 6.1 Radionuclides and their half-lives time over which most of the Earth's landscape has evolved significantly, over which many of its geo- t l t 1/2, half-life , decay , mean morphic features have been produced (see Bierman Nuclide (yr) constant (1/yr) life (years) and Nichols, 2004). 10Be 1.387 106 5.0 10–7 2.00 106 The cosmic rays responsible for the nuclear reac- 14C 5.73 103 1.21 10–4 8.27 103 tions are energetic particles originally accelerated to 26 5 –7 6 Al 7.05 10 1.42 10 1.02 10 great speeds in supernova explosions elsewhere in the 36Cl 3.01 105 2.30 10–6 4.34 105 41Ca 1.04 105 6.67 10–6 1.50 105 cosmos; hence their appellation "cosmic." The parti- 129I 1.56 107 4.44 10–8 2.25 107 cles responsible are originally protons, or the nuclei of hydrogen atoms, which are stripped of charge upon
Source: Half-lives after Prime lab web site, with encountering the Earth's magnetic field to become updated 10Be half-life after Chmeleff et al., 2009; Korschinek neutrons. Neutrons are far less reactive than protons, et al., 2009. so can then penetrate more deeply into the atmos- phere. As these particles descend into the atmosphere they encounter ever-increasing densities of gas, increasing the likelihood and frequency of nuclear 80 interactions. The production rate at first increases with depth into the atmosphere, as there are more 75 atoms with which to react. It then declines, as signifi- cant numbers of reactions sop up energy in the 70 incoming particle rain. An example of the cascade of
(‰) interactions and particles produced from a single 2 65
CO incoming particle is shown in Figure 6.12. Such a 14
D diagram inspires wonder at the complexity of the 60 process. It is the most energetic of these that are most capable of producing the nuclear reactions in 55 atmospheric and near-surface materials. Because these interactions are cumulative, the pro- 2003 2004 2005 2006 duction rate of nuclides depends upon the depth into Date the material, and on the density of the material. In effect, what matters is the probability of the incoming Figure 6.11 Time series and best-fitting filtered curve 14 particle impacting an atom, which therefore depends of Á CO2 from Niwot Ridge, CO (closed diamonds; solid line) and upper tropospheric samples from New England (open boxes; upon the number of atoms per unit volume, and the dashed line) over a three-year period. Downward trend is depth into the material. The parameter characterizing 14 attributable to increasing contributions of C-dead CO2 L to the atmosphere from power plants (from Turnbull this interaction likelihood is , the "mean free path." et al., 2007, Figure 6, with permission from the This may be translated into a characteristic length American Geophysical Union). scale by dividing by the density of the material. For example, if L ¼ 160 gm/cm2, then a length scale over which the production rate declines significantly is
through time once produced. While we will focus on L/r. We call this length scale z* (although in the 10Be and 26Al, much of the discussion is relevant to all literature it is common to see L referred to as a of the nuclides. That these nuclides are produced by length). Given that the difference in density between interaction with common minerals on the Earth's rock or soil and the atmosphere is about 2000-fold,
surface (e.g., quartz, SiO2), and that the half-lives of the characteristic length scales will differ accordingly. the nuclides are of the order of one million years, For most of the nuclides of concern, the length scale
means that they are readily available, ubiquitous, z* within rock is a large fraction of 1 m, while that in and can be used to establish timing on the Earth's the atmosphere is about 1.5 km. That the production surface over most of the Plio-Pleistocene. This is the rate falls off with a 1 m scale in the Earth means
particle has spent within this zone – exactly the top of atmosphere zone in which most geomorphic processes operate. Similarly, that the production rate depends upon altitude with a length scale of 1.5 km means that by 4.5 km, say the mean altitude of the Tibetan Plateau, or of the altiplano in South America, the production rate will be e3, or roughly 25-fold higher than at sea level. Much to the benefit of the field geomorphologist who has to carry out his/her samples from the field, this implies that samples from such places can be much smaller (see Problem 3 on page 157). Production occurs by several mechanisms, each of which may be written as a nuclear reaction. The most important reaction is the splitting of an atom, or "spallation" in which the target nucleus shatters into several smaller shards, as depicted in Figure 6.13.In the reaction shown, 26Al is produced from splitting of air 28 rock Si. The other shards produced are typically smaller, e.g., an alpha (a) particle, or nucleus of 4He. In general, the reactions are numerous and quite com- plex. Another process is neutron capture, which is important for example as one mechanism involved in the production of 36Cl. Here it is slow neutrons, electromagnetic hadronic mesonic braked by the numerous interactions within the component component component atmosphere and near-surface rock, that are now slow enough to be captured into a nucleus, transmuting Figure 6.12 Cascade of particle interactions generated by the 35 36 entrance of a high-energy particle at the top of the atmosphere. Cl to Cl. Cosmogenic nuclides produced both in the atmosphere and in the top few meters of rock most commonly result from at least secondary particles. In situ production profiles within rock In situ production means production in place, in this case within the rock or soil near the Earth's surface. The production rate profile for 10Be and 26Al can be characterized by the sum of three exponential pro-
files, each with its surface production rate, Po, and 28 26 Si(n, p2n) AI with a characteristic decay length scale. These are shown in Figure 6.14 in both linear and log plots. Figure 6.13 Diagram of spallation reaction producing 26Al from 28Si (from D. Granger, website, Purdue). Recall that exponentials appear as straight lines when shown on a log–linear plot. It is apparent that near the surface the spallation mechanism dominates, while below some trade-off depth muogenic produc- that by a depth of several meters the production tion mechanisms dominate. This cross-over depth is rate will be negligible. Since cosmogenic radionuclide several meters, by which time the production rate is accumulation will occur only within this top few down to only a few percent of its surface value. In meters, the concentration of cosmogenic radio- only a few applications must we worry about muo- nuclides will reflect the time a parcel of rock or a sand genic production.
0 production rates at times of low field strength (and 1 presumably a spike in production associated with a magnetic field reversal, during which the dipole 2 spallogenic dominates strength lessens dramatically). It is largely this vari- 3 ation in the magnetic field that wreaks havoc with the 4 CRN-based clock. We can use the tool with confi- 5 dence only when we know the rate at which the clock has ticked in the past. In 14C dating, the 14C dates Depth (m) 6 muogenic have been calibrated against tree-ring and varve 7 dominates chronologies that are independent and that extend 8 almost the entire timescale over which the method is 9 useful (say 40–50 ka). For other nuclides with million- 10 26 10 year timescales ( Be and Al, for example), no such 10–5 10–4 10–3 10–2 10–1 100 101 independent chronometer exists. Much effort in both P (atoms/gm qtz/yr) European and US scientific communities has recently been expended to produce a reliable production rate Figure 6.14 Profiles of production rate due to three main production mechanisms. Bold gray: spallation; black: total history. This was done largely through measurement production. Dashed line is sum of production due to two muogenic of CRN concentrations in surfaces of known processes, which are well approximated by the single exponential age, from which the mean production rate over the shown in the thin solid line. The cross-over depth (dashed horizontal line), given the assumed density, is roughly 3.6 m. age of the surface may be calculated. In the early use of the method, samples of glacially polished bedrock immediately up-valley of moraine-ponded lakes were measured (e.g., Nishiizumi et al., 1989). The independent age of the surface came from Variations in production rate 14C dating of basal peats in the lake sediments. in space and time One other element of reality must be faced in CRN production varies in both space and time. While CRN dating. Topography can shield or partially the flux of cosmic rays arriving from the cosmos in block the cosmic rays responsible for production, the vicinity of the Earth is likely both isotropic and thereby lowering the production rate on a surface. steady, the production of radionuclides in the Earth's We have already noted that production varies with atmosphere and at its surface is neither uniform nor elevation – the atmosphere serves to attenuate the steady. The primary particles involved are protons cosmic ray flux. In calculating the topographic (the nuclei of hydrogen). Because they are charged, shielding at a site, we must assess the fraction of they are affected by magnetic fields. Production of the incoming cosmic ray beam that is blocked by cosmogenic nuclides is therefore modulated by the the local horizon. This is made somewhat compli- spatial pattern of the magnetic field of the Earth, by cated by the fact that the beam is not uniform with temporal variations in it, and by the solar wind which angle from the vertical (the zenith angle). The incom- in turn is modulated by the Sun's magnetic field. ing beam of cosmic rays is presumed to be isotropic. Solar variations give rise to the well-documented But the travel path within the attenuating atmos- 11-year cycle that is documented in atmospherically phere is longer the higher the zenith angle. This produced 10Be measured in ice cores. The Earth's results in a non-uniform contribution to the produc- magnetic field steers charged particles toward the tion of CRNs from different parts of the dome, poles (witness the aurora borealis), and shields them lowest efficiency for near-horizontal angles, highest from equatorial latitudes. This causes higher cosmo- for vertical. It is this bell-shaped pattern that is genic nuclide production at higher latitudes than at truncated by the horizon. The user of the method lower latitudes. The magnetic field of the Earth therefore documents the angle to the horizon in changes in both its effective dipole axis (magnetic pole many directions (commonly 8). For most low-relief location) and strength. Secular variation in the sites the shielding factor is between 0.9 and 1.0, but strength of the Earth's magnetic field results in high can drop significantly in mountainous terrain.
Theoretical backdrop It is the concentration of these rare nuclides in soil and rock that we use to deduce timing in the land- scape. Given that nuclides are both produced and decay, we must take both processes into account in crafting an equation that governs the evolution of their concentration through time. Just as in many other systems described in this book, the word picture for the system is:
rate of change of number of atoms in the dx x x+dx box ¼ production of atoms in the box – decay of atoms in the box Figure 6.15 Production and decay of cosmogenic radionuclides. Cosmic rays interact with atoms in surficial This is illustrated in Figure 6.15. If we cast this math- materials to produce new nuclides (stars). Radionuclides decay ematically, we have (circles) with probability set by the decay constant. The rate of change of the concentration of radionuclides is therefore set by dC dxdydz any mismatch between the birth and death rates. ¼ P dxdydz lC dxdydz ð6:6Þ dt where C is the number of atoms per unit mass of 5 14 quartz, r the density of the rock, P the production C = P/λ 12 rate of new nuclides per unit mass of quartz, l the 10 decay constant for the nuclide, and dxdydz the 8 volume of the box. This can be converted to an equa- slope = P tion for the rate of change of nuclide concentration 6 per unit mass of material (say atoms 10Be per gram of 4 quartz, something we measure) by dividing by the 2
r Be atoms/gram quartz, × 10
mass, dxdydz, leaving 10 0 0123456 dC Time (My) ¼ P lC ð6:7Þ dt Figure 6.16 Approach of 10Be concentration toward We can anticipate the shape of the solution by inspecting secular equilibrium. Initial slope is set by the production rate. this equation. Early in the history of accumulation, low The timescale for the approach is set by the decay constant, l, the inverse of which, the "mean life," is 2.00 Ma. Uncertainty concentrations of nuclides will result in only small in measurement of [10Be] (heights of gray boxes) leads to contributions from the second term, and the rate of uncertainty in age (dashed lines), which increases with the growth of the concentration should be steady, set age. At ages of several times the mean life, the concentration is no longer a good clock. by the production rate P. As concentration increases, however, the decay term will grow, forcing the rate of increase of concentration to decline. Ultimately, the concentration should become high enough that the P/l, at which time the system is said to be in "secular production and decay terms balance, and a steady con- equilibrium." This plot also reveals the timescale over centration should then be achieved. Setting the left-hand which the concentration of a radionuclide will be side of Equation 6.7 to zero, we find that this would useful as a clock. Once the concentration gets close occur when C ¼ P/l. Indeed, the solution to this equa- to P/l, and the concentration is no longer changing tion, shown in Figure 6.16, reveals just this behavior: significantly with time, the concentration can no longer be used to reveal time. The real limiting time ¼ P lt ð : Þ for the method is dictated both by the characteristic C l 1 e 6 8 time of decay (or the "mean life," 1/l) and by our The early growth rate (slope on the plot) is set by the ability to resolve concentrations in the method (see production rate, P. The asymptotic concentration is Figure 6.16, with error bars).
How are the measurements made: sputtered from the sample target are accelerated to processing and AMS measurement great speeds. The ratios of 10Be/9Be or 26Al/ 27Al are then measured by bending the resulting high-speed The measurement of the concentration of cosmogenic beam of ions around corners using magnets that are radionuclides from rocks is not easy. It includes always the root of a mass spectrometer. What is mineral separation, purification of the CRN nuclides remarkable is that we can routinely measure these to generate a target, and measurement of the CRN ratios to levels of 10–15, with errors of a few percent. nuclide concentrations in an accelerator mass spec- It is hard to grasp this number, but here is one way. If trometer. The first step is the separation of quartz from Be atoms were sand grains, with a diameter of 0.1 mm all other minerals in the sample. This in itself is time- (fine sand), and 10Be were red while 9Be were white, consuming, as the properties of quartz are very similar then measuring to levels of 10–15 corresponds to find- to those of feldspar. The most common method entails ing one red grain in a volume of 103 m3, or roughly the slow dissolution of the sand-sized grains in a dilute HF volume of a typical one story house. Enough said! acid cocktail. As the dissolution of feldspar is slightly These are difficult measurements to make. faster than that of quartz, many hours of stirred acid Over the last two decades geomorphologists have baths result in the sample "cleaning up" to pure used the concentrations of cosmogenic radionuclides quartz. After addition of a known quantity of very in surface materials to deduce the rates of many pure 9Be or 27Al spike (the common stable nuclides surface processes. We have dated abandoned bedrock of these elements), the quartz is then dissolved and the surfaces, abandoned depositional surfaces, and cave Si fumed off to leave a scum on a vial that contains all deposits. We have obtained measurements of the the impurities in the quartz, including cosmogenically rates of surface lowering (erosion) at points, and produced atoms of 10Be and 26Al, along with the spike. averaged over basins. We are also beginning to see Isolation of these nuclides by ion chromatographic the method employed to date stratigraphic sections columns results in a sample target into which the going back into the Pliocene (>2 Ma). We will CRN nuclides are tamped. Measurement of the CRN describe briefly each of these applications. concentrations is done in an accelerator mass spec- trometer facility, of which there are not many in the Dating bedrock surfaces world. Most of these facilities are dedicated largely to 14C measurements, mostly for medical analyses; the Once abandoned, a bedrock surface carved by some time windows in which 10Be, 26Al and other nuclides of process, for example glacially or fluvially, will simply importance are available to geomorphologists are accumulate cosmogenic radionuclides at a rate short. The facility is very complex, as revealed in dia- dictated by the local surface production rate. As grammatic form in Figure 6.17. The heart of the pro- the concentration will follow the curve depicted in cedure is a tandem accelerator in which the atoms Figure 6.16, one may deduce an age for the
ExB Velocity Selector
Analyzing Magnet
Switching Magnet Tandem Ion Source Injector Magnet Accelerator Electrostatic Analyzer Gas Ionization Detector
Figure 6.17 Diagram of AMS facility used to measure small concentrations of radionuclides (from PRIME lab website).
abandonment of the surface by solving Equation 6.8 for time. This is simply lC t ¼ t ln 1 ð6:9Þ Po where C is the measured radionuclide concentration and t ¼ 1/l. In the limiting case of young ages and hence low concentrations, the decay term in Equation 6.7 can be neglected and the age is simply the concen- tration divided by the surface production rate,
t ¼ C=Po ð6:10Þ In only rare instances have very old surfaces been discovered. The reason is that all surfaces are experi- Figure 6.18 Sampling bedrock strath more than 100 m above the Indus River, Pakistan, as it incises its Middle Gorge through the encing some rate of surface lowering due to weathering; Himalayas. Exposure dating of this and other scraps of strath original surfaces that have experienced no erosion since terraces revealed rates of incision as high as 1 cm/yr their exposure by an erosion event in the past are (photograph by R. S. Anderson). therefore difficult to find. In general, one must use the existence of some kind of surface indicator, such as the patina of fluvial wear, or glacial polish, whose thick- nesses are very small, to document the lack of surface
lowering since abandonment of a surface. It is just such Exposure Age (ka) glacially polished surfaces that are used to document 10 15 20 25 long-term mean surface production rates; Equation 6.8 1.6 1.5 (a) can just as easily be solved for Po if we know independ- 1.4
ently the surface age. In the case of glacial polish, such 1.3 520 2 14 Lake Bonneville independent ages come from C dating of peats in (b) 500 Shore Elevation (km) Elevation Shore the base of moraine-ponded lakes, which were presum- 480
ably roughly coeval with the formation of the last bit –32 460 Insolation Watts/m O ‰ –38 (c) of polish on the bedrock just up-valley. 18 –40 In Figure 6.18 we show an example of such a bed- –44 YD BA
GRIP δ 100 rock surface, sculpted by the channel of the Indus river, LHML (d) Mean retreat rate = 15.4 m/yr and then abandoned, and now more than 100 m above 80 Elk Creek the river. Cosmogenic radionuclide dating of this and Silverton Molas Lake several other surfaces with similarly river-worn bed- 60 30.2 ± 2.3 ka rock morphology has revealed a spatial pattern of Needleton long-term incision of the Indus river through its middle 40 Tacoma gorge as it tangles with the rapidly uplifting rock near Baker's Bridge
Nanga Parbat (Burbank et al., 1996). (km) Distance Up-valley 20 Using a set of samples collected from glacially Durango Moraines polished surfaces, Guido et al.(2007) documented 0 the deglaciation history of a 90 km long valley 10 15 20 25 Exposure Age (ka) draining the San Juan mountains in southwest Color- 10 ado. The measured Be concentrations were first Figure 6.19 Ages of sites in Animas Valley, San Juan interpreted to be surface ages using Equation 6.9. Mountains Colorado, based upon 10Be concentration in LGM All but one of the surface ages lined up in a mono- terrace, and in glacially polished bedrock (d), and various proxies for climate: (a) Lake Bonneville shoreline elevation; (b) insolation; tonically declining trend with up-valley distance from (c) d18O from GRIP ice core, Greenland. Deglaciation takes at least the terminal moraine complex (Figure 6.19), which 7 ka (after Guido et al., 2007, Figure 3).
lends confidence to interpretation of the trend as a 0 deglaciation history: the samples record the time since 0.5 they were uncovered by retreat of glacial ice from the last glacial maximum. The one outlier, sampled from 1 a resistant quartzite ridge, both illustrates the poten- 1.5 tial problem with the method and its potential utility as an erosion meter. Sampled from roughly half way 2
up the valley, the outlier appeared to be twice as old Depth (m) 2.5 (had twice the 10Be concentration) as most of the samples, making it impossible to interpret as a time 3
since deglaciation of that portion of the valley. It is 3.5 instead interpreted to have experienced too little 4 erosion during the last glacial cycle to remove the full 0 0.2 0.4 0.6 0.8 1 1.2 1.4 10 inventory of Be obtained during exposure to cosmic Concentration (atoms/gram quartz) rays in the last interglacial. Given the decay of
production with depth in rock, characterized by z* Figure 6.20 The expected shifted exponential of (¼ 0.7 m), "full" resetting of the cosmogenic clock cosmogenic radionuclides in a deposit. The gray profile reflects the inheritance of radionuclides, averaged over many grains. requires removal of at least 3z* or 4z* of rock, leaving The bold profile is the total concentration, which includes both –3 –4 3% (e 0.03) to 1% (e 0.01) of the original inheritance and post-depositional grow-in of nuclides. inventory. Lack of full resetting therefore indicates Scatter at any depth largely reflects variation in the inheritance, as post-depositional production should less than say 2–3 m of erosion in the glacial cycle. If be determined well by the depth into the deposit the glacial cycle at that point in the valley lasted (in other words, it is deterministic). 30 ka, this would translate into long-term glacial erosion rates of less than 3 m/30 ka ¼ 0.1 mm/yr. It appears that glacial erosion rates are for the most In addition, the inheritance will inevitably vary from part sufficient to reset the cosmogenic nuclide clock to one grain to another. Each grain has its own history of zero. In only rare geologic settings is significant exposure, and hence will arrive on the surface with its inheritance measured: either in very hard rock such own inheritance. This gives rise to considerable scatter as quartzite, or in massive granites such as some of if one dates single cobbles on a surface. How do we see the joint-free intrusives of Yosemite Valley. through this problem of inheritance? The solution is expensive. A method has evolved in which one measures the concentrations of several samples (hence Dating depositional surfaces the expense) in a vertical profile into the surface, each More common than bedrock surfaces are sediment- sample being an amalgamation of equal mass from capped surfaces, some simply mantling bedrock many clasts. The amalgamation process effectively surfaces with several meters of sediment (marine averages out the inheritance, the stochastic compo- terraces, fluvial terraces, pediments), others filling the nent in the concentration, to which has been added a landscape more deeply (alluvial fans, fill terraces, deterministic component that varies systematically moraines). These too, if sampled well away from edges with depth due to the decline in production with depth. of the surface that might be experiencing significant The expected profile is a shifted exponential, as seen in modification by either erosion or deposition, can be Figure 6.20, the shift being the mean inheritance of dated using cosmogenic radionuclides. There is, how- the deposit, and the exponential being the post- ever, a problem. All of the sediment that accumulated depositional accumulation of nuclides. Once the to form the deposit came from elsewhere, and there- shift is constrained, which is best accomplished with fore spent some time within a few meters of the Earth's one or more samples from several meters depth, surface, and therefore accumulated cosmogenic radio- the remaining exponential can be solved for the time nuclides (e.g., Anderson et al., 1996). This "inherited" since deposition. component can be large compared to that obtained This method has been used to date both marine while sitting on or in the surface we wish to date. terraces (see "Whole landscapes," Chapter 18,asitis
0 significantly too young – they were far lower than FR1 they should have been. The cosmogenic results there- 100 fore imply that the assumed production rates to which the samples were subjected were too low. This 200 can best be explained by the surfaces having been 0 buried by on average at least a meter of material, FR2 presumably loess, since their formation (Hancock
100 et al., 1999). As the loess is not there during the present interglacial, it must have been at least this thick during glacial episodes. 200 This problem of landform evolution is even worse 0 FR3 on landforms that are not flat to begin with. For
Depth (cm) example, the method has been applied to date glacial 100 moraines (see discussion in Briner et al., 2005; and modeling of Putkonen and Hallet, 1994). These 200 moraines begin as relatively sharp-crested landforms, 0 mounds of material deposited around the fringe of a FR4 glacier, which are then abandoned as the glacier 100 recedes. The landform subsequently evolves toward a more rounded form, through processes discussed in 200 the Hillslopes chapter. The peaks decline, and the 0 1.0 2.0 3.0 materials removed from them are deposited low on 10 × 6 Be (atoms/gm) 10 the flanks of the form. This means that a sample taken from the crest of the moraine was once in the Figure 6.21 Profiles of 10Be in four river terraces beside the modern Fremont River, Utah (from Repka et al., 1997, EPSL, subsurface; its production history will not have been with permission from Elsevier). steady, but will have grown through time to its pre- sent maximum value. The opposite is the case well down on the flank of the form. Researchers have employed on the Santa Cruz terraces), and fluvial either (1) ignored the problem altogether, (2) acknow- terraces. In a fluvial example, the terraces that bound ledged the direction in which this process would push both the Wind River in Wyoming and the Fremont their age, and assert that the age deduced from the River in Utah show this shifted exponential well concentration is a lower limit, or (3) chosen the largest (Figure 6.21). As we will see below, the inheritance possible boulders on the moraine crest and hoped that itself can be used to constrain rates of exhumation in these boulders reflect the original crest of the moraine. the landscape contributing sediment to the river. While we have pointed out problems with the One must exercise caution in applying this method, method and its application to date geomorphic as even flat landforms can fool us into thinking that surfaces of various types, we stress that this method they are static. For example, Hancock et al.(1999) is often the only quantitative absolute dating method showed that the very extensive, flat terraces of the available. The materials used, quartz, are almost Wind River in Wyoming, while flat and immune to ubiquitous. hillslope erosion or deposition, have been buried sig- nificantly by loess. This windblown mantle of silt can Exhumation rates apparently come and go. We find on many present terraces, in valleys subjected to high winds in the We have already admitted that it is difficult to find Pleistocene, that loess or silt caps are common in surfaces that are not eroding by one or another soils. These might only be a few tens of centimeters mechanism, for example, by weathering and removal thick. In attempting to date such surfaces, where we of grains. Rather than sulk about not having well- had an independent date from a volcanic ash, the age behaved geomorphic surfaces, we can make the best we predicted from cosmogenic concentrations was of the situation and use the cosmogenic radionuclide
concentration to document this rate of lowering. (a) 4 Imagine the production rate history that would be experienced by a rock that is slowly and steadily being 3 exhumed at a rate e. We show in Figure 6.22 that as 2
the surface approaches the rock parcel, the produc- Depth (m) 1 tion rate will climb exponentially. Ignoring for the 0 0 100 200 300 400 500 600 moment any decay of nuclides, the cosmogenic radio- (b) Time (ka) nuclide concentration ought simply to climb at an ever-increasing rate. Upon being sampled at the 0.8 surface, the concentration will be the integral of 0.6 the production rate history. Cast mathematically, 0.4 the concentration upon exposure will be 0.2 1ð 0 Be (million atoms/gm) 0 100 200 300 400 500 600 ¼ e t=z ¼ =e ð : Þ 10 C Poe dt Poz 6 11 Time (ka) 0 Figure 6.22 Numerical simulation of (a) depth history Here we have neglected decay. The concentration of a block of bedrock as it is exhumed at a rate of 5 microns/year simply equals the product of the surface production over 600 ka, and (b) its history of accumulation of 10Be atoms. The rate of accumulation accelerates as it comes closer to the surface, rate, Po, with the time it takes the rock parcel to pass e reflecting the rise in the production rate. Solid dot corresponds through the last z* to the surface, i.e., z*/ .Ifwe to the analytic solution ignoring decay (Equation 6.11). include the decay of nuclides, the equation is slightly Dashed line: concentration history including decay of altered to nuclides; open dot represents the analytic solution (Equation 6.12). P C ¼ o ð6:12Þ ðÞþe=z l By inspection of this equation, one can see that in 20 the limiting case of very low l (long half-life), or Beartooth very rapid erosion, high e, this equation reduces to 15 Equation 6.11. In other words, as long as decay is Front Wind River Range Sierra minor over the relevant time it takes to erode through 10 Nevada the region in which production occurs, we are safe using Equation 6.11.
Erosion rate (m/Ma) 5 This method has been used to determine bedrock erosion rates at many sites around the world (e.g., see 0 an early summary in Bierman, 1994). Examples of Sample bedrock lowering rates in the alpine and desert areas Figure 6.23 CRN-based erosion rates from bedrock in of western North America reveal very slow rates of western US mountain ranges. Rates shown are based on 10Be; exhumation that are all only a few microns per year, 26Al-based ages are comparable. Note that the average rate is et al EPSL or a few meters per millions of years (Figure 6.23; 7 m/Ma or 7 microns/year (after Small ., 1997, , with permission from Elsevier). see also work on Australian desert landscapes by Bierman and Caffee, 2002). Researchers have also placed constraints on the amount of erosion accomplished by the Last Glacial the surface sufficiently to reset the profile to near- Maximum (LGM) ice sheet covering the eastern edge zero. It had inherited 10Be from prior interglacial of Baffin Island (Briner et al., 2006). As shown in exposure, meaning that LGM erosion was less Figure 6.24, they found that on the inter-fjord flats than a few meters. In contrast, on the subaerial walls the 10Be concentrations were much higher than could of the adjacent fjords, the 10Be concentrations could be accounted for by post-glacial accumulation of 10Be. be interpreted as reasonable deglaciation times, Instead, the LGM ice sheet must not have beveled meaning that all inheritance from exposure during
5 upland erratics highly weathered zone intermediately weathered zone
0 1.5 freshly eroded zone bedrock 0 freshly eroded zone 1.5 intermediate weathered zone bedrock erratic Relative probability 0 blockfield highly weathered zone 1.5 lake bedrock glacial deposit tor 0 0 30 60 90 120 150 Age (ka)
Figure 6.24 CRN dates on glacial erratics and exposure ages on bedrock in the fjorded edge of eastern Baffin Island. (a) Age distributions from sites in various landscape positions shown schematically in (b). Low erosion rates on highly weathered upland surfaces result in high effective ages there, contrasting with low (LGM) ages from the freshly eroded fjords. LGM ages of glacial erratics on the upland surfaces speak to gentle (non-erosive, and presumably cold-based) ice covering the inter-fjord highlands, while erosive (presumably temperate) ice gouges the fjords (after Briner et al., 2006, Figures 2a and 5).
previous interglacial periods had been removed. Ice in rate e is a common 10 m/Ma, and the length scale
the fjords is therefore more erosive than ice on the z* ¼ 0.7 m, then the timescale is 0.7/10 Ma, or 70 thou- inter-fjord flats, meaning that the ice is sliding less sand years. One must be aware of this timescale in rapidly on the inter-fjord flats than in the deep fjords. interpreting a single rate, and in comparing this rate This may imply that ice was thinner and therefore with others. polar (frozen to the bed) on the inter-fjord flats, and, Granger et al.(1996) have successfully and ele- being frozen to its bed, could accomplish no erosion. gantly tested this method in a small basin in Nevada in which a small alluvial fan built out onto and covered a shoreline of Lake Lahontan (Figure 6.25). Basin-averaged erosion rates As this shoreline was independently dated (using 14C) The method of documenting erosion rates at points in to be 14000 years old, the volume of the fan could be a landscape has now been extended to obtain average divided by this time to derive a long-term (14 ka) rates of exhumation within whole basins. Granger average erosion rate in the contributing basin. Using et al.(1996) showed that the measured average con- this same sediment, the cosmogenic radionuclide con- centration of many sand grains can be used to deduce centration could be used to solve for the basin aver- the average erosion rate of the contributing basin. age erosion rate. The rates derived using the different This is a very powerful tool, as it achieves at once methods were amazingly similar. both a spatial and a temporal average rate. The The method has since been used in many settings to spatial average comes from the fact that sand grains map out the role of lithology, climate and tectonics representing myriad sites within the basin can be in setting the basin-averaged lowering rates. Such sampled using just one sample from, say, the point cosmogenic radionuclide-based long-term average bar on the exit stream. As you can hold a million sand rates have been contrasted with stream sediment- grains in your hand, the average should be a very gaging records to suggest that the gaging records miss robust one! The temporal average is inherent in the large rare events (Kirchner et al., 2001; Figure 6.26). calculation of the lowering rate from any particular Cosmogenic radionuclide-based basin erosion rates bedrock site. The averaging time is the time it takes in Sri Lanka (von Blankenburg et al., 2004) are sur-
the parcel to traverse z*. For example, if the erosion prisingly low, despite the fact that Sri Lanka has both
Figure 6.25 Field site in which the basin-averaged erosion rate method employing 10Be was tested on two adjacent catchments draining the edge of an escarpment on the Fort Sage Mountains. Fan volumes accumulated on a Lahontan Lake 1 B A 1 shoreline were documented to 2 deduce basin-averaged erosion 3 2 since 16 ka. Isopachs at 1 m 3 4 intervals are shown in the 10 26 4 deposits. Be and Al 5 concentrations were measured in the same fan sediments, from which the basin-averaged erosion rate may be estimated. tan shoreline Lake Lahon The estimates from the two 3 2 4 1 methods correspond very well 3 2 for both catchments: 5.8 cm/ka 1 for catchment A and 3.0 cm/ka 1 for catchment B (after Granger et al., 1996, Figure 1, Journal of Geology, with permission from the University of Chicago Press).
high relief and huge amounts of rainfall, both of which materials (quartz), and that 26Al decays roughly twice 10 have been thought to drive high erosion rates. The as fast as Be (tBe10 ¼ 2.00 Ma; tAl26 ¼ 1.02 Ma). If method has also been employed to derive basin- quartz-rich sediment is washed into the cave by the averaged erosion rates in the past (paleo-erosion river responsible for the dissolution of the rock, and is rates). In this twist on the method, researchers utilize then sequestered far enough underground to prevent the concentration of radionuclides attributable to further production of radionuclides within the sedi- inheritance from river terraces as the measure of ment, then the differential decay of the nuclides basin-averaged erosion rates at the time of deposition results in decay in the ratio of their concentrations. of the terrace. As shown in Figure 6.27, if the fluvial This ratio can then be used as a clock. Mathematic- system has many terraces, one can document the ero- ally, the ratio may be expressed as sion rate history of the basin contributing sediment t=tBe NoBee =t (e.g., Schaller et al., 2004). R ¼ ¼ R e t R ð6:13Þ t=t o The key here is that documentation of these NoAle Al rates has not been available to geomorphologists until where very recently. Surface lowering rates of the order of t t several microns per year are not measurable on PhD t ¼ Be Al ð : Þ R t t 6 14 (or funding) timescales. Stream-gage-based sediment Be Al
transport rates, from which erosion rates can be calcu- Here Ro is the initial ratio (most often taken to be the lated, are spotty in space, and are at best 100 years in production ratio, 6.75), and the mean life of the ratio,
duration. tR, is 2.08 Ma. In principal, therefore, the ratio can be used to date sediment as old as several of these timescales, or roughly 5 Ma. This is the essence of Burial ages what has become known as the "burial age method" We can even date cave deposits using cosmogenic (Granger et al., 1997; Granger and Muzikar, 2001). radionuclides. The method is based upon the fact that As with most methods, it has several advantages, but both 10Be and 26Al are produced in the same a few drawbacks. One principal advantage is that the
(a) (a) 1000 long-term averages
/yr) Simpleveld-2
100 Margraten
0.01 Sibbe-1&2 150 m Valkenburg-1 10 Valkenburg-2
short-term averages Geertruid-1 Denudation rate (mm/yr) Sediment yield (T/km 0.001 West Meuse Valley Geertruid-2 1 Geertruid-3 5 Pietersberg-2 0.1 10 1000 10 Pietersberg-1 Drainage area (km2) Pietersberg-3 100 m
(b) Gravenvoeren
Cosmogenic Apatite Rothem-2 Rothem-1 0.1 nuclides fission tracks Caberg-1 Caberg-2 Eijsden-Lanklaar 50 m 0.01 Caberg-3 Mechelen a/d Maas Sediment Holocene floodplain trapping and NW SE Denudation rate (mm/yr) 0.001 gaging
1 100 104 106 108 (b) 100 Time scale (yr)
80 Figure 6.26 Comparison of sediment-gaging-based average sediment yields and cosmogenic 10Be-based erosion rate estimates for catchments draining the Idaho 60 batholith. (a) 10Be-based methods (filled circles) suggest rates an order of magnitude higher than those derived from sediment 40 gaging and trapping (closed squares), independent of drainage area. (b) Results from these methods are compared with several million-year average exhumation rates deduced rate (m/Ma)Erosion 20 from apatite fission tracks, which support the higher rates based on cosmogenic radionuclides (after et al 0 Kirchner ., 2001, Figures 1 and 2). 0 0.5 1.0 1.5 Age (Ma)
Figure 6.27 The River Meuse displays a number method is immune to any temporal variations in the of terraces up to more than 100 m above the modern river, (a) dating back to more than 1.3 Ma. 10Be concentration profiles production rate, as it is simply decay that is the clock. on the alluvial cover from these terraces can be used to deduce The necessary depth of burial is a couple of tens of basin-averaged erosion rates in the contributing basin. Analysis meters, deep enough to prevent production by both of many such profiles yields a history of erosion rate in the River spallogenic and muogenic processes; most caves are Meuse headwaters. (b) In general, these show roughly twofold acceleration of the erosion rate from early to middle Pleistocene, at least this deep. The cave sediment must be quartz- with the highest rates in late Pleistocene (redrawn from Schaller rich, meaning that somewhere in the headwaters of et al., 2004, Figures 2 and 6, with permission from the the cave there must be a quartz-rich source of sedi- University of Chicago Press). ment. As it is only the inherited nuclides that are counted in this method, the concentrations of cosmo- to only roughly 10% (6.75 being the presently accepted
genic radionuclides start out small, and decay from value of the ratio of production rates, Ro), the ratio there, halving every 2 Ma. Finally, we must know well can only be known to that level. This places a lower
the initial ratio of the nuclides, Ro. That we know this limit on the utility of the method. Enough decay must
37° 20' Pearlsburg
37° 15' 5 km
80° 45' 80° 35' (b) 40 Figure 6.29 Phreatic passage in Snailshell Cave, Borneo. Such passages were at one time completely filled with water, leaving the walls sculpted from roof to floor (photograph by Greg Stock, with permission to reproduce). 30 30.2 m/M.y.
20 caves bounding other rivers in the Ohio-Mississippi 19.7 m/M.y. drainage, including the extensive Mammoth cave system in Kentucky (Granger et al., 2001), date back 10 at least 2 Ma, and reveal a story of river incision that Elevation above river (m) possibly reflects rearrangement of the Ohio drainage 27.3 m/M.y. in glacial times. 0 0 0.5 1.0 1.5 Caves in the Sierra Nevada also tell a story of Burial age (M.y.) river incision history, in this case dating back to more than 3 Ma (Stock et al., 2004, 2005). Most Figure 6.28 (a) New River, Virginia (triangles ¼ caves) are surprised to hear that there are caves in the as it passes through the valley and ridge province of the Sierras at all; in fact there are more than 300 mapped Appalachian mountains. Gray ¼ ridges. (b) Incision history based upon the ratio of cosmogenic radionuclides (burial dating) of caves. While the bulk of the Sierras are granitic, the sediments in caves along the river. Bold line represents wall rocks of the batholiths were significantly meta- average incision rate based on all data (redrawn morphosed, in places taking limestones up to from Granger et al., 1997, Figures 1 and 3). marbles. It is in these marble septa in the walls of the river valleys draining the western slope of the Sierras that useful caves have been developed. The situation is ideal for burial dating because the gran- have occurred to lower the ratio by more than 10% in itic sediment in the headwaters is quartz-rich, and order to have any confidence in the age. Practically, has been washed into several of these caves. Users this means that caves less than 300 ka cannot be dated of this method must be careful in selecting proper reliably at the moment. Nonetheless, that we can date sedimentary deposits within the cave. Only those these voids beneath the Earth's surface at all is in passages that are demonstrably carved by the remarkable, and opens up their use as a means of stream itself are useful if the goal is to determine documenting the rates of incision of the streams river incision rates. Such passages are called phrea- responsible for them. tic passages, and have a distinctive cross-sectional Several cave systems have been dated in this way. shape, with evidence of full occupation by rapidly The original study was that of Granger et al.(1997), flowing water. The photograph in Figure 6.29 shows who demonstrated that the New River in Virginia has a particularly clear-cut example of such a passage been incising at rates of 27 m/Ma averaged over the from a cave in Borneo. The incision of Kings last million years (Figure 6.28). Subsequent work on Canyon has been documented using several caves
(a) 3 South Fork Kings River Canyon 2.5 2 1.5 B 1 Elevation (km) 0.5 051015 20 Distance (km) (b)
400 Incision rate (mm yr–1) Bat Cave 26AI/10Be burial sample 3.09 +/– 0.30 Ma
Cliff Cave 300
100 Boyden Cave Elevation above river level (m) 1.40 +/– 0.08 Ma Windy Cliff Cave 0.033 Bear Cave 0.022 0 0.32 +/– 0.08 Ma Modern river cobble –0.06 +/– 0.10 Ma
(c) 500 South Fork Kings 400 Yucca Creek South Fork Kaweah 300 0.15 mm/yr 0.2 mm/yr 200 100 0.02 mm/yr
Elevation above river (m) 0 01234 Burial age (Ma)
Figure 6.30 Use of cosmogenic radionuclide burial dating of sediments in Sierran caves to derive an incision history for the Kings River Canyon. (a) Cross section of Kings Canyon, indicating location of the caved inner gorge detailed in (b) and shown in photograph at left. Ages of caves shown span the last 400 m of incision. (c) Plot of cave ages vs. elevation emphasizes that the rate of incision was rapid from 3–1.5 Ma, and slowed thereafter (after Stock et al., 2004 and 2005, EPSL, with permission from Elsevier).
at varying heights above the modern river. In effect, in the Quaternary. This information greatly enriches these caves act as strath terraces internal to the our understanding of Sierran erosion, as prior to this mountain. Using ages of four such caves, as shown work only the long-term rate of incision, between in Figure 6.30, the incision is revealed to have been 9 Ma and the present, was known from dates on rapid from 3–1.5 Ma, and then slowed significantly volcanic flows that came down ancient valleys.
26 10 (a) (b) of Al is greater than that of Be. The history of the 0 0 concentrations of both nuclides, and of the ratio, 26Al might therefore look something like those shown in Figure 6.31. 5 10 5 Be This method can be pushed back to the beginning of the Plio-Pleistocene glaciations. Recently, Balco 10 10 et al.(2005a,b,c) have used a modified version of the burial method to date buried tills from the great 15 15 ice sheets that once covered the Midwest of North America. They target the paleosols developed in the Depth (m) Depth (m) tops of the tills, as these were stable surfaces for 20 20 some potentially long period of time, and were subse- quently buried quickly by a later till. The first feature 25 25 allows the initial concentrations to be high. The second feature prevents any significant production
30 30 of nuclides during the burial process – it is effectively 0 0.5 1 1.5 2 0 2 4 6 instantaneously buried, just as sediment is instantan- 10 26 10 Be (million atoms/gm) [ Al] / [ Be] eously deeply buried in a cave. They report the age of the earliest glacial till to be around 2.4 Ma Figure 6.31 Profiles of 10Be and 26Al concentrations (a) and their ratio (b) in an accreting deposit. Initial (Figure 6.32; Balco et al., 2005a), which corroborates concentrations (inheritance) are 1 105 and 4.8 105 atoms/gram interpretations of onset of North American glaciation 10 26 quartz for Be and Al, respectively. Porosity of the deposit is from the d18O record from the Gulf of Mexico. Later taken to be 35%. The deposition rate is 20 m/Ma, and the total duration of deposition is 1.5 Ma. The production rate ratio is tills of 0.7 to 1.5 Ma are also reported at other sites taken to be 4.8. Concentrations initially increase with depth (Balco et al., 2005b). while the sediment is in the zone of significant production, and then decline once decay overwhelms new production. Dashed curves ignore decay, and reveal the pattern due simply to accumulation in the production zone. The ratio monotonically declines with depth. Shallow geothermometry: establishing long-term rates of exhumation
We often wish to determine how rapidly a landscape Use to date stratigraphy has evolved over long timescales. In general, one Only very recently has this burial method been would like to know how long it took a parcel of rock adapted to explore the depositional history of a sedi- to reach the surface from some fixed depth. Ideally, mentary deposit. The twist here is that the burial to one would like to measure the time since the rock depths needed to halt all new production is not crossed below a certain pressure. Knowing how to instantaneous, as in caves, but is progressive. Many derive depth, z, from pressure (P ¼ rgz), we could deposits lack material that would allow dating by any then determine the long-term exhumation rate (total other means. In particular, loess sequences, which in depth/time). Unfortunately, information about pres- China go back to at least 3 Ma, are commonly dated sure is not well preserved in a rock. Instead, a set of using only magnetic stratigraphy (depths at which the methods has been developed to enable us to docu- samples are magnetically reversed and normal). Thick ment the time since a rock crossed below a particular piles of gravel also lack datable material. As the temperature, as proxies for this are preserved within sample is buried within the accumulating deposit, it rocks. If we know how to deduce temperature from continues to accumulate new nuclides until it falls depth, then this serves the same purpose. We will below the production zone. Beyond this depth, the see that the interpretation of information derived rate of decay exceeds the rate of production, and the from these temperature proxies requires awareness concentration of each nuclide begins to decline. Only of what controls the thermal field within the Earth, then does the ratio begin to decline as the rate of loss
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(a) their pressure–temperature (P–T) paths through an orogen. Minerals form at specific P–T conditions.
I One method therefore entails dating when the S 40 S I mineral formed, and hence when it crossed this S S IP phase boundary. In general, these reactions take P I place at high temperatures and tell us little about R . the lower temperature history of the rock as it then Musgrove clay pit is exhumed. The revolution in the last few decades has involved the development of proxies for tem-
ISS O perature histories of minerals subsequent to their M UR I R. formation at great pressures and temperatures. The temperature proxies come in two flavors: fission 38 N tracks and trace gases. Both are born of the decay of radioactive elements in mineral grains. Fission 92 90 W tracks are damage zones in a crystal lattice gener- ated by the rare splitting of atomic nuclei (mostly U) (b) into two pieces. The trace gasses are small shards 1.5 4 100 % CaCO3 0 from the fission itself, in particular He nuclei (two neutrons and two protons) that are trapped within the crystal. In both cases the method is based upon the fact that the mineral grains can "forget" that
Age (Ma) these fission events ever happened if the grain is maintained above a certain temperature: the fission tracks heal, or anneal, and the small gas atoms 2.0 diffuse out of the mineral. As the events are "remembered" at all lower temperatures, the proxies become a recording of how much time the grains have spent below these critical temperatures. The methods differ in the critical (or closure) tempera- tures (Table 6.2). Therefore, if used in concert with one another, they can document the times at which a sample crossed several temperatures en route to 2.5 the Earth's surface, producing a temperature history (see example of this in Figure 6.33). 0–1–2–3 d18 O Fission tracks
Figure 6.32 Dating the onset of North American glaciation from Fission tracks (FT) are generated in any mineral in burial ages in early till sequence of the mid-west USA. (a) Map which elements capable of nuclear fission occur d18 238 of sample site north of Missouri River, (b) O and CaCO3 time ( U). These events are rare – about one for every series from deep sea cores, and cosmogenic date from sample (bar with shaded error band) (after Balco et al., 2005a, Figure 1, two million alpha-decay events. But the heavy shards 238 with permission from the American Association for the of the original U nucleus (usually with mass Advancement of Science). numbers of about 90 and 140) do considerable damage to the crystal. The tracks are invisible, but can be revealed by lightly etching a polished section and how this evolves in time beneath an evolving of the grain. Fission tracks began to be used as landscape. a chronometer in the 1960s by Naeser (1967) (see We have learned to make use of metamorphic reviews in Gallagher et al., 1998, and Tagami and minerals in rocks that emerge at the surface to deduce O'Sullivan, 2005). The minerals commonly used in
Table 6.2 Common thermochronometers and temperature ranges
System Mineral Precision (%, 1 sigma) Closure temperature ( C) Activation energy (kJ/mol)
(U-Th)/Pb Zircon 1–2 >900 550 Titanite 1–2 550–650 330 Monazite 1–2 700 590 Apatite 1–2 425–500 230 40Ar/39Ar Hornblende 1 400–600 270 Biotite 1 350–400 210 Muscovite 1 300–350 180 K-feldspar 1 150–350 170–210 Fission track Titanite 6 380–420 440–480 Zircon 6 230 210 Apatite 8 90–120 190 (U-Th)/He Titanite 3–4 160–220 190 Zircon 3–4 160–220 170 Apatite 3–4 55–80 140
(after Reiners et al., 2005, Table 1; see this for full references to the methods)
1000 and the track number and lengths are documented. zircon U/Pb The concentration of U in the sample must be meas- 800 ured as well, as a measure of how rapidly fission titanite U/Pb tracks should be produced in the mineral. The tem- 600 perature below which the tracks heal at such slow hornblende 40Ar/39Ar rates that they do not change significantly over 400 biotite 40Ar/39Ar geologic time is called the closure temperature. 40 39 Temperature (°C) plagioclase Ar/ Ar The temperature range within which the healing rate 200 zircon (U-Th)/He apatite apatite FT (U-Th)/He is fast enough to alter track lengths but not fast
0 enough to erase them over the time spent at that 60 50 40 30 20 10 0 temperature is called the partial annealing zone, or Age (Ma) PAZ. In Figure 6.34 we reproduce an example in which this partial annealing zone has been docu- Figure 6.33 Temperature history of the Speel pluton, Alaska, derived from several thermochronometers. The data mented in samples taken from a borehole. Samples reveal rapid cooling after emplacement, followed by rapid cooling taken along vertical transects in mountainous topog- caused by exhumation after 10 Ma (after Reiners, 2005, raphy have also been used to detect exhumation of Figure 3A and references therein, with permission from the American Geophysical Union). such a PAZ by recent erosion.
Ar/Ar thermochronometry shallow thermochronometry are apatite and zircon. The damage zones, or fission tracks, are initially 40K is a radioactive isotope, constituting roughly about 15 microns long. Above a certain temperature, 0.012% of the naturally occurring potassium, and the tracks shorten in length by annealing at each tip, decays to 40Ar with a half-life of 1.25 Ga. Ar is a the rate of annealing being a strong function of the noble gas capable of diffusing out of mineral grains temperature. In modern FT analyses, both the either through the lattice of the grain (volume diffu- number and the length distribution of tracks are sion), or along grain boundaries (grain boundary measured. The mineral grain is mounted, beveled, diffusion). As in all diffusion problems, the rate of etched lightly with acid to widen the tracks suffi- diffusion is strongly dependent upon the temperature ciently to make them visible in a lab microscope, of the medium. At high temperatures the daughter
Sample fission track age (Ma) ° 050100 150 Tc muscovite = 350 C Outcrop (10 °C) T biotite = 320 °C 10 c
Temperature Tc closure temperature 50 86 °C
partial annealing zone (PAZ) 92 °C Time Temperature (°C)
100 40 109 °C 20 40Ar 0 0 10 0 5 10 15 Track Mean track length (microns) length (microns) open closed system system Figure 6.34 Fission track analyses of samples from a depth profile in the Otway Basin, southeastern Australia, shown against present temperatures. Stratigraphic closure age Time age of the package is 120 Ma (thin dashed line). Track length distributions (right) shown for several of the samples, and mean Figure 6.35 40Ar/39Ar system. The system is open to diffusive track lengths for all samples (middle) show significant shortening loss of 40Ar at high temperatures, becoming progressively more of track lengths above temperatures of about 55 C, defining closed and retentive of 40Ar as temperature cools during the lower temperature boundary of the partial annealing zone exhumation. (PAZ). Vertical dashed lines: mean original track lengths ( 16 microns). For greater temperatures and hence depths, the age deduced from track numbers (left) will under-represent the true age of the rock grains, promoting the popularity of new detrital (after Gallagher et al., 1998, Figure 6, with methods. We illustrate in Figures 6.36 and 6.37 the permission from Annual Reviews). use of rock samples in deriving an exhumation history from a site in Nepal. Here Wobus et al.(2008) analyzed biotite grains separated from rock samples 40Ar is free to escape the grain, and the system collected in a vertical transect on a steep valley wall. behaves as an open system. As the temperature drops, As shown in the schematic in Figure 6.36, the slope however, and 40Ar continues to be produced, more on the age–elevation plot may be interpreted as and more of it is retained in the grain, and the system the exhumation rate. At this site in Nepal, the exhum- moves toward being quantitatively closed. Below a ation rate appears to have increased dramatically at closure temperature of roughly 350 C, muscovite is about 10 Ma. effectively closed to such diffusion, and quantitatively With the development of the 40Ar/39Ar laser retains the gas. The closure temperature for biotite is microprobe, we can now assess the quantity of Ar in slightly lower, 320 C(Figure 6.35). Note that this is individual grains. This has greatly refined the ability a relatively high closure temperature. In contrast, to date not only whole rocks but many individual the 4He atoms generated by alpha-decay in the grains within a single rock. And it has also opened U/Th system discussed below are much smaller and the door to the use of this tool in detrital thermo- can diffuse much more readily in crystal lattices. chronometry, in which the distribution of cooling This results in the low closure temperatures of the ages of individual grains from a sedimentary deposit, U/Th-He thermochronometer. (They are the He or from a modern fluvial system, can be compiled. bicycles, as opposed to the Ar trucks, cruising down Armed with this information, we can now invert the the narrow crystal lattice streets.) cooling age distribution for the cooling history of the The 40Ar/39Ar tool has been traditionally used to catchment comprising the headwaters from which infer cooling ages of rocks. More recently analytical the sample was derived. The target grains are K-bearing improvements have allowed its use on individual minerals of K-feldspar, muscovite and biotite. Ages
(a)(b) (c) pdf of elevations Relief, R 3
2 Elevation
pdf of ages Depth
closure depth 0 t1 t2 t3 Cooling age Cooling age
Tc Temperature
Figure 6.36 (a) Use of elevation profiles to infer exhumation rate in a steady-state landscape. (b) Expected age-elevation distribution in a uniformly eroding landscape such that the rate of erosion in the valley bottom is identical to that at the crest of the mountain. The long-term erosion rate can then be determined to be R/Át, where Át is the difference between ages at the mountain crest and valley trough. (c) The detrital method in which the pdf of ages from a sediment sample in a stream draining this landscape is interpreted in the light of the pdf of elevations in the catchment (its hypsometry). In a uniformly eroding landscape and steadily eroding landscape the pdf of elevations should map directly onto the pdf of cooling ages. Note that the interpretation of the cooling ages depends upon one's assumption about the geothermal gradient, which dictates the assumed cooling depth (after Brewer et al., 2003, Figure 2, with permission from Blackwell Publishing).
4400 from each of more than 100 grains are determined by laser fusion of the grain, from which a graph of the sort shown in Figure 6.36 is obtained. This is the sum 4200 of the probability density function of the individual 0.01 mm/yr grain ages (the synoptic pdf, or SPDF). It is this 4000 graphic that is then interpreted in the geological con- text of the sample. Several uses of the 40Ar/39Ar method relevant to 3800 geomorphology have evolved over the last decade (summarized in Hodges et al., 2005). These include Elevation (m) 0.5 mm/yr (1) determination of the provenance of a sample, 3600 where the various bumps on the SPDF are attributed to rock from a portion of the catchment with a known
3400 age distribution; (2) timing of the exhumation in the catchment; (3) determining the lag time between exhumation and deposition, which in the case of small lag allows estimation of the mean exhumation 510152025 Age (Ma) rate in the source region; and (4) constraining modern erosion rates in a catchment (e.g., Brewer et al., 2003). Figure 6.37 Elevation profile of biotite 40Ar/39Ar ages from As noted by Stock and Montgomery (1996), in this Langtang, Nepal, showing 50-fold acceleration of exhumation rate method one must acknowledge the probability distri- at roughly 10 Ma. Bars depict full range of measured ages from ten laser-fusion analyses at each elevation (after Wobus et al., bution of elevations from which the grains have been 2008a, Figure 3, EPSL, with permission from Elsevier). sampled within the catchment (the hypsometry,
concentrations of the parent nuclides U and Th are then measured in ICP-MS. The basis of a U-Th/He geochronometer is that the 4He concentration in a mineral grain is a clock. The basic equation governing the system is a modification of the radioactive decay equation that tracks not the parent but the daughter population:
rate of change of daughter population ¼ rate of gain by decay of parent(s) – loss by diffusion Normalized Probability 1000 30 In symbols, this becomes 20 Cool100 ing Age (Ma 10 c dD dN 10 0 ysiographi ¼ n diffusion ð6:15Þ –10 f Ph dt dt ) 1 –20 n (km) Distance N o Transitio where n is the number of daughter atoms generated by the decay of a single parent atom. We can also Figure 6.38 Distributions of 40Ar/39Ar muscovite proceed by recognizing that the number of daughter ages from detrital samples from small catchments north atoms will be the product of n with the number of and south of a major physiographic break in the Himalayan front, central Nepal. The 20–30-fold drop in detrital age across the parent atoms that have decayed to that time, i.e., physiographic break (arrow) implies significant activity on a lt surface-breaking fault, driving rapid exhumation of rocks D ¼ nðNo NÞ¼nðNo Noe Þ et al ð6:16Þ to its north (after Hodges ., 2005, Figure 5; see also ¼ ð ltÞ Wobus et al., 2003). nNo 1 e by appeal to the radioactive decay equation for parent shown in Figure 6.36). And (5) documenting the loca- atoms. The problem is that in this system what we tion of strong gradients in exhumation, which in turn measure is not the number of original parent atoms, can illuminate the presence of geologic structures that but the present number, N. Solving the radioactive decay
are otherwise difficult to observe (e.g., Hodges et al., equation for No and replacing No in Equation 6.14 yields 2004; Wobus et al., 2008a, as shown in Figure 6.38). D ¼ nNeltð1 e ltÞ¼nNðelt 1Þð6:17Þ (U-Th)/He method Knowing n, and measuring N and D, we can invert this equation for time, t. For the (U/Th)/He system, Several 4He nuclei, or a particles, are emitted in the this becomes (see Reiners, 2005, Equation 1): decay of U and Th to stable Pb. As in fission track l l analyses, both zircon and apatite are the primary 4He ¼ 8238U½e 238t 1 þ7235U½e 235t 1 ð : Þ minerals of choice, although some use titanite (sphene). l 6 18 þ 6232Th½e 232t 1 The closure temperature of apatite to He loss is 68 5 C (Farley quoted in Reiners, 2002). This The number of 4He or alpha particle daughters pro- roughly 70 C closure temperature is much less than duced from decay of these three parents varies from the 110 C closure temperature for fission tracks in six to eight. apatite, making (U-Th)/He the lowest temperature ther- Use of such shallow, low-temperature geochron- mochronometer in common use at present. This ometers requires that we acknowledge another source method can reveal the time it has taken a rock parcel of complexity in our interpretation of the concentra- to move through the last 2–3 km to the Earth's surface tions. As we show in Figure 6.39, because the 70 C (70 C/(25 C/km) ¼ 3 km), making it relevant to the isotherm is only about 2–3 km deep, it is likely to be evolution of mountainous topography whose relief is significantly warped within high mountainous relief. often of the same order. The method entails mass spec- When interpreting the results to derive an exhuma- trometric measurement of the concentration of 4He tion rate for a particular sample, one must therefore from a very small mass of the mineral, sometimes pay particularly close attention to the thermal struc- derived from laser ablation of the surface. The ture through which these rocks have passed en route
0 Although the exposure to such temperatures is brief, and can cause damage to the rock surface (see 1000 Weathering chapter), they can again cause loss of He. 2000 This fact has been cleverly used to determine the ages of natural coal-seam fires in Wyoming, where brush 3000 w/o radioac fires sweeping across the landscape have ignited out- 4000 cropping Cretaceous coal seams. The fires burn
5000 tive sources downward along the dipping coal seam into the sub- surface, dying only when they can no longer obtain Depth (m) 6000 enough oxygen. The result is the baking of the sur- 7000 rounding shales to red clinker (Figure 6.40). As these rocks are the hardest in the landscape, red clinker hills 8000 dominate the subtle landscape around the edges of 9000 northern Wyoming's Powder River Basin. He dating of these beds has revealed how long ago these fires 10 000 0 50 100 150 200 250 300 occurred, documenting the timing of late Cenozoic Temperature (°C) basin exhumation (Heffern et al., 2008). Because the closure temperature is so low, it is also Figure 6.39 Modeled temperature profiles in the upper crust demonstrating the role of possible for the clock to be reset by surface processes minerals containing short-lived radioactive elements involving heating, in particular wild fires (Mitchell in altering the thermal structure. Steady profiles show curvature and Reiners, 2003). While this represents a problem reflecting the need to pass increasing amounts of heat as the surface is approached. Mantle heat flux taken to be 41 mW/m2; for documenting exhumation, the problem can be conductivity of rock ¼ 2 W/m-K. Radioactive heat source is turned around and used to assess the intensity of past –5 3 distributed exponentially, with surface source of 10 W/m , wildfires. The depth in the rock to which the resetting and scale for decay of source with depth of 3 km. Arrow depicts the difference in depths at which a thermochronometer occurs reflects the temperature of the fire and its would pass through its closure temperature (here taken duration, as we discuss in Chapter 7 on weathering. to be 120 C). The use of two minerals with different closure tem- peratures allows documentation of the penetration of the thermal wave (Mitchell and Reiners, 2003). to the surface. The isotherms are not only warped by Another characteristic of these systems is that the the topography, they will evolve as the topography ejected alpha particle travels a fair distance from the evolves. A rock parcel exhumed in the valley bottom original atom in the fission event. This is indeed will have experienced significantly greater thermal the root of the fission track method. But it poses prob- gradients than a rock exhumed on the ridge crest. In lems in the (U-Th)/He method because some fraction modeling the thermal structure, it is also important to of the fission events will result in ejection of the alpha acknowledge the role of radiogenic elements in the particle from the mineral grain. This loss must be shallow crust, as their concentration in the top few corrected for. While the calculation is somewhat km of the rock column will increase the thermal involved, the principle of the correction is straightfor- gradient, which in turn alters the depth at which a ward, as illustrated in Figure 6.41. One must assume a particular isotherm is crossed. One can see this bend mineral shape and know the characteristic stopping in the geotherm easily in Figure 6.39. distance, call it d. Assuming that the ejection angle It is also possible for the temperatures of rocks at is random with respect to the grain boundary, statis- the surface itself to be raised above 70 C by natural tically speaking those events that occur within processes (see our discussion of hot processes in the half the stopping distance of the wall will result in Weathering chapter). If the surface of the rock is dark ejection. Calling the stopping distance d, the expected enough, direct exposure of the rock surface to the sun under-representation of 4He in the grain is simply can raise the surface temperature to nearly 70 C, the fraction of the grain volume represented by a inducing some diffusive loss of He. Forest or brush shell of thickness d/2. Consider an idealized case. fires burn at much higher temperatures than this. Let the grain be spherical of diameter D. Then the
expected loss is the shell volume divided by the grain diameter, meaning the smaller the grain the larger volume: the correction. Yet another twist on this method has evolved in the ð =2Þ4pD2=4 F ¼ ¼ 3 ð6:19Þ last couple of years. Researchers are now beginning to p 3= D 6 D exploit the concentration profiles within apatite crys- tals as a signal of thermal history to which the rock Calculations for other more realistic grain shapes has been subjected (Shuster and Farley, 2003, 2005). result in similar expressions. They always go as d/D. The method is called the 4He/3He method because the The effect is inversely proportional to the grain
clinker of Wyodak-Anderson coal zone cross-section A-A' 0.502 0.615 +/– 0.028 vertical exaggeration 30:1 0.120 +/– 0.035 sandstone/siltstone/shale +/– 0.008 0.01 +/– 0.001 coal A 1500 m North Antelope clinker Figure 6.40 Clinker beds Rochelle Mine ZFT age (Ma) seam A' from the natural burning of the coal 1400 m outcrop of the Wyodak coal ZHe age (Ma) 4km seam, Powder River Basin, Wyoming. Ages shown are derived from FT and U/Th-He on clinker beds (after Heffern et al., 2008).
profile is documented by much more precise measure- ment of the ratio of He isotopes. The 3He is emplaced δ uniformly in the crystal by intense radiation, usually in a medical irradiation laboratory. This method can theoretically allow us to deduce temperature D paths down to about 30 C. On a typical geotherm (25–30 C/km) this temperature range is within about δ/2 1 km of the surface, meaning that the method should record the timing of the last 1 km of erosional history Figure 6.41 Alpha rejection. Fission of of the rock. Documentation of the concentration radioactive U and Th atoms results in rejection of some fraction of the 4He atoms (alpha particles) from the mineral. profile within apatite grains is done indirectly, by Statistically, half of the events within an outer shell progressively increasing the temperatures to which of width d/2 will produce alpha particles that the apatite grains are subjected; the first gases emitted are lost from the mineral.
Warren Peak Surprise Valley ′ A 16–14 Ma A 3 basalt flows 3 Sample WR104 2 2
26–17 Ma tuffs 34 Ma tuff 1 and lava flows 1
Elevation (km) Granite Eocene to Oligocene volcaniclastic boulder horizon S.L. and sedimentary rocks S.L.
Figure 6.42 Cooling history derived from thermochronometric analysis of a single sample from normal-faulted Warren Peak bounding Surprise Valley, Nevada
Temperature (°C) (top cross section). Geologic cross section suggests faulting-related exhumation and associated cooling must begin after 16–14 Ma basalt flows cap the landscape. Three methods (fission track, (U-Th)/He, and 4He/ 3He) are combined in a single model to deduce the cooling history. Two episodes of rapid cooling are separated by a period of slower cooling (after Colgan Time (Ma) et al., 2008, Figures 1 and 5).
are from nearest the grain edges, the later gases from thermochronometers shown in Figure 6.43, revealing the interiors of the grains. This method has recently rapid exhumation initiated in the Mio-Pliocene. been used to deduce erosion of a fjord into the moun- The history of exhumation can also be deduced tains of British Columbia, telling us for the first time by applying thermochronometry to populations of that at least at this site the fjord was rapidly emplaced detrital grains within a stratigraphic section. An at around 1.8 Ma (Shuster et al., 2005). example is illustrated in Figure 6.44. The two-stage exhumation history of a mountain To derive depth histories and hence exhumation range in the Basin and Range province of the western histories, the temperatures obtained from thermo- USA has also been assessed by using multiple thermo- chronometric methods must be turned into depths. chronometers, including the 4He/3He method This requires either simplifying assumptions about (Figure 6.42; Colgan et al., 2008). The long-term the thermal structure of the Earth, or models of how exhumation history of the eastern margin of Tibet the thermal structure will evolve over the course has also been assessed by application of several of the exhumation. For high-temperature methods
(a) (b)
20° 32 60° 110° 70° 90° 100° (c) A-A¢: Min Shan region 31 6
4 30 2 Elevation (km) 0 0 100 200 300 400 500 100 102 104 Distance (km) (d) (e) 500 B-B¢: Longman Shan region 400 6 biotite 300 4 T (°C) 200 k-feldspar 2 zircon 100 Elevation (km) apatite 0 0 0 100 200 300 400 500 0 50 100 150 200 250 Distance (km) Time (Ma)
Figure 6.43 Topography of eastern margin of Tibetan Plateau shown in (a), detailed in (b). Swath profiles of topography roughly perpendicular to the margin of the plateau in two areas (c and d). (e) Thermal history of the region as deduced from several thermochronometric systems. Lines connect data from individual samples. The thermal data indicate slow cooling through the Mesozoic and early Cenozoic, with rapid cooling, implying rapid exhumation of the Tibetan margin, in the Mio-Pliocene (after Kirby et al., 2002, Figures 1 and 9, with permission from the American Geophysical Union).
(a) Nyadi River Detrital Sample (2002) 0 40Ar/ 39Ar Muscovite Data 16 0.03 n=111 1 Frequency 12 2 40 0.02 3 75 8 4 Probability 100
Depth (km) 110 0.01 5 4 6 160 7 2 4 6 8 10 12 14 16 18 0246810 Age (Ma) Distance (km) (b) Figure 6.44 Example SPDF of detrital 40Ar/39Ar muscovite ages. 160 Relatively young ages suggest rapid exhumation in the headwaters et al valley of the Nyadi River, Nepal (after Hodges ., 2005, Figure 1, 120 with permission of the Mineralogical Society of America).
80 involving closure temperatures of > 200 C, it is rea- ridge 40
sonable to assume that the thermal field at the time of Temperature (°C) closure was simple (essentially one-dimensional), that 0 the isotherms were horizontal and were uninfluenced 1086420 by the details of the topography above. One must Time before present (Ma) (c) only assume a geothermal gradient at the time. If 0 90 enough information exists in vertical transects of mul- 80 Temperature (°C) 1 70 tiple thermochronometers, it is possible to constrain 20 60 2 20 20 50 this paleo-gradient. 40 3 40 40 The picture is not so simple in lower closure tem- 40 30
Depth (km) 60 perature systems. Here the proximity of the Earth's 4 60 60 20 80 10 surface warps the isotherms. In general, isotherms are 5080 80 012345678910 compressed beneath valleys, and expanded beneath Distance (km) ridges as the movement of heat becomes two- to three-
dimensional rather than one-dimensional. Because the Figure 6.45 Thermal structure in a steadily eroding topography influences the thermal structure to depths ridge-valley transect (a) and the resulting temperature history of perhaps twice the total relief in the landscape, it is of rocks emerging on the ridge and in the valley (b). Isotherms corresponding to FT (110 C) and (U-TH)/He thermochronometers also possible that evolution of the topography will (75 C) are shown as bold lines. Isotherms are compressed beneath influence the signal (see Safran, 2003). This is shown the valley floor and are expanded near the ridge crest. Closure in Figure 6.45. In other words, one must acknowledge temperatures are shown as bands in (b). A valley bottom sample would cross through the FT closure temperature at 4.5 Ma, and not only the spatial distribution of temperatures in through the (U-Th)/He closure temperature at 2 Ma (after the rock mass, but the evolution of the topography Safran, 2003, Figure 2, with permission from the American and co-evolution of the thermal field in the interpret- Geophysical Union). (c) Steady-state temperature field in a cross section through mountainous topography. Mantle heat ation of these temperature data. Most recently, these flux is taken to be uniform in space, and no radioactive elements complexities have led to the development of numer- contribute to the heat flux. Isotherms, reported in 20 contours, ical codes in which both topography and thermal are compressed beneath valleys, and are expanded beneath ridges. The effect of topography is minor below a depth fields are modeled to predict the T-paths of rocks equivalent to the relief of the valley (calculation courtesy emerging at any location on the Earth's surface of Dylan Ward). (e.g., Braun, 2005; Ehlers, 2005; Ehlers and Farley, 2003; see discussion in Reiners et al., 2005).
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A technological revolution within the last two decades has development of models of landscape evolution that can now placed geomorphology on a much more rigorous timeline that be rigorously tested. extends millions of years, covering the timescale over which On the flip side, the careful interpretation of the cosmo- most landscapes have evolved. The materials that constitute genic and thermochronometric data requires increasingly the clocks now in use are commonly available in most rocks, sophisticated models in which we acknowledge the geo- from the quartz used in CRN dating to the accessory minerals morphic setting of a sample. The evolution of the landscape employed in thermochronometry. We can now assess how governs both the production rate history and the thermal rapidly mountains are being exhumed at timescales of millions history of any particular sample. The models require yet again of years to thousands of years. We can compare basin- that we craft formal equations for the conservation of some averaged rates to rates at a point. This revolution in the quantity, in this case the numbers of elements, nuclides, or documentation of timing has gone hand in hand with fission tracks.
1. You are asked to date a shoreline that is found These are datable using U/Th methods, and have well above the margin of a present day lake. This yielded good dates back to several hundred thou- is important paleoclimatic information for the sand years. One of these terraces 120 m above basin, and you need to know how long ago the modern mean sea level is dated to be 80 ka. lake was at this elevation. You find a tree stump (a) What is the long-term rate of rock uplift on and sample its outer rings to date using the radio- this coastline? carbon method. You have this analyzed at Beta (b) What assumptions have you made in per- Analytic Labs in Florida, and they report back forming this calculation? that only 21% of the original 14C remains in the sample. 3. You are asked to date a moraine in the Indian (a) How old is it, given that the half-life of 14Cis Peaks of the Front Range, Colorado, and choose 5730 40 years? (This was determined by to do so using 10Be concentrations in moraine a set of researchers at Cambridge, and is boulders. One of the key calculations you have known in the community as the Cambridge to make in this business is how big a sample to half-life.) collect. (b) Given the range of estimates of the half-life, (a) Using the following constraints, estimate calculate the associated range in age esti- how much sample you will need to collect. mates. (This will be a minimum estimate of Report the answer in kilograms. The AMS the range for several reasons, in part because method requires that you provide them with there will be additional error associated with two million atoms of 10Be. error in the lab analysis.) The moraines are located out in the valley well away from any valley walls that 2. On the Huon Peninsula of Papua New Guinea, would mask the radiation from cosmic a set of coral marine terraces grace the coastline. rays.
We expect the moraines to be last glacial 10Be concentration 26Al concentration maximum (LGM) in age. Depth (m) (atoms/g) (atoms/g) The rock type is a granite with roughly 5 6 30% quartz. 0 4.05 10 2.43 10 0.5 2.43 105 1.47 106 The elevation at the site is 8236 ft. 5 6 0.9 1.66 10 1.05 10 10 The Be production rate at sea level is 4.8 1.3 9.81 104 5.97 105 atoms per gram of quartz per year. 1.7 4.81 104 3.06 105 The rate of increase of production rate with elevation is an exponential with a length scale of 1.5 km. (In other words, 6. Using the Speel River pluton cooling history the equation for production rate is shown in Figure 6.33 (from Reiners, 2005), esti- ðz=z Þ Po ¼ Posealevel e , where z* ¼ 1.5 km) mate at what depth the pluton is likely to have (b) If this mass is a cube, what is the length of a been emplaced. Assume a geothermal gradient of side of the cube, in meters? (In other words, 30 C/km. If the surface temperature is 0 C, just how big is this?) what is the minimum exhumation rate implied by the U/Th-He date at 8 Ma? 4. You have now gone through the agony of pro- cessing the sample and have had it analyzed at 7. Weathering rinds. The average weathering rind the Purdue PRIME lab. They report back thickness on basalt clasts on an 8 ka flow is and provide a concentration from which you 1.2 mm. If the rind thickness grows as the square calculate that the concentration of 10Be is root of time, what is the age of a moraine on 6.1 105 atoms/g of quartz. How long has the which basalt clasts are measured to have average boulder been exposed to cosmic rays – in other rind thicknesses of 2.2 mm? words, how old is the moraine? Do the calcula- tion in two ways: 8. Ratio clock. Quartz-rich sediment in a cave in the (a) assuming that there is no decay of 10Be over Sierras is measured to have a 26Al/10Be ratio of the age of the boulder; 2.8, and the 10Be concentration is 2.2 104 (b) taking into account radioactive decay. (Hint: atoms/g in quartz. here you will have to use a slightly modified (a) What is the age of the deposit (and hence the equation to calculate the age. See the text for age of the void in which they are found)? guidance.) (b) Calculate the inherited concentration upon deposition, and estimate the basin-average 5. Dating a terrace.Giventhe10Be concentrations erosion rate in the catchment, having calcu- in the table below, and assuming sediment bulk lated that the average production rate in the density r ¼ 2100 kg/m3 and characteristic attenu- catchment is 35 atoms/(g yr) in quartz. ation length [fast neutrons] L ¼ 1600 kg/m2, (a) determine the best fitting shifted exponential 9. You are in Pakistan and do not have access to the profile; web. You are sampling a strath terrace above the (b) calculate the age of the surface, assuming Indus in its Middle Gorge near Nanga Parbat, that the local production rate Po ¼ 30 and need to know how large a sample to collect. atoms/(g yr) in quartz, and ignoring decay. You cannot simply say "I don't know" and take a (c) Using the inheritance determined from sample the size of your vehicle, because (i) there is the shift in the profile, calculate the basin- limited space in the vehicle, and (ii) it costs money averaged erosion rate, assuming the same to ship the samples home. You are 200 m above production rate. the river at an elevation of 3700 m. The rock you
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are sampling appears to be a highly meta- Wyoming to be 9.2 105 atoms/g in quartz. morphosed granite. List all the assumptions The elevation is 3500 m, and the sea level you make in the calculation. production rate of 10Be is 6 atoms/(g yr) in quartz. What is the local erosion rate of the 10. Plot the expected profile of 10Be in the sandy outcrop? cover deposits on a marine terrace. The local production rate of 10Beis6atoms/(gyr)in 13. What is the effective "averaging time" implicit quartz. The age of the deposit is 130 ka. Let's when you use 10Be concentrations to deduce assume that this terrace is young enough to erosion rates in a catchment? Assume that the ignore decay. Plot the profile for the following relevant length scale for decay of production two cases: rate with depth is 0.7 m. Calculate this for ero- (a) assume that there is no bioturbation; sion rates of 10 and 100 microns/yr. (b) assume that bioturbation homogenizes perfectly the top 50 cm of the deposit. 14. Thought question. List and discuss the several means by which cosmogenic radionuclides have 11. Estimate how many sand grains you can hold been employed in the last two decades. Contrast in your hand. (This is how many presumably the state of the art of establishing timing in the independent samples you have of erosion rates landscape in the absence of this tool. at spots in the landscape when you sample a handful of sand from a river draining a basin.) 15. Thought question. Compare the timescales over which we document average exhumation 12. You measure the 10Be concentration of a rates when using 10Be, apatite fission tracks, sample from a granitic outcrop atop Mt and U/Th-He methods. Assume that the real Osborn in the Wind River Mountains of exhumation rate is 10 m/Ma.
Bierman, P. R., 2007, Cosmogenic glacial dating, 20 years Granger, D. E. and P. Muzikar, 2001, Dating sediment and counting, Geology 35 (6): 575–576. burial with cosmogenic nuclides: theory, techniques, This is a quick review of the method as employed in glacial and limitations, Earth and Planetary Science Letters 188 settings, looking back on progress over the first two decades (1–2): 269–281. of its use. A valuable resource for practitioners, this review article provides sufficient information to allow full understanding of Cerling, T. E. and H. Craig, 1994, Geomorphology and the methods and pitfalls of the use of cosmogenic nuclides to in-situ cosmogenic isotopes, Annual Review of Earth and date buried surfaces (e.g., caves, deep deposits). Planetary Sciences 22: 273–317. This is an early snapshot of the use of this method, with Reiners, P. W. and T. A. Ehlers, eds., 2005, attention to both stable and radionuclides. Thermochronology, Reviews in Mineralogy and Geochemistry 58. Gosse, J. C. and F. M. Phillips, 2001, Terrestrial in situ The editors have brought together articles that collectively cosmogenic nuclides: theory and application, review the field of thermochronology as it is used to assess Quaternary Science Reviews 20: 1475–1560. the thermal history of the shallow crust. It is a very useful This is a lengthy but useful review of use of nuclides in entrance point into the literature, which is fast-evolving with geomorphology as of the turn of the century. new methods.
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