<http://purl.org/dc/terms/description>
"Marine sediment cores from the continental margin off Western Canada (48\u00B0 to 50\u00B0N,\r\n125 to 128\u00B0W) yield evidence of dramatic changes in oceanographic conditions over the last 16\r\nkyr. During the late Glacial the accumulation of marine organic matter was greatly reduced.\r\nThis reflects low primary and export production because glacial-mode atmospheric circulation\r\ndid not drive coastal upwelling. At the start of the Boiling warm period (-14.3 kyr B.P.), and\r\ncoincident with the retreat of glaciers from the continental shelf, there was a substantial increase\r\nin the burial of organic matter. However, much of this material was \"old\" terrestrial organic\r\ndetritus derived in part from the erosion of shelf sediments. It was not until the Allerod (-13.5\r\nto 12.6 kyr B.P.) that the accumulation of marine organic matter increased substantially. Other\r\npaleoproductivity proxies (i.e., % biogenic Ba, % opal and alkenone abundances) also indicate\r\nvery high marine productivity at this time. Since primary production in the region is controlled\r\nby the upwelling of nutrient-rich subsurface waters, this suggests upwelling was enhanced\r\nduring the Allerod. A decrease in benthic-planktonic age differences (i.e., older planktonic\r\nforam ages) supports this interpretation. There was a brief return to glacial conditions (i.e.,\r\nlower primary and export production) during the Younger Dryas, followed by a slight rise in\r\norganic carbon burial between -11 and 10 kyr B.P.. In general, the accumulation of organic\r\ncarbon was low throughout the Holocene despite high primary productivity. Low sedimentation\r\nrates and the resulting long oxidant exposure times as well as extensive biological recycling\r\nappear to be the primary causes of low organic carbon burial, although lower productivity\r\nrelative to the Allerod cannot be ruled out.\r\nChanges in the vertical settling flux of organic carbon to the sediment had a direct\r\nimpact on sedimentary redox conditions at Site JT96-09 (920 m water depth). At present, near-surface\r\nsediments become suboxic within millimetres of the sediment-water interface and Re\r\nenrichment is observed below the depth of bioturbative mixing. However, anoxic conditions do\r\nnot develop despite relatively low bottom water oxygen concentrations (0.3 ml/1) and relatively\r\nhigh organic carbon flux to the sea floor. The situation was much different during the deglacia During the Allerod, when the organic flux to the sediment peaked, Mo enrichment is observed\r\n(&gt;2 ug/g) implying that near-surface sediments were anoxic at this time. The benthic\r\nforaminifera assemblage (i.e., the dominance of Bolivina spp.) likewise suggests that sediments\r\nwere more oxygen-depleted. These results imply that the bottom waters at Station JT96-09\r\nwere more oxygen-depleted during the Allerod and since JT96-09 is located within the oxygen\r\nminimum zone (OMZ), it follows that the OMZ was more intense. The increase in marine\r\norganic carbon accumulation and the lack of evidence of decreased ventilation suggests that\r\nOMZ intensification was a response to increased primary production and carbon export.\r\nDeglacial intensification of the OMZ is documented along the entire length of the California\r\nCurrent System, but it appears to have been delayed by ~1500 years off Vancouver Island. It is\r\nprobable that the presence of the Laurentide and Cordilleran ice sheets continued to influence\r\natmospheric circulation, and in turn oceanic circulation, in the region until at least the Allerod.\r\nIt is possible to use redox-sensitive trace metals as palaeo-proxies in the deglacial\r\nsediments of Core JT96-09 because sedimentation rates were quite high (&gt;100 crn/kyr). In\r\ncontrast, in the slowly deposited Holocene sediments (5 cm/kyr) the trace metal record is\r\ncorrupted due to the precipitation of trace metals decimetres below the sediment-water interface.\r\nThis enrichment obscures the original palaeo-signal, which formed shortly after sediment\r\ndeposition. Rapid fluctuations in the sedimentation rate can also complicate trace metal\r\nrecords. In the deglacial deposits of Core JT96-02 the formation of Mo and Re concentration\r\nspikes is the direct consequence of episodic deposition of turbidites that restricted oxygen\r\ninflux into the sediment and thus allowed anoxic conditions to develop.\r\nIntriguing results were obtained from the measurement of sedimentary Ag concentration\r\nin near-surface sediments. Unlike the other redox-sensitive trace metals, for example Cd which\r\nhas a similar geochemical behaviour, the primary control on Ag concentration is not\r\nsedimentary redox conditions. Rather, it seems that Ag accumulates in, and is transferred to the\r\nsediment by, settling organic particles in much the same way that Ba is. Hence, measuring the\r\nAg concentration of sediments may provide another means of assessing palaeoproductivity."@en
<https://open.library.ubc.ca/cIRcle/collections/ubctheses/831/items/1.0052403>