Abstract 
 
 Widespread late Pleistocene lake systems of the Basin and Range Province indicate substantially greater moisture availability during glacial periods relative to modern times, but the climatic factors that drive changes in lake levels are poorly constrained. To better constrain these climatic forcing factors, we present a new lacustrine paleoclimate record and precipitation estimates for Lake Surprise, a closed basin lake in northeastern California. We combine a detailed analysis of lake hydrography and constitutive relationships describing the water balance to determine the influence of precipitation, evaporation, temperature, and seasonal insolation on past lake levels. At its maximum extent, during the last deglaciation, Lake Surprise covered 1366 km2 (36%) of the terminally draining Surprise Valley watershed. Using paired radiocarbon and 230Th-U analyses, we dated shoreline tufa deposits from wave-cut lake terraces in Surprise Valley, California, to determine the hydrography of the most recent lake cycle. This new lake hydrograph places the highest lake level 176 m above the present-day playa at 15.19 ± 0.18 calibrated ka (14C age). This significantly postdates the Last Glacial Maximum (LGM), when Lake Surprise stood at only moderate levels, 65–99 m above modern playa, similar to nearby Lake Lahontan. 
 
 To evaluate the climatic factors associated with lake-level changes, we use an oxygen isotope mass balance model combined with an analysis of predictions from the Paleoclimate Model Intercomparison Project 3 (PMIP3) climate model ensemble. Our isotope mass balance model predicts minimal precipitation increases of only 2%–18% during the LGM relative to modern, compared to an ∼75% increase in precipitation during the 15.19 ka highstand. LGM PMIP3 climate model simulations corroborate these findings, simulating an average precipitation increase of only 6.5% relative to modern, accompanied by a 28% decrease in total evaporation driven by a 7 °C decrease in mean annual temperature. LGM PMIP3 climate model simulations also suggest a seasonal decoupling of runoff and precipitation, with peak runoff shifting to the late spring–early summer from the late winter–early spring. 
 
 Our coupled analyses suggest that moderate lake levels during the LGM were a result of reduced evaporation driven by reduced summer insolation and temperatures, not by increased precipitation. Reduced evaporation primed Basin and Range lake systems, particularly smaller, isolated basins such as Surprise Valley, to respond rapidly to increased precipitation during late-Heinrich Stadial 1 (HS1). Post-LGM highstands were potentially driven by increased rainfall during HS1 brought by latitudinally extensive and strengthened midlatitude westerly storm tracks, the effects of which are recorded in the region's lacustrine and glacial records. These results suggest that seasonal insolation and reduced temperatures have been underinvestigated as long-term drivers of moisture availability in the western United States. 

 During the Last Glacial Maximum, large lakes (light blue) covered many of the now dry desert basins of Nevada, Oregon and California. Credit: Daniel Ibarra (Phys.org) —A new study by Stanford scientists solves a longstanding mystery of how ancient lakes in the western United States grew to such colossal sizes. 
 
 The research, published in the journal Geological Society of America Bulletin, found that the lakes were able to grow large – rivaling the Great Lakes – during the peak of the last Ice Age 21,000 years ago, a period known as the "Last Glacial Maximum," because evaporation rates were significantly lower than today. 
 
 "It was previously thought that the lakes grew because there was more rain and snowfall during this period of the Earth's history," said Daniel Ibarra, a graduate student in Stanford's Department of Environmental Earth System Science and the first author of the study. 
 
 More quantitative studies of past climate could help refine the computer models used by the Intergovernmental Panel on Climate Change (IPCC) to simulate Earth's atmospheric conditions under changing atmospheric conditions, said Kate Maher, assistant professor of geological and environmental sciences, who headed the project. 
 
 "The IPCC uses climate models to simulate past and future climate, so knowing that some of the models do a better job of simulating past changes gives us more confidence that we understand the physics involved," Maher said. "That can give us more confidence in the models we use to simulate future climate change." 
 
 During the Last Glacial Maximum (LGM), giant lakes covered large sections of California, Nevada, Oregon and Utah, including where Salt Lake City is today. Earth scientists have long been puzzled by how these ancient lakes, now completely dry, grew so large. The prevailing theory was there was more rain and snowfall during this time period. But recent evidence from paleoecology and climate model simulations indicates that precipitation rates were actually relatively low compared to later periods. 
 
 To resolve the discrepancy between computer models and the interpretation of geologic evidence, Ibarra collected more than 80 samples of tufa – a limestone created by the evaporation of mineralized water – from different locations around the edges of Lake Surprise, a moderate-sized fossil lake in Surprise Valley, California. 
 
 The layered shorelines of California's Lake Surprise date back to the peak of the last Ice Age and provide a record of lake level changes over thousands of years. Credit: Anne Egger "The smaller lakes can tell you about the regional climate and can serve as a water gauge for the bigger lakes," Maher said. 
 
 By measuring the decay of radioactive carbon-14 and uranium in tufa samples, the team reconstructed the ancient shorelines of Lake Surprise at different times in the past. Their findings showed that at the height of the LGM, Lake Surprise had a surface area of about 390 square miles, roughly the size of San Francisco Bay. 
 
 With support from other laboratories at Stanford, Ibarra also used a mass spectrometer to precisely measure the amounts of two slightly different forms of oxygen in the tufa samples: oxygen-16 and the slightly heavier oxygen-18. Both isotopes of oxygen are present in water, but oxygen-18 water evaporates at a slower rate than oxygen-16 water. By knowing the ratio of oxygen-16 to oxygen-18 in the tufa samples, the scientists were able to calculate Lake Surprise's water balance through time. 
 
 Cooler temperatures 
 
 Their analyses revealed that 21,000 years ago, the evaporation rate at Lake Surprise was nearly 40 percent lower than today, with precipitation rates similar to the modern era. These results are consistent with previously run climate simulations that show Earth's climate was cooler during the LGM. 
 
 The cooler global temperatures would have reduced evaporation rates, allowing the lakes to gradually grow over time through inflows from streams and rivers. 
 
 "Lake Surprise is located in a closed basin. All streams flow into the lake, but there is no outflow. The only way for water to escape is through evaporation," Ibarra said. 
 
 The team's dating and isotope measurements also show that precipitation rates in the region increased for a brief period after the LGM. The tufa measurements indicated that Lake Surprise reached its largest size – around 530 square miles – 15,000 years ago. 
 
 Thus, the enormous lakes that once dotted the western United States initially grew large during the peak of the LGM due to reduced evaporation, but didn't reach their maximum sizes until several thousand years later, when rain and snowfall increased. 
 
 Knowledge about the past precipitation patterns of the region could be used to test the accuracy of the differing climate models scientists currently use to simulate global climate conditions. 
 
 "We can actually rank the models now," Ibarra said. "Our findings have implications for evaluating the models, and deciding which models successfully reproduced the past precipitation patterns we observe." 
 
 Explore further: Study of past climate change in Pacific Northwest lakes may presage future drought 
 
 More information: "Rise and fall of late Pleistocene pluvial lakes in response to reduced evaporation and precipitation: Evidence from Lake Surprise, California," Geological Society of America Bulletin, B31014.1, first published on June 2, 2014, DOI: 10.1130/B31014.1 
 

– During the period known as the Last Glacial Maximum some 21,000 years ago, mammoth lakes spread out across large swaths of the West, and scientists have long wondered why the now-dry lakes used to be so big. Mystery solved: A new study finds that the reason has nothing to do with higher precipitation—a long-running theory unraveled by recent analysis that found rain and snowfall levels were relatively low at the time—but rather slower evaporation. Analyzing limestone samples taken from the edges of Lake Surprise, a fossil lake in California, geologists found more amounts of the isotope known as oxygen-18 than oxygen-16, which is slightly lighter. Oxygen-18 water evaporates more slowly, and the ratio of the two isotopes indicates that evaporation rates during the Last Glacial Maximum were a whopping 40% lower than what we experience today. "Lake Surprise is located in a closed basin," lead author Daniel Ibarra at Stanford tells PhysOrg. "All streams flow into the lake, but there is no outflow. The only way for water to escape is through evaporation." Because precipitation rates in the region appear to have gone up for a brief period 15,000 years ago, right around when Lake Surprise reached its largest size (530 square miles), the giant lakes in the West probably grew large initially due to lower evaporation, and then reached what would be their maximum sizes when rain and snowfall increased several thousand years later. (It may not be the sexiest answer, but at least this mystery can't be blamed on a meteor.)