PREDICTING CONTROLS ON THE WATER CYCLE IN THE NORTHWESTERN GREAT BASIN DURING THE LAST DEGLACIATION: SURPRISE VALLEY, CALIFORNIA

Evidence of late Pleistocene lakes in the Great Basin indicates greater moisture availability during Pleistocene glacial periods. We dated shoreline tufa deposits from wave-cut lake terraces in Surprise Valley, California to determine the hydrography of the most recent lake cycle. To further evaluate the climatic forcings associated with the lake cycle, we use an oxygen isotope mass balance model combined with an analysis of model predictions from the Paleoclimate Model Intercomparison Project 3 (PMIP3) climate model ensemble. We dated 111 sub-samples from 22 tufa samples using ²³⁸U-²³⁰Th geochronology, and pair these analyses with 15 radiocarbon ages. Our new lake hydrograph places the highest lake level ~176 m above present-day playa at >15.2 ka. During the Last Glacial Maximum (LGM, 19 to 26 ka) Lake Surprise stood at moderate levels (~80 m depth). The Lake Surprise highstand postdates the Lake Lahontan highstand, and corresponds to several post-LGM highstands and stillstands of smaller lake systems further east. The isotope mass balance model predicts minimal precipitation increases of only ~2-20% during the LGM relative to modern, compared to a ~75% increase in precipitation during the Heinrich Stadial 1 (HS1) highstand. The PMIP3 climate model ensemble results corroborate these findings and predict an average precipitation increase of only 6.5% at the LGM relative to modern, accompanied by a 28% decrease in total evaporation and a 7°C decrease in mean annual temperature. LGM climate model simulations also suggest a seasonal decoupling of runoff and precipitation, shifting peak runoff to the late spring.

Reduced evaporation, and not increased precipitation, likely facilitated moderate lake levels during the LGM. This primed smaller, isolated basins, such as Surprise Valley, to respond rapidly to increased precipitation during HS1. Increased precipitation in the northern Great Basin was potentially driven by intensification and broadening of the southern arm of the split polar jet stream (‘mean position’ ~ 41°N). Seasonal insolation, in particular the effect of summer insolation on lake evaporation, appears to be a previously under-investigated long-term driver of moisture availability in the western United States.