The widespread lake systems of the Basin and Range during the late Pleistocene indicate substantially greater moisture availability during glacial periods relative to modern. To determine the hydrography of the most recent lake cycle, we dated shoreline tufa deposits from wave-cut lake terraces in Surprise Valley, California. The lake hydrograph is constrained by 230Th-U ages on 22 tufa samples paired with 15 radiocarbon ages. This new lake hydrograph places the highest lake level 176 m above the present- day playa at >15.23 ± 0.36 ka cal BP (14C age). During the Last Glacial Maximum (LGM, ~19 to 26 ka), Lake Surprise stood at moderate levels, 65 to 99 m above modern playa. Temporally, the Lake Surprise highstand slightly postdates the Lake Lahontan highstand and corresponds to several post-LGM highstands and stillstands of smaller lake systems farther east. To further evaluate climatic forcings 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 2.5 to 18.2% (average = 9.5%) during the LGM relative to modern, compared to an approximately 75% increase in precipitation during the 15.23 ka highstand when lake surface area increased by 138%. LGM PMIP3 climate model simulations corroborate these findings, predicting an average precipitation increase of only 6.5% relative to modern, accompanied by a 28% decrease in total evaporation propelled by a 7°C decrease in mean annual temperature. LGM climate model simulations also suggest a seasonal decoupling of runoff and precipitation, with peak runoff shifting to the late spring. Based on our coupled analysis, we propose that moderate lake levels during the LGM were driven by reduced evaporation, a result of reduced summer insolation, and not by increased precipitation. Reduced evaporation primed Basin and Range lake systems, particularly smaller, isolated basins such as Surprise Valley, to rapidly respond to increased precipitation during late-Heinrich Stadial 1 (HS1, ~14.5 to 19 ka). Post-LGM highstands were potentially driven by increased rainfall during HS1 brought by latitudinally extensive and strengthened mid-latitude westerly storm tracks, the effects of which are recorded in the lacustrine and glacial records as far south as ~32°N. These results suggest that seasonal insolation, in particular the effect of summer insolation on lake evaporation, provides a previously under-investigated long-term driver of moisture availability in the western United States.
Precipitation rates and atmospheric heat transport during the Cenomanian greenhouse warming in North America: Estimates from a stable isotope mass-balance model
