CLIMATE VARIABILITY IN THE WESTERN GREAT BASIN RECORDED BY GEOCHEMICAL SIGNALS IN CLOSED-BASIN LAKE SEDIMENTS

We have been carrying out geochemical studies of several closed-basin lakes in the Great Basin, including Death Valley playa, and Owens, Mono, Pyramid, Alkali and Harney Lakes, in a S-N transect mainly along Sierra Nevada. The methods involve U-series dating and measurements of d18O, d13C, trace elements, carbonate mineralogy in lake deposits. Between 900 and 1200 AD (Medieval Climatic Anomaly) climate in the Great Basin was dry. It turned to wet and cold during the early Little Ice Age (LIA) between 1250 and 1500 AD. During 1500-1600 AD, the climate shifted from wet/cold to dry/cold, and remained dry in the late LIA of 1600-1850 AD. During the late LIA, the decadal climatic records showed a difference between the northern basins (Harney and Alkali) and southern basins (Mono and Owens), due perhaps to the influence of jet-stream shift on precipitation at different locations. In addition, climate fluctuated more frequently after ~1500 AD. The Holocene climate in the Great Basin was dry during Young Dryers, cool/wet in early Holocene, and warm/dry in middle Holocene, shown by relatively high lake level at 9 Ka and low level at 6 Ka. Apparently, the effective moisture budget in this region reached its height during the N. Hemisphere summer insolation maximum at 9 ka, and the situation was reversed at 6 ka. The last interglacial (stage 5) in the region appeared drier than stages 4 and 6. Stages 5b and 5d were generally wet as shown by the overflowing lake conditions. Stages 5a, 5c and 5e are characterized by low lake levels and hydrological closure. The glacial/interglacial climatic patterns we deduced are different from those based on the loess-paleosol sequence in China. The difference may be caused by the fact that unlike temperature, precipitation is linked more subtly to the orbital elements. Being more prone to be affected by changes in atmosphere-ocean circulation patterns, precipitation could exhibit more substantial regional and short-term variations.