HIGH-RESOLUTION PAGE

HIGH-RESOLUTION STUDIES OF LAKE SEDIMENTS

Since 1992, the Arid Regions Climate (ARC) Project has been involved in high-resolution studies of sediments from the Owens Lake basin, California, the Pyramid Lake basin, Nevada, and the Mono Lake basin, California. Researchers involved in these studies include:

L V. Benson (U.S. Geological Survey), James W. Burdett (Cornell University), Michaele Kashgarian (Lawrence Livermore National Laboratory), Steve P. Lund (University of Southern California), Joseph P. Smoot (U.S. Geological Survey), Scott Mensing (University of Nevada, Reno), Robert O. Rye (U.S. Geological Survey), and Tim P. Rose (Lawrence Livermore National Laboratory).

Climatic and Hydrologic Oscillations in the Owens and Mono Lake Basins, California

Oxygen isotope and total inorganic carbon values of sediments from the Owens Lake basin, California, show that Owens Lake overflowed much of the time between 52,500 and 12,500 radiocarbon years ago, indicating that the climate of the late Pleistocene was much wetter than the climate of the historical period. Magnetic susceptibility and organic carbon content of sediments from the Owens Lake basin also demonstrate that about 19 Sierra Nevada glacial oscillations occurred between 52,500 and 23,500 years ago. Glacial advances were usually accompanied by decreases in the amount of discharge reaching Owens Lake, suggesting that when summer temperatures decreased allowing, the advance of mountain glaciers, precipitation in the Sierra Nevada decreased. These data indicate that climate oscillated on a millennial scale between cold-dry and warm-wet states. Sediment cores from the North Atlantic ocean have revealed that massive iceberg discharges (Heinrich events) from the Laurentide and other Northern Hemisphere ice sheets occurred about 35,000 (H4), 27,000 (H3), 21,000 (H2), and 14,000 (H1) radiocarbon years ago. The Heinrich and other iceberg discharge events are recorded as anomalously high concentrations of rock fragments (lithics) in North Atlantic sediments. Comparison of the timing of Sierra Nevada glacial oscillations with the lithic record of North Atlantic core V23-81 shows that the number of mountain glacial cycles and North Atlantic lithic events were about equal between 39,000 and 23,500 years ago. In addition, the sedimentary record from the Owens Lake basin shows that Owens Lake desiccated during Heinrich H1 and ceased spilling during Heinrich event H2. These data suggest but do not prove the hypothesis that North Atlantic climate oscillations may have directly influenced the climate of the western United States during the late Pleistocene.

Evidence for Great Basin millennial-scale climate change from the Mono basin, California, has tended to confirm this hypothesis. Oxygen-18 values of sediments from the Wilson Creek formation, Mono basin, California, indicate three scales of temporal variation (Milankovitch, Heinrich and Dansgaard-Oeschger) in the hydrologic balance of Lake Russell between 35,500 and 12,900 radiocarbon years ago. During this interval, Mono Lake experienced four lowstands each lasting between 1000 and 2000 years. Each lowstand was initiated and terminated by abrupt changes in wetness. Paleomagnetic secular variation data indicate that three of the four lowstands occurred at the same times as Heinrich events H1, H2 and H4 in the North Atlantic Ocean. Oxygen-isotope data also indicate that the two wettest intervals occurred 18,000 and 13,500 years ago, corresponding to the pre- and post-glacial-maximum passages of the mean position of the polar jet stream over the Mono Basin.

With the retreat of Northern Hemispheric ice sheets during the Last Glacial Termination (15,000 to 10,000 radiocarbon years ago), the climate of the North Atlantic region experienced a series of abrupt oscillations that were recorded as the Oldest Dryas/Bolling/Older Dryas/Allerod/Younger Dryas vegetational sequence in European terrestrial sediments. Evidence of irregular cold-warm oscillations has also been found in Greenland ice cores and has been equated with the Oldest Dryas-Holocene vegetational sequence. Although questions remain as to the geographic extent of many of these oscillations, the last major cold event (the Younger Dryas event) appears to have been global in extent. Stable-isotope data from cored sediments taken from the Owens Lake basin, California, evidence four major oscillations in the hydrologic balance of this surface-water system during the Last Glacial Termination. The data demonstrate that climatic transitions in western North America were nearly synchronous with climate transitions occurring in the North Atlantic. When the climate of the North Atlantic region was cold and dry, the climate of the Owens Lake basin was also dry. This implies that climate variability in the two regions was linked by an atmospheric teleconnection. The linkage further suggests that cooling of the North Atlantic Ocean, which accompanied decrease in thermohaline circulation, caused a downstream cooling of the North Pacific which in turn decreased the amount of moisture reaching the western United States.

These three studies demonstrate that when the climate of the North Atlantic Region is cold and relatively dry (Heinrich and Dryas events) the climate of the Great Basin of the western United States is also cold and relatively dry, indicating a near synchroneity of climate change in the northern sector of the Northern Hemisphere during the late Pleistocene.