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Show A TALE OF THREE LAKES Larry V. Benson U. S. Geological Survey 3215 Marine St., Boulder CO 80303 Until recently the prevailing paradigm of past climate variability was rooted in the astronomical theory of climate change. The first indication that climate change had occurred abruptly on scales smaller than those dictated by astronomical forcing was first observed in 8180 records obtained from Greenland ice cores. Initially, the abrupt changes in the 5180 records were attributed to disturbance of the ice's stratification. In 1992, however, S. Johnsen published four continuous 5180 profiles from four Greenland cores each of which indicated the same irregular pattern of centennial- and millennial- scale climate change, demonstrating that ice core records were reproducible and reliable. A paradigm shift had occurred. The ' Pacemaker of the Ice Ages' had been disrupted by climatic fibrillation in the North Atlantic region. H. Heinrich had previously ( 1988) noted evidence of six last- glacial ice rafting events in the northeast Atlantic Ocean. In 1992, G. Bond identified the same six ' Heinrich events' in another core, demonstrating that each of these events recorded rapid decreases in sea surface temperature. In 1993, Bond used sea surface temperature proxy records from two North Atlantic sediment cores and an air temperature proxy record from the GRIP ice core to show that North Atlantic sea surface temperature oscillations could be matched with the ice- core record for the past 90 kyr. This recognition of a link between ice sheet behavior and ocean- atmosphere temperature changes raised questions as to the cause, extent and mode of propagation of climate perturbations occurring in the North Atlantic region. Systematic studies of the rise and fall of Great Basin lakes were initiated in the late 1950s and early 1960s by W. Broecker, P. Orr, R. Flint, W. Gale, and A. Kaufman. These and other studies that followed usually resulted in low- resolution ' envelopes' of lake- level variation which are unsuitable for comparison with existing high- resolution ice and marine records. To provide high- resolution climate ( hydrologic balance) records from lakes of the western Great Basin, we obtained continuous sediment records from the Owens, Mono, and Pyramid Lake basins ( Fig. 1). The samples ( which integrate between 20 and 100 yr of record) have been analyzed for 5180, 513C, and a variety of other chemical and physical parameters. The 5180 value of a lake represents a balance between amounts and 5180 values of water input to and lost from a lake. When the lake overflows, 5180 is primarily |
