9709576 Kaufman Proxy climate indicators that record purely temperature or precipitation are rare but are critical for separating the two signals to compare with the output from climate models and for resolving the relative contribution of each to the hydrologic budget of Pleistocene lakes. This study follows previous research that has demonstrated the utility of biomolecules preserved in fossils as paleothermometers. I propose to use the extent to which chemical reactions, particularly amino acid racemization, have progressed in fossil molluscan shells from extensive late Quaternary lake deposits in the Great Basin, Western United States, as a measure of paleotemperature. The extent of amino acid racemization in a fossil is proportional to the length of time elapsed since the organism lived and to the ambient postdepostional temperature. Coupled with independent age control though AMS 14C, luminescence, and tephrochronology, the amino acid data can be used to assess the timing and magnitude of paleotemperature changes. By using shells of different ages, the temperature of successive time intervals bracketed by the shells can be calculated. Because protein diagenesis is an integrative measure of the entire postdepositional temperature history, it reflects long-term temperature changes that complement the geologically instantaneous paleoenvironmental evidence contained within the deposits themselves. During the past four years, the Amino Acid Geochronology Laboratory at Utah State University has gained considerable experience analyzing fossil snail shells from across the northern Great Basin. Preliminary results indicate that the effective diagenetic temperature (EDT) in the Bonneville basin for the period 25-15 ka (calibrated yr) was 1.5(2.0 (C, which is about 9 (C lower than the present mean annual air temperature (MAAT). The uncertainty in the temperature estimate could be reduced through additional laboratory heating experiments and analysis of 14C dated Holocene snails that wi ll serve to refine the kinetic equation. The technique will also benefit from a newly developed analytical procedure and instrumentation that resolves multiple amino acids including some that react at higher rates than the single amino acid (isoleucine) that was used previously. In addition, because the sample-size requirements are nearly an order of magnitude less for the new analyzer, and because reducing uncertainties imposed by analytical and inter-shell variability. The questions addressed in this study focus on calculating temperature differences (thermal gradients and temperature changes between time intervals), which are inherently more precise than absolute-temperature estimates. The aim is to attain precision better than ( 0.5( C, which is more than sufficient to assess regional-scale characteristics of past temperatures and to compare with fundamental features of GCM-and regional-model-simulated climate, such as: (1) full-glacial, MAAT anomalies that increase about 2-3( C northward and are lowered by 3-5( C relative to the present; (2) full-glacial, mean annual water-to-air temperature difference of about 3-4( C; and (3) postglacial warming that progresses northward during the period 15-12 ka (14C yr). Preliminary amino acid data on 14.7 ( 0.3 ka (calibrated yr) snails from the Franklin basin (40( 10( N) and the Chewaucan basin (42( 40( N) suggest that the average late Quaternary temperature anomalies increase northwestward by 3( C, whereas the present MAATs at these sites differ by < 1( C. To evaluate whether the steeper paleotemperature gradient reflects either greater early/middle Holocene warmth in the north or greater late Pleistocene cooling in the south will require partitioning the EDT estimates of the late Quaternary into shorter time intervals using shells of bracketing ages, such as those that have been previously dated at numerous sites across the Great Basin. In addition, new tephra (A. Sarna-Wojcicki, USGS, Denver), luminescence (S. Forman, University of Chica go), and AMS 14C analyses will be conducted as part of this study. The shorezone deposits rimming the basins provide an ideal datum for collection because they contain similar-age shells and because they occur at similar elevations, thereby avoiding differences in thermal histories resulting from different submergence durations. On the other hand, contemporaneous shells from nearby localities but from different elevations afford a means of assessing the difference between past water and air temperatures, a major control on evaporation rate. Similarly, shells from a restricted area but of a range of ages can be used to date climate transitions, such as the last glacial-to-interglacial warming. On the inter-basinal scale, widely distributed tephras previously reported in Pleistocene lake deposits provide ideal stratigraphic markers that, in conjunction with amino acid analyses, can be used to calculate paleotemperature gradients across the area of the eruptive plume.
