RELICT SOILS AND PALEOCLIMATE: LESSONS FROM THE WIND RIVER BASIN, WYOMING

Stable isotopes in carbonates of buried soils have provided exciting insights into paleo-vegetation, climate, and atmospheric chemistry, and their importance in deciphering environmental conditions throughout geologic time continues to expand. However, the preservation of landforms (terraces, fans) with relict soils increases in the late Cenozoic, and these soils provide an additional isotopic record of continental climate that, when combined with recent advances in micro-sampling and isotopic analyses, provide insights into the timing and magnitude of recent environmental change. A distinctive aspect of our approach to using relict soils is the exploitation of carbonate coats on gravels, which afford stable isotope records of high temporal resolution because (in some cases) they preserve an orderly microstratigraphy that’s amenable to precise dating over > 100,000 years. In this talk, we review our work on the carbonate-dominated soils of the cold, arid Wind River Basin, and summarize their morphological development and their U-series and O isotope chemistry. We have hypothesized that O isotope shifts in the Quaternary represent changes in atmospheric circulation (Amundson et al.,1996. Geology 24:23-26)). Recently, we have improved our sampling methods and, through U-series dating (Sharp et al. 2003. Quaternary Research 59:139-150), we have greatly refined the chronology of the O isotope changes. We show that soil carbonates in the Basin had very consistent δ¹⁸O values of 21 to 22 ‰ (SMOW) between 55 and 20 Ky BP that were then followed by a shift to values of about 19 ‰ after 17 Ky BP. In this talk, we will discuss the interpretation of this record (atmospheric circulation change, soil water evaporation effects, temperature changes), and consider opportunities and challenges for future work on relict soils.