SOIL GEOCHEMISTRY-BASED PROXIES FOR INTRA-ANNUAL VARIABILITY IN EVAPORATION MINUS PRECIPITATION, AND AN EXAMPLE OF THEIR APPLICATION TO PALEOSOLS SPANNING THE PETM IN THE BIGHORN BASIN OF WYOMING

Water availability in soils plays a fundamental role in determining their chemical and mineralogical characteristics. The presence of water allows for the chemical breakdown of parent material, and the preferential removal of soluble ions relative to insoluble ions from weathered soil horizons, while the absence of water can result in the formation of authigenic minerals such as calcite. In turn, water availability in soils is influenced largely by (1) the amount of water added to them via precipitation, (2) the amount of water lost from them via evaporation, and how (3) how precipitation & evaporation vary over the course of a year. Thus it should be possible to use the ratio of insoluble to soluble ions in soils (as represented by the ‘chemical index of weathering’, or CIW), and the occurrence of authigenic carbonate in soils, as proxies for evaporation minus precipitation (E-P), and as proxies for intra-annual variations in E-P.

A initial goal of this project is to test these hypotheses and develop a new set of proxies by comparing CIW from a suite of extant North American soils, and the presence/absence of carbonate in these soils, to estimates of E-P at sub-annual timescales from the same localities. Support for a link between geochemistry and climate come in the form of good correlations between CIW and summer E-P, winter E-P, and the number of months a year when E-P is less than zero. The presence/absence of soil carbonates also correlates with CIW, with winter E-P, and with the number of months a year when E-P is less than zero.

To study the utility of modern-based proxies to infer climate of the past, they are applied to a stratigraphic sequence of paleosols that span the Paleocene-Eocene Thermal Maximum (PETM) in the Bighorn Basin of Wyoming. Temporal changes in winter E-P, and in the number of months a year when E-P is less than zero, are consistent with other proxies and match predictions made by previous workers. There are, however, inconsistencies with the climatic conditions associated with authigenic carbonate formation that need to be explored. Once refined, the ultimate goal of this research is to integrate proxy climate data from multiple Laramide basins with predictions made using global climate models run with different topographic boundary conditions in order to study paleoelevation of western North America during the early Eocene.