EPISODIC ARIDITY IN WYOMING DURING THE EARLY EOCENE: MULTIPLE-PROXY RECORDS AND POSSIBLE ORBITAL FORCING

Recent research indicates that short-term variations in climate, in particular temperature, occurred over western North America during the early Eocene. Records of amounts of precipitation, however, are lacking. Furthermore, causes of such relatively short and small climatic variations are not well understood. To address these issues, we analyzed stable isotope data collected from several mammalian taxa that covers the first ~1.4 Ma of the Eocene. Differences in oxygen and carbon isotope ratios among taxa that are inferred to ingest water from rivers versus plant leaves provide a qualitative measure of aridity, and thus amount of precipitation. In addition, major element data were collected from paleosols associated with these fossil remains. Elemental ratios from these soils are related to the extent of chemical weathering, which in turn is tied to amount of precipitation.

Oxygen and carbon isotopic data indicate that the first 1.4 Ma of the Eocene was characterized by three episodes of significant aridity, as identified by an increase in the isotopic offset between riparian (e.g. Coryphodon) and open forest (e.g Hyracotherium) taxa. The occurrence of these dry periods appears to be confirmed by a second, and completely independent, measure of aridity. In this case, paleosols found associated with mammal remains have higher ratios of mobile to immobile elements, which is consistent with reduced chemical weathering and thus water flux, during these arid episodes. All of these dry intervals predate a known cool period in the Bighorn basin, and have not been described heretofore.

In an effort to identify the cause of these geochemical, and thus climatic, variations over time, we conducted time series analyses of our records. Preliminary results indicate that orbital variations, particularly those associated with eccentricity, played a role in driving these changes in aridity.