STRONTIUM CONCENTRATIONS IN PALEOSOL CARBONATES: A POSSIBLE RECORD OF HYDROLOGIC CHANGE IN THE BIGHORN BASIN, WYOMING, DURING EARLY EOCENE HYPERTHERMALS

Hyperthermals are short transient increases in global temperatures that occurred during the early Cenozoic. These events are associated with pronounced decreases in carbon isotope ratios of carbon reservoirs, and dramatic shifts in plant and animal assemblages. The decreases in carbon isotope ratios are termed, Carbon Isotope Excursions (CIEs). The Paleocene Eocene Thermal Maximum is the largest and most studied hyperthemal/CIE event, but two smaller hyperthermals followed, ELMO and H2.

The aim of my research is to identify the ELMO and H2 events in the terrestrial environment, and to investigate the possible impact of these events on groundwater hydrology. I collected 61 carbonate nodules and associated matrix samples over 130 vertical meters in the McCullough Peaks area of the Bighorn Basin of Wyoming. Carbon isotope data reveal a pronounced negative ~ 4‰ CIE which can be correlated to the H2 event in marine records. In an effort to investigate groundwater hydrology before and during this event, Sr concentrations in carbonate nodules were measured, and they exhibit an increase from in Sr from ~280 to 580 and back to ~280 p.p.m across the H2 interval. This relative spike of Sr is interpreted to represent increased residence times of ground waters as they flow through soils. In turn, such an increase in groundwater residence time is thought to correlate to a change in precipitation regimes: a pre-H2 condition where precipitation is more evenly distributed over the course of the year and thus groundwater through-put is constant, to a H2 condition where precipitation is highly seasonal and groundwater through-put is episodic.