STABLE CARBON ISOTOPE TRENDS ACROSS PALEOSOL HORIZONS ASSOCIATED WITH AN EARLY EOCENE HYPERTHERMAL EVENT (WILLWOOD FORMATION, BIGHORN BASIN, WYOMING)

Marine stable isotope records indicate the presence of repeated early Eocene hyperthermal events in the form of negative carbon isotope excursions (CIEs). These CIEs vary in magnitude with some in excess of -1 ‰, such as the Paleocene-Eocene Thermal Maximum (PETM) and Eocene Thermal Maximum 2. High magnitude CIEs have been detected in terrestrial deposits, such as the alluvial sediments of the Willwood Formation (Bighorn Basin, WY), but lower magnitude CIEs may be difficult to identify in alluvial strata due to variation in inorganic δ¹³C imposed by pedogenic and hydrologic processes. We measured the δ¹³C of soil carbonate nodules from the McCullough Peaks (Bighorn Basin) and observed a low magnitude CIE stratigraphically above a mature paleosol with distinct B sub-horizons. We tentatively correlate the CIE to the 55.165 Ma C24rH7-F hyperthermal event in the global marine record. δ¹³C values from the mature paleosol complicated our characterization of the CIE over the composite section by exhibiting significantly greater variance (σ² = 0.53 vs. σ² = 0.24 for all other strata combined) and a decreasing trend in δ¹³C from the base to the top of the B-horizon. Paleosol horizons and sub-horizons were distinguished using grain size distributions, nodule concentration, mottling intensity, pedogenic textures, and sediment color, a proxy for iron oxide and hydroxide minerals. δ¹³C values from the lower B-horizon were significantly more positive (average δ¹³C = -9.37 ‰) than those from the upper B-horizon (average δ¹³C = -9.87 ‰) and diffuse A-B horizon boundary (average δ¹³C = -10.62 ‰). The δ¹³C trend with paleosol horizon is likely related to water table fluctuations and corresponding gradient in soil water evaporation instead of reflecting solely a pattern of paleoclimate change. The change in δ¹³C values (∆δ¹³C = -1.25 ‰) across paleosol horizons exceeds the hyperthermal CIE magnitude (∆δ¹³C = -0.55 ‰), calculated using the average δ¹³C for the entire red paleosol, but may be influenced by greater seasonality in precipitation during the hyperthermal, as observed during the PETM. We are currently measuring nodule δ¹³C and precipitation proxies from mature paleosols before and after the hyperthermal event to better separate the “soil signal” from the “climate signal” in our stable isotope record.