Floodplain response to Eocene Thermal Maximum 2 in the southern Bighorn Basin, Wyoming, U.S.A.

Paleosols in the Willwood Formation of the Bighorn Basin, Wyoming contain a sedimentary and geochemical record of several early Eocene hyperthermal (rapid, global warming) events including the Paleocene-Eocene Thermal Maximum (PETM) and the Eocene Thermal Maximum 2 (ETM2). Numerous studies of the PETM indicate environmental shifts including an overall decrease in precipitation and soil moisture, but the hydrologic response to the subsequent smaller Eocene hyperthermals remains poorly understood. In order to estimate potential precipitation changes during ETM2, I sampled floodplain paleosol horizons from Willwood Formation strata below, within, and above the stratigraphic carbon isotope excursion (CIE) that marks the ETM2. Paleosol profiles are overall thinner and more weakly developed in the ETM2 CIE interval, which may suggest a higher sedimentation rate on the floodplain during the hyperthermal. Geochemical proxy measurements of mean annual precipitation (MAP) show variability across the ETM2 CIE interval, with the driest intervals ~850 mm/yr and wetter intervals ~1300 mm/yr. MAP estimates average ~1175 mm/yr in the pre-ETM2 interval, ~1150 mm/yr for the duration of ETM2, and ~1100 mm/yr in the post-ETM2 phase. These MAP values are in the same range as previous PETM studies, but unlike the PETM there is no definitive systematic shift in MAP that occurs in conjunction with the ETM2. The hydrologic response of continental interiors does not appear to scale linearly between hyperthermal events of differing magnitudes based on the Bighorn Basin data set