ECCENTRICITY-PACED EARLY EOCENE HYPERTHERMAL EVENTS COEVAL WITH EXPANSIONS OF HYPERSALINE GOSIUTE LAKE, GREEN RIVER FORMATION, WY

The terrestrial response to transient warming during the Paleocene-Eocene thermal event is well documented and suggests that warming led to enhanced weathering and paleoriver discharge. Recent ocean cores show that multiple lesser magnitude d13C ‘hyperthermal’ excursions occurred at eccentricity maxima across the Early Eocene climatic optimum, however, their precise timing in relation to terrestrial lithofacies is poorly understood. The Wilkins Peak Member of the Green River Formation in southwest Wyoming has a prominent 10–30-meter cyclicity between intervals of siliciclastic strata deposited in alluvial floodplain environments and intervals of carbonate and evaporite strata deposited from a saline-alkaline lake. To better understand how these cycles relate to astronomical solutions for insolation variation and hyperthermal events, we used radioisotopic dating and astrochronology to precisely compare the marine record to a suite of proxy records from basin center cores and outcrops of the Wilkins Peak Member. These results show that maximum dD values for leaf waxes and minimum 87Sr/86Sr ratios occur just above the transitions from fluvial marker beds to overlying lacustrine lithofacies. U-Pb and 40Ar/39Ar geochronology, including a new date for a tuff in the upper Wilkins Peak Member, indicate these transgressive transitions correspond to eccentricity maxima. High dD values likely correspond to increased advection of Gulf of Mexico atmospheric moisture to the western US. Coeval low 87Sr/86Sr ratios suggests greater chemical weathering of the western part of the paleolake’s catchment area, where low-87Sr/86Sr carbonates are exposed across the fold and thrust belt of the North American Cordillera. Surface exposures of the uppermost surfaces of alluvial marker beds show abrupt transitions to lacustrine lithofacies and are in places marked by unusual meter-scale sand wave bedforms overlying truncated fluvial bedforms, consistent with rapid transgression. These findings collectively suggest that hyperthermal events promoted advection of high-dD atmospheric moisture to the catchment of Gosiute Lake from the Gulf of Mexico that led to lake level rise. Higher rainfall is consistent with observed intense paleosol development in alluvial strata updip of the lake.