Continental hydroclimate during the late Paleocene-early Eocene in the central Rockies: insights from sedimentological and geochemical records preserved in the greater Green River Basin, southwestern Wyoming, U.S.A.

Min Gao ^1,2,3* Majie Fan ² Guangsheng Zhuang ⁴ DOMENICO CHIARELLA ³

• ¹ Research Institute of Petroleum Exploration and Development (RIPED), Beijing, China
• ² Department of Earth and Environmental Sciences, College of Science, University of Texas at Arlington, Arlington, Texas, United States
• ³ Department of Earth Sciences, Royal Holloway, University of London, Egham, United Kingdom
• ⁴ Department of Geology and Geophysics, College of Science, Louisiana State University, Baton Rouge, Louisiana, United States

The late Paleocene to early Eocene (LPEE, ~59-50 Ma) greenhouse condition is the most recent geologic analogue for future climate change induced by increase in atmospheric CO2 concentration (pCO2). Recognition of the hyperthermals and reconstruction of the continental hydroclimate and atmospheric pCO2 during this warm period are fundamental to the understanding of Earth’s surface responses to high pCO2. Here we study paleosol morphology, bulk organic matter δ13Corg, and leaf wax n-alkanes δ13Cn-alk and δDn-alk, and reconstruct mean annual precipitation (MAP) based on chemical index of alteration (CIA-K) proxy from the fine-grained floodplain deposits in the greater Green River Basin, western U.S.A., to examine the hydroclimate evolution in the continental interior during the LPEE. The Paleocene Eocene Thermal Maximum (PETM) was identified as a ~4-5 ‰ negative carbon isotope excursion (CIE) in the bulk organic δ13Corg record and leaf wax n-alkanes δ13Cn-alk record, and a 30 to 50 ‰ increase in leaf wax n-alkanes δDn-alk record. Well drained green paleosols and water-logged histosols dominated the floodplain deposition during the LPEE, and poorly drained red paleosols and carbonate-rich calcisols characterized the hyperthermals. Our estimated MAP is generally high (900-1600 mm) during majority time of the LPEE, but seems low (520 mm) during the PETM. Our reconstructed atmospheric pCO2, by integrating bulk organic δ13Corg and paleosol carbonate δ13Cc values, is generally in the range of 600-900 ppm during the early Eocene, 1-2 times higher than the preindustrial level. These records suggest that the continental hydroclimate was generally humid and warm during the LPEE, and transient drying likely happened during the PETM.