PALEOENVIRONMENT RECONSTRUCTION OF THE EOCENE SOUTHEASTERN TETHYS USING GEOCHEMISTRY OF SEDIMENTARY ROCKS

The early Eocene is considered as the warmest time interval of the Cenozoic Era, with global temperatures more than 10 degrees warmer than today. Despite its tectonic significance, the paleoclimate history of the southeastern Tethys during the early Eocene remains poorly known. Here, we report new geochemical data that reveal the paleoclimate condition of the early Eocene (~ 53.6 to 52.6 Ma) from the Qumiba section in the Tingri region in southern Tibetan Plateau, China, which likely represents the youngest marine strata in the southeastern Tethys Ocean. The studied Qumiba section consists of the Enba Formation characterized by silty marl, mudstone and lithic sandstone and the Zhaguo Formation dominated by mudstone and a few thin layers of lithic sandstone. The depositional history of the Qumiba section is reconstructed using major, trace and rare earth elements. Redox and nutrient conditions of the southeastern Tethys are evaluated in relation to the abundance of calcareous nannofossils, and are considered to be affected by elevated temperature and higher precipitation. Stable carbon and nitrogen isotopes of bulk marine organic matter suggest the warming signal is registered in the sediments and may have led to anomalous response of the carbon and nitrogen cycle in the eastern Tethys. The Early Eocene hyperthermals (H1, ca. 54.09 Ma and H2, ca. 54 Ma) may have been recorded in the Qumiba section based on the observed abrupt negative excursions in carbon isotopes from carbonate and organic matter (-2‰ in δ¹³C_carb and -4.5‰ in δ¹³C_org). Chemical weathering proxies (i.e., chemical index of alteration) suggest more intense chemical weathering due to warmer temperatures across the presumed hyperthermal event. Meanwhile, redox proxies (EF_U and EF_V) suggest that the bottom water in the depositional basin experienced dysoxic to slightly anoxic bottom conditions. The coeval increase of the nutrient proxies (e.g. EF_P, EF_Ba, EF_Ni and EF_Co values) suggests an increase in nutrients availability and primary productivity due to the elevated pCO₂ and continental weathering. More research elsewhere in the eastern Tethys need to be done on the paleoenvironment reconstruction associated with these hyperthermal events.