Molecular Fossils Reveal the Control of Late Palaeocene Climate Cooling Event on Watermass Conditions and Biological Sources in the Lacustrine Subei Basin

ABSTRACT The late Palaeocene climate cooling event has been reported in the organic‐rich sediments of the second member of the Funing Formation (E 1 f 2 ) in the Subei Basin. However, there is a lack of in‐depth research on the constraints of paleoclimate changes on paleoenvironment and biological sources that remain unclear. This study presents detailed molecular geochemical analyses from the lacustrine mudstones of the E 1 f 2 in the Subei Basin. The proxies of paleosalinity (gammacerane/C 30 hopane, β ‐carotene/ n ‐C 20 and extended tricyclic terpane ratio) and redox conditions (pristane/phytane, phytane/ n ‐C 18 and dibenzothiophene/phenanthrene) suggest climate cooling enhanced a higher salinity and anoxic (ferruginous) water column during the lower unit of the E 1 f 2 deposition. As the climate shifted to warmer and more humid conditions, water salinity and reducing conditions noticeably decreased. Proxies for biological source (maximum n ‐alkane, C 27 /C 29 sterane, 4‐methylsterane/C 29 regular sterane and steranes/hopanes) indicate that cooling climate constrained biodiversity. A transition is observed from the dominance of halophilic algae in the lower unit to a bloom of phytoplankton and prokaryotes in the upper unit. In addition, high abundance aryl isoprenoids and heavy organic carbon isotope compositions suggest the occurrence of photic zone anoxia in the lower unit, potentially providing a new case of green sulphur bacteria that thrived in a ferruginous lacustrine environment. Our research provides perspectives for the evolution of watermass conditions and biological sources in the Subei Basin during the late Palaeocene, highlighting the important role of climate changes in the evolution of lacustrine environment conditions.