The evolution of organic matter sources and sedimentary environments in lakes during the Eocene Climatic Optimum

Abstract

The global climate experienced many rapid and long-term warming events during the early–middle Eocene, and these warming events had a significant impact on the evolution of the global paleoecosystem and led to the widespread development of organic-rich fine-grained sediments worldwide during the same period. To elucidate the regulatory effects of early–middle Eocene warming events on the organic matter (OM) sources and sedimentary environments of paleolakes, in this study, which is based on a detailed analysis of organic petrology, mineralogy, bulk geochemistry and biomarkers of typical wells, combined with a previous understanding of paleoclimate and hydrothermal activities, the influences of warming events on the OM sources and sedimentary environments of early–middle Eocene lakes were systematically investigated. The results demonstrate that units I and III, which were deposited under warm and humid climates, experienced massive freshwater influxes, highstand lacustrine conditions, and dysoxic–anoxic bottom waters, fostering the codominance of algae and terrestrial vascular plants. In contrast, units II and IV record lacustrine salinization events under warm semiarid–semihumid climates and synsedimentary hydrothermal activity, marked by increased salt-tolerant/thiophilic bacteria and euxinic bottom conditions linked to salinity stratification and hydrothermal sulfur cycling. The climatic transition from warm–humid to hyperthermic and semihumid–semiarid conditions during deposition of the Wenchang Formation triggered the amplification of lacustrine biodiversity and expansion of the oxygen minimum zone. Salinization-driven ecosystem restructuring, coupled with elevated primary productivity and exceptional preservation, resulted in organic-rich mudstones with high hydrocarbon generation potential. This study fully reveals the effects of paleoclimate fluctuations during warm periods on the evolution of the ecological environment of paleolakes, thus providing key geological information for deciphering the OM enrichment mechanism in early–middle Eocene paleolakes.