Late Eocene–early Oligocene transition as a trigger for organic matter accumulation

The Eocene–Oligocene transition (EOT, ∼34 Ma) is a major cold event, but its paleoenvironmental impacts on organic matter (OM) accumulation remain debated. This study applies mineralogical, inorganic, and organic geochemical analyses to organic-rich rocks of the Eocene Shahejie Formation (Bohai Bay Basin) to address this gap. Results reveal EOT-induced paleolimnological reorganization (lake shallowing, enhanced carbonate sedimentation, and laminated lithofacies) and concurrent salinity/floral shifts that boosted productivity and OM preservation. The key innovation is identifying coupled paleosalinity–humidity dynamics as the governing mechanism: elevated salinity promoted halotolerant algal blooms and organo-clay protection, while cold-humid climates prolonged stratified anoxia and enhanced P delivery primarily via physical erosion and aeolian deposition, jointly optimizing TOC burial. This synergy explains the unique positive carbon isotope excursion (exceeding marine–terrestrial analogs) driven by evaporative dolomite deposition. The study provides a predictive reference for lacustrine source rock assessment in EOT-aged extensional basins with similar geological backgrounds.