Hydrological evolution and biogeochemical feedbacks controlling organic carbon accumulation in an Eocene saline lake

Organic carbon (OC) accumulation in lacustrine basins is commonly attributed to sustained anoxia and high primary productivity, yet the role of dynamic hydrological reorganization in carbon preservation remains poorly constrained. Here, we integrate multi-proxy sedimentological and geochemical data from the Eocene Kongdian Formation mudstones of the Cangdong Sag, Bohai Bay Basin to reconstruct salinity stratification, metalimnetic oxygen minimum (MOM) dynamics, and OC accumulation in a saline lake system. The results reveal a halocline-controlled redox architecture in which a salinity gradient favored the maintenance of a MOM that locally extended into the photic zone to form a metalimnetic euxinic zone (MEZ), whereas the hypolimnion remained predominantly suboxic during the stratified stages. Climate-driven hydrological evolution drove the lake through three successive states: (1) relatively stable stratification restricting vertical nutrient exchange and favoring organic matter preservation, resulting in moderate OC accumulation; (2) mixing-stratification oscillations reactivating internal nutrients, stimulating algal productivity, expanding the MOM, and enhancing the incorporation and preservation of methane-derived carbon, collectively yielding highest total organic carbon (TOC) content (up to 10.5%); and (3) progressive lake shallowing weakening stratification, destabilizing the MOM–MEZ system, increasing bottom-water ventilation, and reducing organic carbon preservation. These results demonstrate that OC accumulation in greenhouse saline lakes responds nonlinearly to hydrological evolution. Maximum TOC enrichment occurred during an intermediate hydrological state, when episodic mixing replenished nutrient supply without fully disrupting the redox stratification favorable for OM preservation. These findings suggest that OC accumulation in greenhouse saline lakes is regulated by hydro-biogeochemical feedbacks linking nutrient renewal, redox stratification, the retention of methane-derived carbon, and OM preservation.