Middle Eocene Climatic Optimum drove palaeoenvironmental fluctuations and organic matter enrichment in lacustrine facies of the Bohai Bay Basin, China

Abstract

The Middle Eocene Climatic Optimum (MECO) is a transient warming episode during an interval of long-term Cenozoic cooling that is recorded in deep-sea marine sediments and marine basins; however, the MECO is rarely reported in terrestrial contexts, complicating its comparison with marine settings. In this study, we describe well-dated core samples from the Shahejie Formation, Dongpu Depression, Bohai Bay Basin, China, using thin-section observations, isotope geochemistry, and elemental geochemistry to determine lacustrine MECO records and associated palaeoenvironmental changes. Based on previously published age models, we identified that the MECO event (40.7–40.07 Ma) in the Dongpu Depression occurs at a core depth of 2945–3046 m and we report two δ¹³C_carb excursions and one δ¹⁸O_carb excursion. During the MECO event, the palaeoclimate was warm and humid, characterized by reduced salinity and increased water depth, consistent with contemporaneous marine and lacustrine palaeoenvironmental changes, indicating the global importance of MECO-induced environmental fluctuations. In conjunction with palaeoenvironmental changes, we established models for organic matter (OM) enrichment at our site. OM enrichment was influenced by palaeosalinity and water depth. The warm, humid climate, increased water depth, reduced salinity, and elevated atmospheric CO₂ concentrations provided favorable conditions for algal growth. Additionally, the anoxic conditions at the bottom of the lake facilitated the preservation of OM. This study provides new insights into palaeoenvironmental changes driven by climatic events and offers perspectives for predicting organic carbon sequestration in modern lakes.