Middle Eocene terrestrial paleoweathering and climate evolution in the midlatitude Bohai Bay Basin of eastern China

Abstract

The middle Eocene climatic optimum (MECO, ca. ∼42 Ma) is a key time period for understanding Cenozoic cooling of the global climate. Still, midlatitude terrestrial records of climate evolution during MECO epoch are rare. In this study, continuous high-resolution record of shale sediments in mid-Eocene Shahejie Formation (MES shales) in the Bohai Bay Basin were performed with major-element and wavelet analysis. The midlatitude paleoweathering and paleoclimatic evolution during MECO epoch were analyzed in this study. The MES shales experienced weak-moderate paleoweathering under a subtropical monsoon paleoclimate with mean annual temperature of 8.3–12.9 °C and mean annual precipitation of 685–1100 mm/yr. The MES shales record a mixed provenance involving intermediate igneous rocks, and low compositional maturity. The nutrient-rich environment led to enrichment in organic matter in the MES shales. Wavelet analysis revealed good periodicity about the paleoclimate and weathering during MECO epoch. In the stage I of MES shales depositional process, the paleolake was high in nutrients, and the MES shales experienced high chemical weathering due to a relatively warmer and more humid climate. In contrast, the climate in stage II was relatively cold and dry, and the maturity of the MES shales was relatively high during this stage, suggesting a relatively stable tectonic background. This work provides more terrestrial records of MECO epoch for midlatitude region, and is benefit for better understanding of the palaeoenvironment when MES shales formed. The implication of organic matters enrichment in this study is meaningful for the shale oil/gas exploration in Nanpu Sag.