Quantitative geochemical reconstruction of Eocene paleoenvironment in Fushun Basin, northeast China

The quantitative estimation of key parameters of paleotemperature and paleoprecipitation is crucial for paleoclimate reconstruction. Geochemical data from modern sediments are highly consistent with climate data, and their relationship can provide an important reference for the quantitative reconstruction of the paleoclimate. In this study, detailed inorganic geochemical analysis was carried out using high-precision sampling of the Paleogene (LFD-1 well) Guchengzi, Jijuntun and Xiloutian Formations in the Fushun Basin located in the mid-latitudes of the Northern Hemisphere. The Eocene Guchengzi Formation (54.51–47.8 Ma) and Jijuntun Formation (47.8–41.2 Ma) in the Fushun Basin were found to have been deposited under a humid climate. The lower (41.2–40.1 Ma) and upper (40.1–37.8 Ma) parts of the Xiloutian Formation were characterized by semiarid and semihumid–semiarid climates, respectively, which is very similar to the paleoclimatic information reflected by organic carbon isotopes. The Eocene Thermal Maximum 2 (ETM2, ~ 53.7 Ma), Early Eocene Climatic Optimum (EECO, ~ 53.1–46.5 Ma, Eocene Thermal Maximum 3 (ETM 3, ~ 52.8 Ma), and Middle Eocene Climatic Optimum (MECO, ~ 40.7–40.1 Ma) events significantly enhanced chemical weathering during these periods. The rapid increase in pCO₂ concentration leads to an increase in temperature, precipitation, and surface runoff, exhibiting strong chemical weathering. The mean annual temperature (MATa) and mean annual precipitation (MAPa, MAPb, and MAPc) were estimated using parameters, such as the corrosion index without potassium (CIA-K), CaO/Al₂O₃, and (Na₂O + K₂O)/Al₂O₃. Comparing MAPa, MAPb, and MAPc with the MAP estimated using pollen data, MAPa and MAPb were found to be more sensitive to the climate during high precipitation periods (precipitation > 1000 mm, Guchengzi Formation), and the recovered average precipitation was similar to MAP. In contrast, MAPc was more sensitive to the climate during low precipitation periods (precipitation < 1000 mm, Jijuntun, and Xiloutian Formations), with higher accuracy. To fully consider the influence of soluble inorganic salts Ca²⁺ and Na⁺, multivariate linear equations of CIA-K and CaO/Al₂O₃ with CIA, and CIA-K and CaO/Al₂O₃ with MAP were constructed, namely MAPd and MAPe. The results show that MAPe has the highest performance and can be effectively used to estimate the change of paleoprecipitation in Northeast Asia.