Middle Eocene Climatic Optimum sensitivity of a continental lake basin from geochemical records of the Fushun Basin, Northeastern China

Abstract

The dating of continuous Eocene sediments in the Fushun Basin, NE China, provides a good opportunity to study the evolution of the land paleoclimate during extreme high-temperature events in mid-latitude regions. Here, the lower part of the Eocene Xiloutian Formation in the Fushun Basin was sampled, and isotope geochemistry, element geochemistry, organic geochemical analysis, magnetic susceptibility, and sediment color analysis were conducted to determine the evolution of paleoclimate in the continental lake basin during the Middle Eocene Climatic Optimum (MECO) period. The oxygen (δ18O) and carbon (δ13C) isotope data indicated the presence of phases during the MECO, with 401.5–374.3 m (41.2–40.42 Ma) representing the main stage and 374.3–371.2 m (40.42–40.32 Ma) the recovery stage. In addition, evaluation of the opening and closing of lakes using δ18O and δ13C showed that the lakes in the lower part of the Xiloutian Formation in the Fushun Basin are currently in a transition period between open and closed. The reconstructed paleotemperature and precipitation in the lower part of the Xiloutian Formation were characterized by an initial high trend followed by a low trend, with the highest values of both occurring during the main MECO stage. The lower part of the Xiloutian Formation is characterized by an oxidative oxygen environment of brackish water, and MECO events did not affect the salinity and redox state of the water. However, the total organic carbon (average 5.24%) in the dark mudstone during the main MECO stage was significantly higher than that during the cooling period (average 4.67%), indicating that the MECO promoted carbon fixation and enrichment of organic matter. Climate comparison showed that the climatic types in eastern and western regions of China during the Eocene were similar. Furthermore, climatic evolution may be influenced by the uplift of the Tibetan Plateau, and the retreat of the inland proto Paratethys Sea.