The oxygen and carbon isotope records of East Asian climate variations during the Eocene warm periods from Weihe Basin, central China

Eocene warmth has been used as one of the best analogues for future anthropogenic warming. How East Asian hydroclimate responds to the increased temperature during the Eocene is still elusive. Here, we present a combined element and isotopic geochemistry study of an Eocene lacustrine sequence covering the period 46-33 Ma from Weihe Basin, central China. Based on the formation process of lake carbonate and the paleosol CO₂ barometer equation, a calculation model of lake carbonate carbon isotope (δ¹³Ccarb) that is suitable for open lake basins with low productivity is proposed. The sensitivity analysis of the Eocene lacustrine carbonate δ¹³Ccarb  show that the SRF is the main influencing factor of the carbon isotope fractionation. The reconstructed SRF of the Eocene is generally high, with an average value of ~215 g C/m²/yr, revealing a relatively warm and humid environment in the Weihe Basin. After ~41 Ma, the SRF gradually decreased, indicating that the climate in the Weihe Basin gradually became colder and drier. This trend is consistent with the global cooling, especially at ~36 Ma, ~33 Ma (the Eocene-Oligocene transition EOT) showing the most significant reduction. The reconstructed precipitation oxygen isotope (δ¹⁸Op) in the Eocene warmth is characterized by a positive value (~-6 ‰) in the northwest inland region, and relatively negative  values in the central region such as the Weihe Basin and Lanzhou Basin (~-10 ‰), and the Qinghai-Tibetan Plateau area (~-11 ‰). This kind of distribution is similar to modern precipitation δ¹⁸Op, indicating that a prototype of the East Asian summer monsoon circulation has probably formed in the Middle Eocene.