Stable isotope record of middle Eocene summer monsoon and its instability in eastern China

Abstract

Abstract Recent studies based on proxy records and paleoclimate simulations have suggested monsoon climate in Asia as early as the Eocene. However, good proxy record of Eocene monsoon is rare in eastern Asia where East Asia summer monsoon prevails. Here we study middle Eocene lacustrine deposits of the Shahejie Formation in the Bohai Bay Basin in eastern China to understand middle Eocene paleoclimate and its stability. Our study integrates new δ13C and δ18O data of authigenic lacustrine carbonates, lake water salinity proxy of Mg/Ca ratio, and published carbonate minerology, chemical weathering proxy data of CIA* and Ln(Al2O3/Na2O), and lithofacies interpretations from two cores to provide new understanding of paleoclimate elements. Our results show that in most part of the record, the authigenic calcite δ18O values in laminated mudstone-dominated lithofacies display frequent variations, up to 2.9‰, while Mg/Ca ratio and dolomite content remain low and nearly stable. These features suggest frequent variations of lake water level driven by changes in runoff amount because of changing intensity of summer monsoonal precipitation during the middle Eocene. The comparison of stable isotope variation with modern precipitation stable isotope record in the study area and seasonal precipitation of the laminated lithofacies both support the interpretation of seasonal climate. We interpret that the lake water and carbonate δ18O values decreased when the East Asia summer monsoon was strong and increased when the monsoon was weak. During two periods of the studied middle Eocene interval, the lake water experienced periodic intense evaporation, causing high Mg/Ca ratios and dolomite content, and high authigenic dolomite δ18O values up to 0‰. These periodic drying events likely reflect millennial-scale droughts that are common today to the East Asia summer monsoon because of warming in the Indo-Pacific Warm Pool. We infer that the frequent droughts in the two periods of the middle Eocene were resulted from very warm climate during the two periods. We conclude that the East Asia summer monsoon existed in eastern China during the middle Eocene, and the climate state was not stable.