Paleoclimate changes of the last 1000 yr on the eastern Qinghai–Tibetan Plateau recorded by elemental, isotopic, and molecular organic matter proxies in sediment from glacial Lake Ximencuo

Abstract

Total organic carbon (Corg) and nitrogen (Ntot) concentrations and isotope compositions and n-alkane and n-alkan-ol molecular biomarker compositions were measured in two parallel sediment cores from Lake Ximencuo, a typical glacial lake on the eastern Qinghai–Tibetan Plateau (QTP), to reconstruct local climatic variations during the past 1000yr. Concentrations of Corg and Ntot vary similarly with changes in precipitation recorded by tree rings in Dulan, northeastern QTP, indicating their close relation to precipitation. Carbon Preference Index (CPI) values of C22–C33n-alkanes and C22–C28n-alkan-1-ols and Average Chain Length (ACL) values of C27–C33n-alkanes generally negatively correlate with δ13Corg and δ15Ntot and also vary comparably with the oxygen isotope temperature record from the Dunde ice core, northeastern QTP. These patterns indicate that high CPI values and negative shifts of the δ13Corg and δ15Ntot correspond to warmer conditions that favor the growth of vascular land plants and that lower values and positive shifts correspond to cooler conditions in the Lake Ximencuo locality. This observation contradicts previous interpretations from peat and modern soil studies that conclude that the CPI indices were controlled by microbial degradation under different climate conditions. Here we propose that the changes of organic matter sources under different climate regimes, combined with physiological adjustments of vascular plants to different air temperatures, might yield different CPI responses to climate changes in glacial plateau lakes. Air temperature and precipitation increases and decreases reconstructed in this 1000-yr study were generally independent of each other, but they both occurred at centennial or multi-centennial scales. The interaction between subtropical (Asian Monsoon) and mid-latitude (Westerly) atmospheric circulation systems probably dominated the local precipitation variations in the Lake Ximencuo catchment, whereas solar insolation, volcanism and greenhouse gas variations were likely responsible for the air temperature changes that are recorded in the lake sediments and that are consistent with regional temperature variations in the eastern QTP.