Carbon and Oxygen Isotopic Compositions: How Lacustrine Environmental Factors Respond in Northwestern and Northeastern China

Abstract

Abstract:Surface lake sediments, 28 from Hoh Xil, 24 from northeastern China, 99 from Lake Bosten, 31 from Ulungur and 26 from Heihai were collected to determine δ13C and δ18O values. Considering the impact factors, conductivity, alkalinity, pH, TOC, C/N and carbonate‐content in the sediments, Cl, P, S, and metal element ratios of Mg/Ca, Sr/Ca, Fe/Mn of bulk sediments as environmental variables enable evaluation of their influences on δ13C and δ18O using principal component analysis (PCA) method. The closure and residence time of lakes can influence the correlation between δ13C and δ18O. Lake water will change from fresh to brackish with increasing reduction and eutrophication effects. Mg/Ca in the bulk sediment indicates the characteristic of residence time, Sr/Ca and Fe/Mn infer the salinity of lakes. Carbonate formation processes and types can influence the δ13C‐δ18O correlation. δ18O will be heavier from Mg‐calcite and aragonite formed in a high‐salinity water body than calcite formed in freshwater conditions. When carbonate content is less than 30%, there is no relationship with either δ13C or δ18O, and also none between δ13C and δ18O. More than 30%, carbonate content, however, co‐varies highly to δ13C and δ18O, and there is also a high correlation between δ13C and δ18O. Vegetation conditions and primary productivity of lakes can influence the characteristics of δ13C and δ18O, and their co‐variance. Total organic matter content (TOC) in the sediments is higher with more terrestrial and submerged plants infilling. In northeastern and northwestern China, when organic matter in the lake sediments comes from endogenous floating organisms and algae, the δ13C value is high. δ13C is in the range of −4%o to 0‰ when organic matter comes mainly from floating organisms (C/N<6); in the range of −4‰ to 8‰ when organic matter comes from diatoms (C/N=6 to 8); and −8‰ to −4‰ when organic matter comes from aquatic and terrestrial plants (C/N>8).