Carbon isotopes in surface-sediment carbonates of modern Lake Qinghai (Qinghai–Tibet Plateau): Implications for lake evolution in arid areas

To further investigate the significance of carbon isotopes of lake carbonates in arid areas, we examined the carbon isotopic composition of ostracods, bulk carbonate, fine-grained carbonates, and associated water DIC (dissolved inorganic carbon) from Lake Qinghai and several small lakes and ponds surrounding Lake Qinghai. We obtained three major results. 1) The carbon isotopic compositions of ostracods, bulk carbonate, and fine-grained carbonates in the lakes and ponds are clearly correlated with water δ13CDIC values, which vary with water salinity in the Lake Qinghai area. 2) The variation in the δ13CDIC values of lake water is mainly controlled by CO2 exchanges between the atmosphere and lake waters in the Lake Qinghai area. 3) Ostracods, bulk carbonate and fine-grained carbonates show consistent trends of isotopic composition in the study area, and the differences in carbon isotopic composition between authigenic carbonates and ostracods may be explained by the differences in carbon isotopic composition between the DIC of surface water and that of the water near to the sediment–water interface as well as the ‘vital offsets’ of ostracods.Our results suggest that variations in the δ13C values of carbonates in Lake Qinghai and other lakes in this arid, high-altitude area are primarily controlled by the carbon-isotope ratios of the lake water DIC, which in turn are related to water salinity. Therefore, changes in carbonate δ13C values may be used to indirectly indicate changes in water salinity in the Lake Qinghai area.

► The carbon isotope of carbonates and DIC was systematically studied in Lake Qinghai.
► The carbon isotope of DIC was mainly controlled by CO2 exchanges in Lake Qinghai.
► The carbon isotope of carbonates was controlled by DIC isotope in Lake Qinghai.
► The carbonates δ13C may indirectly indicate salinity change in Lake Qinghai area.