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 delta C-13(DIC) values, which vary with water salinity in the Lake Qinghai area. 2) The variation in the delta C-13(DIC) 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 &#39;vital offsets&#39; of ostracods. Our results suggest that variations in the delta C-13 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 delta C-13 values may be used to indirectly indicate changes in water salinity in the Lake Qinghai area.</p

Stable oxygen isotope of ostracods in recent sediments of Lake Gahai in the Qaidam Basin, northwest China: The implications for paleoclimatic reconstruction

The oxygen isotopic composition of ostracod shells in lakes has been used as a useful indicator in palaeolimnological research and has provided some important contributions to the understanding of lacustrine systems. Usually, the oxygen isotopic compositions of ostracods from the lake sediments are interpreted as changes in effective precipitation, temperature and evaporation/input water ratio in a sub-arid or arid area. Here, we compare a 150-year-long oxygen-isotope record that was derived from ostracod carbonate from the sediments of Lake Gahai in the Qaidam Basin with meteorological data (precipitation) and tree-ring evidence for changing precipitation. Our results show that the oxygen isotopic compositions of ostracod shells are related to precipitation over the past 150 years. In general, increased precipitation accompanied a shift to less positive delta O-18 values in the lake water, and thus in the ostracod shells, whereas decreased precipitation coincided with the opposite in Lake Gahai over the past 150 yr. Therefore, we conclude that the oxygen isotopic compositions of ostracod shells can be used to indicate changes in precipitation over a short time scale in lake Gahai.</p

Salinity control on long-chain alkenone distributions in lake surface waters and sediments of the northern Qinghai-Tibetan Plateau, China

Long-chain alkenones in lacustrine settings are potentially excellent biomarkers for the reconstruction of past terrestrial environmental conditions, and have been found in many different types of lakes around the globe. A wider range of factors influence the occurrence and distribution of alkenones in lake sediments and waters when compared to marine systems. Lake environmental conditions, such as temperature (in particular) and salinity, are among the key factors controlling alkenone distributions in lacustrine settings. Here we investigated alkenone distribution patterns in lakes of the northern Qinghai-Tibetan Plateau, China, and their possible relationship with environmental conditions, by analyzing paired samples of suspended particulate matter in surface waters and surface sediments. Salinity of investigated lake waters ranges from almost 0 to similar to 100 g/L, while temperature variation among the lakes is minimal, effectively eliminating temperature effects on the alkenone distribution patterns observed here. We show that (1) alkenone concentrations vary substantially between the lakes, yet controlling mechanisms remain elusive; (2) C(37)/C(38) ratios are substantially lower in the lakes of the Qaidam Basin than in the Lake Qinghai region, probably indicating different alkenone producers in the two regions; and (3) large variations in %C(37:4) (the percentage of the C(37:4) alkenone), determined from both surface waters and sediments, are negatively correlated with salinity. We suggest that the %C(37:4) index could be used as a salinity indicator at least on a regional scale, with careful considerations of other potentially complicating factors. However, potential reasons for why salinity could significantly affect %C(37:4) values need further investigation.</p

Total organic carbon isotopes: A novel proxy of lake level from Lake Qinghai in the Qinghai-Tibet Plateau, China

The isotopic compositions of total organic carbon (TOC) in lakes have been widely used to interpret paleoclimatic changes and the depositional environments of lake sediments. However, the main factors that affect the carbon isotopes of TOG (source of organic material, water condition and others) may vary in different lake sediment records, which have limited the applicability of organic carbon isotopes in explaining biogeochemical and environmental changes in lakes. In this study, the organic carbon isotopic compositions of aquatic plants and surface sediments from Lake Qinghai and the living terrestrial plants and surface soils around the lake were systematically investigated to identify the sources of TOC in the sediments and the significance of the organic carbon isotopes of sedimentary TOG. We found that the aquatic plants in the deep water areas (&gt;10 m) were primarily dominated by Cladophora, but submerged plants (Potamogeton and Ruppia L) are the dominant species in shallow water (&lt;10 m). The Cladophora have negative delta C-13(org) values (-33.6 parts per thousand to -28.6 parts per thousand) that are caused by C(3-)ike photosynthesis, but the submerged plants have enriched delta C-13(org) values (-17.8 parts per thousand to -15.4 parts per thousand) that are caused by C-4-like photosynthesis. In addition, the delta C-13(org) values of Cladophora become more negative with increasing water depth because of the slow photosynthetic rate caused by the weak light intensity at depth. The isotopic data indicate that the carbon isotopes of organic material in the surface sediments are primarily controlled by the types of aquatic plant and that the delta C-13(org) values of TOC can be used to indicate the variation of the water depth (lake level). The organic carbon isotopic data from the IF core showed that the water was shallow (&lt;10 m) because of intense evaporation related to high temperatures even though precipitation sharply increased in the warm period during the early-mid Holocene. The lake level reached its maximum level at 3 ka.</p