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

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