Stable isotopes in bulk carbonates and organic matter in recent sediments of Lake Qinghai and their climatic implications

Multi-proxy indices on annual/decadal scales during the past 600 years were developed from the surface sediments of Lake Qinghai based on 210Pb and 137Cs geochronology. The δ13Ccarb, δ18Ocarb, and total carbonate content are consistent with one another in trends and their variations have been mainly ascribed to regional temperature. It is suggested that the strong evaporation contributes in modifying δ18O of lake water and δ13C of the dissolved inorganic carbon of the surface water, and appears to be responsible for the covariance between δ13Ccarb and δ18Ocarb. Photosynthesis of the aquatic plants appears to have played a significant role in determining δ15N of organic matter. The discrimination of 14N and 15N during photosynthesis may have triggered a positive linkage between temperatures and δ15Norg (after removal of the stratigraphic trend).Variations of total organic carbon (TOC), C/N ratio, and δ13Corg have been ascribed to local precipitation. TOC (after removal of the stratigraphic trend) is positively correlated with the C/N ratio (r = 0.45, α < 0.01), and negatively correlated with δ13C of organic matter (δ13Corg) (r = − 0.55, α < 0.01); while δ13Corg is negatively correlated with the C/N ratio (r = − 0.48, α < 0.01). The climatic significance of these multi-proxy indices has been verified by comparing with the meteorological records and the climates inferred from tree rings in adjacent regions.