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 δ¹³C_carb, δ¹⁸O_carb, 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 δ¹³C_carb and δ¹⁸O_carb. 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 δ¹⁵N_org (after removal of the stratigraphic trend). Variations of total organic carbon (TOC), C/N ratio, and δ¹³C_org 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 (δ¹³C_org) (r = - 0.55, α < 0.01); while δ¹³C_org 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.