Sources and cycling of dissolved organic and inorganic carbon on the northern Qinghai-Tibetan Plateau: Radiocarbon results from Qinghai Lake.

Tibetan Plateau lakes are sensitive to climate variabilities and affect regional temperature, precipitation and ecosystems. In this study, we investigated the concentrations and carbon isotope (C and C) compositions of dissolved organic and inorganic carbon (DOC and DIC) along with the concentrations of major lithologic ions (Na, Mg, K, Ca) and dissolved silicate (DSi) in a large lake, namely, Qinghai Lake, and its seven inflowing rivers on the northeastern Qinghai-Tibetan Plateau of China. Our results revealed large differences in concentrations and isotopic compositions between DOC and DIC, as well as differences in the concentrations of major ions between the rivers and lake. The chemical weathering of carbonate and silicates in the catchments played important roles in controlling the fates of major ions in the rivers. In Qinghai Lake, evaporation resulted in 40-290 times the accumulation of Na, Mg, and K observed in the rivers, while biological uptake and precipitation reduced the concentrations of Ca and DSi in the lake by 4-17 times compared with those in the rivers. In addition, the concentrations of DOC and DIC in the lake were 6-7 times those in the rivers. The carbon isotope (δC and ΔC) signatures of DOC and DIC revealed that the rivers transport millennium-aged DOC (1254 ± 316 years) and DIC (1513 ± 857 years), as influenced largely by the chemical weathering of rocks and preaged soil carbon on the plateau. In contrast, in Qinghai Lake, the ages of DOC are significantly younger (684 ± 378 years) than the ages of riverine DOC, as regulated by newly biologically produced modern DOC. Moreover, the DIC ages in Qinghai Lake are all modern, indicating that exchange with atmospheric CO is the dominant process controlling the lake DIC. The accumulation and removal of riverine-aged DIC in Qinghai Lake, however, are not well understood, and the unbalanced radiocarbon in lake DIC remains a mystery that needs further study.