Aged dissolved organic carbon exported from rivers of the Third Pole

Global river export of dissolved organic carbon (DOC) to the oceans (250 Tg-C yr-1; Tg=1012g) is a key component of the global carbon cycle and is subject to climate forcing. Polar ecosystems are experiencing dramatic warming in the recent decades, potentially mobilizing large amounts of ancient permafrost carbon into contemporary rivers. However, no information is available for rivers of the climate sensitive Tibetan Plateau, which is the highest plateau on earth, water tower of Asia, and another region with extensive permafrost cover. We present the radiocarbon age and export of dissolved organic carbon from the Tibetan headwaters of three large Asian rivers (Yellow, Yangtze and Brahmaputra) during high flow. Dissolved organic carbon concentrations were lower than for Arctic rivers. Consequently, the carbon yield (0.41 gC m-2 yr-1) was lower than for Arctic rivers (1.6 gC m-2 yr-1), despite Tibetan rivers having higher annual discharge (280 kg m-2 yr-1 vs 215 kg m-2 yr-1). Average radiocarbon ages (511±294 years before present) were older in the Tibetan rivers than for Arctic and tropical rivers. Explanations were sought for Tibetan river carbon's old age. A positive correlation between radiocarbon age and permafrost watershed coverage was observed, indicating that old carbon (656±177 years before present) is exported during high flow from permafrost regions of the Tibetan Plateau. Additionally, Inputs of fossil carbon (2,265±1009 years before present) from polluted South and East Asia may also contribute to the old age of Tibetan river carbon.