| Summary: | Climatic warming causes permafrost carbon pool destabilization, with part of the thawed carbon being released to the fluvial systems. As transport pipes and active reactors of terrigenous carbon, rivers and streams emit a significant amount of gaseous carbon and transport a large flux of dissolved or particulate carbon in waterways. Yet the export patterns, magnitudes, and impacts of riverine carbon in permafrost regions are not well understood. Here we investigated the concentrations, fluxes, and carbon isotopes ( 14 C and 13 C) of various carbon species, including dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), and carbon dioxide (CO 2 ), from permafrost rivers on the Qinghai-Tibet Plateau (QTP). Each of the carbon species showed heterogeneous spatial and seasonal patterns. The active layer dynamics and hydrological regime change had strong controls on riverine carbon. The integration of terrestrial carbon fluxes with the aquatic carbon flux show that aquatic carbon export can offset 14-30% of the land carbon sink. Radiocarbon dating indicated that DOC and DIC have depleted 14 C signatures corresponding to millennial ages, both of which were positively correlated to permafrost temperature and affected by active layer dynamics. Dual-carbon isotope mixing model was used to partition the riverine carbon from different age categories. Approximately 58% of annual pre-aged DOC and DIC were exported in summer. The monsoon climate-induced high discharge and maximum active layer thaw depth in summer enhanced the large fluvial export of millennial-aged carbon. As climate warms, more permafrost carbon export is expected and may affect the river ecosystem function and health.
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