Spatiotemporal Variability and Sources of DIC in Permafrost Catchments of the Yangtze River Source Region: Insights From Stable Carbon Isotope and Water Chemistry

Abstract Riverine dissolved inorganic carbon (DIC) exports play a central role in the regional and global carbon cycles. Here, we investigated the spatiotemporal variability and sources of DIC in eight catchments in the Yangtze River source region (YRSR) with variable permafrost coverage and seasonally thawed active layers. The YRSR catchments are DIC‐rich (averagely 25 mg C L −1 ) and export 3.51 g m −2 yr −1 of DIC. The seasonal changes of temperature, active layer, flow path, and discharge can alter DIC and stable carbon isotope of DIC ( δ 13 C‐DIC). The most depleted δ 13 C‐DIC values were found in the thawed period, suggesting the soil‐respired CO 2 during the active layer thaw period can promote bicarbonate production via H 2 CO 3 weathering. Spatially, δ 13 C‐DIC values increased downstream, likely due to CO 2 outgassing and changed permafrost coverage and runoff. We found that evaporite dissolution and silicate weathering in the seasonally thawed active layer contributed 44.2% and 30.9% of stream HCO 3 ‐ , respectively, while groundwater and rainwater contributed 16.7% and 7.3% of HCO 3 ‐ , respectively. Pure carbonate rock weathering played a negligible role in DIC production. These results were compatible with δ 13 C‐DIC source approximation results. Silicate weathering increased from initial thaw to thawed period, reflecting the active layer thaw and subsequent hydrology change impacts. Silicate weathering consumed 1.25 × 10 10 mol of CO 2 annually, while evaporite dissolution may produce CO 2 and neutralize this CO 2 sink. This study provides new understanding of the riverine DIC export processes of the YRSR. As permafrost degrades, the quantity, sources, and sinks of riverine DIC may also change spatiotemporally.