Mobilization of Aquatic Carbon from Permafrost: Tracking the Ancient Carbon Signal.

Abstract:

Hydrological and carbon biogeochemical responses to permafrost thaw are well recognized and documented for arctic and subarctic river systems and include increased infiltration, groundwater flow, stream base flow, and yield of mineralized dissolved inorganic carbon (DIC). However, the anticipated concomitant signal of increased export of aged dissolved organic carbon (¹⁴C depleted DOC) is conspicuously small or absent at the river basin scale. Delivery of permafrost-derived DOC to stream and river networks is controlled by combinations of the amount of carbon stored in permafrost soils; hydrologic connectivity; subsurface physical conditions, including the potential for rapid thaw; and the chemical composition and degradability of the DOC released from permafrost. We examine these conditions in the discontinuous permafrost region of Yukon Flats, Alaska. Results indicate variable coupling among the factors controlling carbon release from permafrost, with greatest aged DOC source and biodegradability in regions that have low potential for immediate thaw or development of well-connected hydrologic networks. Conversely, regions having high potential for impending thaw and enhanced hydrologic connectivity tend to have smaller aged DOC sources and/or are dominated by modern DOC sources. This emphasizes the importance of the water-surface to atmosphere emission of permafrost carbon, with a majority of thawed permafrost organic carbon mineralizing to carbon gases in isolated thaw depressions, wetlands, and upland lakes and limited downstream delivery to river networks.