Discontinuous Permafrost Impacts the Contributions of Terrestrial Dissolved Inorganic Carbon (DIC) Sources to Headwater Streams at a Watershed in Interior Alaska

Abstract

Headwater streams are tightly linked to the terrestrial environments and receive large amounts of terrestrially-derived carbon. However, in permafrost environments, terrestrial-aquatic linkages are likely to shift as ongoing permafrost thaw progresses and extent decreases. As solutes exist in varying quantities throughout soil profiles, changing dissolved solutes in streams can be used to identify shifts in terrestrial flow paths. This research aimed to identify terrestrial sources that contribute carbon to the headwaters using a Bayesian end-member mixing model and 𝛿¹³C-DIC (stable carbon isotope of dissolved inorganic carbon) to compare source contributions with permafrost distribution. In a boreal catchment underlain by discontinuous permafrost, we found that deep and shallow terrestrial sources have distinct solute concentrations and isotopic signatures compared to the stream surface water. Further, there were differences in source contribution among the sub-catchments with varying permafrost extents. The stream with the highest catchment-wide permafrost had the largest amount of shallow soil water contribution to streamflow and experienced the flashiest flow regime. As expected, other headwater streams in catchments with little to no permafrost received higher amounts of deep groundwater to support baseflow than in the high permafrost catchment. Further, we identified multiple deep groundwater sources, which appear to be spatially distinct and contribute to baseflow in different sub-catchments. We found evidence that permafrost influences terrestrial flow paths and the proportional contribution of terrestrial sources, thus dictating stream chemistry. As permafrost thaw progresses, there will likely be shifts in solute delivery to high-latitude headwater streams.