Carbon degradation and CO2 production within onshore and nearshore zones of eroding permafrost coasts

Permafrost coasts are a key interface between the terrestrial and marine system in the Arctic. Alongside with major Arctic rivers, erosion transfers large amounts of organic matter and sediment into the ocean. Warmer temperatures, permafrost thaw and sea ice area decrease promote coastal erosion due to a longer open-water season, higher sea surface temperatures, sea-level rise, and wave action. Upon thaw and erosion a massive pool of permafrost organic carbon (OC) is being exposed and OC released into the Arctic Ocean, where it can be metabolized, sequestrated or transported offshore. Although the understanding of biogeochemical processes upon gradual permafrost thaw on land and permafrost OC transport on the shelf are improving, little is known about the immediate biogeochemical response during the abrupt coastal erosion process and associated greenhouse gas production.

In this study, we mimicked different modes of coastal erosion onshore and in nearshore waters with incubation experiments by mixing in-situ permafrost and eroded permafrost debris (i.e. mud lobes, cliff toe debris) with ambient seawater. Carbon dioxide (CO₂) production was measured over a 2-months period at 4°C and OC, nutrients (N), carbon isotopes (¹³C, ¹⁴C) as well as biomarkers (n-alkanoic acids, n-alkanes) were analysed prior and after incubations to comprehend potential OC degradation and CO₂ dynamics within the coastal rim; both onshore and within nearshore waters. Our results show that large amounts of CO₂ are produced in all simulated erosion modes and that CO₂ production increases with the presence of seawater, especially for in-situ permafrost and cliff toe debris incubated. Although C/N-ratios and stable carbon isotope signatures do not show significant differences prior and after incubation, carbon preference index and high-to-low molecular weight ratios of n-alkanoic acids and n-alkanes indicate ongoing degradation of OC which correlates to the observed CO₂ production.

Our results show that permafrost OC mobilized by coastal erosion is rapidly degrading upon thaw and that this process is accelerated by the presence of seawater. We conclude that erosion of permafrost coasts, onshore and nearshore, is potentially a major source of CO₂ to the atmosphere and an "incubator" of terrestrial organic matter before release into the offshore marine system. Although the fate of OC upon release into seawater is hard to assess by laboratory experiments, we emphasize the importance of coastal erosion for carbon budgets and models, especially under the Earth's current climate trajectory and the accelerated environmental forcing on Arctic permafrost coasts.