Permafrost carbon degradation and transport pathways at thermokarst coasts in the Arctic

Climate change has a strong impact on permafrost coasts in the Arctic. With increasing air and water temperatures, ice-rich permafrost coasts will thaw, which will lead to enhanced thermokarst and erosion. Upon erosion, large amounts of organic carbon previously stored for thousands of years are remobilized and either emitted as greenhouse gases to the atmosphere, redeposited within the landward nearshore zone, or released into the ocean. Yet, little is known about carbon degradation before the organic matter enters the nearshore zone of the ocean. The objective of this study was to investigate these processes at ice-rich thermokarst coasts, by focusing on retrogressive thaw slumps. The study aimed at determining the quantity of organic carbon and nitrogen in undisturbed and non-disturbed (thermokarst affected) coastal stretches, to detect its degradation and accumulation pattern after thawing, as well as its fate in the nearshore of the ocean.
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\nA retrogressive thaw slump located on Herschel Island (Yukon Territory, Canada) was sampled systematically along transects from the undisturbed parts (tundra, permafrost headwall) to disturbed parts (mudpool and slump floor) and the nearshore zone (marine sediments). These thermokarst landforms are ideal study sites as they spatially expose different transport and accumulation stages of thawed permafrost sediments before entering the ocean.
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\nTotal and dissolved organic carbon (TOC and DOC) as well as total and dissolved nitrogen (TN and DN) were analyzed to quantify carbon and nitrogen loss. C/N-ratios, stable carbon isotope concentrations (δ13C-TOC and δ13C-DOC), nutrient concentrations (ammonium, nitrite, nitrate), and lipid biomarkers were analyzed to estimate degradation, carbon metabolization, as well as nitrification and plant assimilation processes. Furthermore, dating of lead isotopes (Pb-210) in nearshore sediments and conductivity-temperature-depth (CTD) profiles of the sea water in front of the slump were analyzed to assess the possible fate of the organic material in the nearshore zone.
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\nOur results show a general decrease of TOC and DOC as well as TN and DN contents from undisturbed to disturbed zones. TOC/TN-ratios are lower in disturbed zones, especially when comparing to permafrost sediments only. DOC/DN-ratios are highest in the tundra and slump floor but in general lower in disturbed zones. Stable carbon isotopes differ only slightly with lower values in disturbed zones, especially when comparing disturbed areas with permafrost only. Nitrate and nitrite concentrations are highest in disturbed areas, while ammonium concentrations are highest in permafrost and mudpool sediments. In the marine sediment core, Pb-210 values hinted towards a well-mixed environment and non-continuous accumulation. CTD surveys showed frequent brackish and mixed water column conditions.
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\nThese results lead to the assumption that sediments released through thermokarst activity are subject to strong degradation, which is supported by lower quantities of TOC and DOC as well as lower C/N-ratios in the disturbed zone. However, slightly lower values of stable carbon isotopes indicate that carbon is less degraded in the disturbed zone. High ammonium values in permafrost and mudpool sediments reflect an increasing activity of bacteria metabolizing organic material. No nitrate and nitrite was found in undisturbed parts, whereas detectable concentrations were found in disturbed parts, leading to the assumption that organic material has been subject to metabolization by bacteria. Lower DN-values in the slump floor reflected the nitrogen fixation by plants that recolonize the disturbed zones.
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\nWe suggest that before entering the nearshore zone permafrost organic carbon and nitrogen is subject to substantial degradation and metabolization. Within the nearshore zone, the accumulated sediments are remobilized frequently and transported either along the shore or further offshore.

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