No abstract

Permafrost is thawing extensively due to climate warming. When permafrost thaws, previously frozen organic carbon (OC) is converted into carbon dioxide (CO 2 ) or methane, leading to further warming. This process is included in models as gradual deepening of the seasonal non-frozen layer. Yet, models neglect abrupt OC mobilization along rapidly eroding Arctic coastlines. We mimicked erosion in an experiment by incubating permafrost with seawater for an average Arctic open-water season. We found that CO 2 production from permafrost OC is as efficient in seawater as without. For each gram (dry weight) of eroding permafrost, up to 4.3 ± 1.0 mg CO 2 will be released and 6.2 ± 1.2% of initial OC mineralized at 4°C. Our results indicate that potentially large amounts of CO 2 are produced along eroding permafrost coastlines, onshore and within nearshore waters. We conclude that coastal erosion could play an important role in carbon cycling and the climate system. Plain Language SummaryThe permanently frozen soils of the Arctic, known as permafrost, store large amounts of organic carbon, which accumulated over millennia due to slow decomposition in the cold Arctic regions. With climate warming this frozen organic carbon reservoir thaws and microbes recycle it quickly into greenhouse gases, which in turn support further warming. A slow and continuous thaw is currently used in models to project future greenhouse gas release from permafrost. Yet along the rapidly eroding coastlines of the Arctic Ocean, which make up 34% of the Earth's coastlines, whole stretches of the coast simply collapse, sink or slide into the ocean, including the previously frozen organic carbon. We simulated greenhouse gas release in response to coastline collapse in a laboratory experiment by simply mixing permafrost with seawater. We show that large amounts of carbon dioxide are being produced during the Arctic open-water season. Our study indicates that eroding permafrost coasts in the Arctic are potentially a major source of carbon dioxide. With increasing loss of sea ice, longer open-water seasons, and exposure of coasts to waves, we highlight the importance of coastal erosion for potential carbon dioxide emissions.

Ice-rich permafrost coasts in the Arctic are highly sensitive to climate warming and erode at a pace that exceeds the global average. Permafrost coasts deliver vast amounts of organic carbon into the nearshore zone of the Arctic Ocean. Numbers on flux exist for particulate organic carbon (POC) and total or soil organic carbon (TOC, SOC). However, they do not exist for dissolved organic carbon (DOC), which is known to be highly bioavailable. This study aims to estimate DOC stocks in coastal permafrost as well as the annual flux into the ocean. DOC concentrations in ground ice were analyzed along the ice-rich Yukon coast (YC) in the western Canadian Arctic. The annual DOC flux was estimated using available numbers for coast length, cliff height, annual erosion rate, and volumetric ice content in different stratigraphic horizons. Our results showed that DOC concentrations in ground ice range between 0.3 and 347.0 mg L À1 with an estimated stock of 13.6 ± 3.0 g m À3 along the YC. An annual DOC flux of 54.9 ± 0.9 Mg yr À1 was computed. These DOC fluxes are low compared to POC and SOC fluxes from coastal erosion or POC and DOC fluxes from Arctic rivers. We conclude that DOC fluxes from permafrost coasts play a secondary role in the Arctic carbon budget. However, this DOC is assumed to be highly bioavailable. We hypothesize that DOC from coastal erosion is important for ecosystems in the Arctic nearshore zones, particularly in summer when river discharge is low, and in areas where rivers are absent.