Fate of organic matter mobilized from eroding permafrost coasts

Permafrost, defined as ground that remains frozen for at least two consecutive years, is a prominent feature of polar regions. In the Northern Hemisphere, approximately 23 million km2 of the ground are affected by permafrost. Climatic warming, which has a greater effect on the Arctic than on any other region on Earth, leads to permafrost thaw, caused by gradual deepening of the seasonal unfrozen layer (active layer), thermokarst formation (i.e. land subsidence due to ground ice loss) and thermo-erosion. In the course of thaw, formerly freeze-locked organic carbon (OC) is mobilized and mineralized into greenhouse gases (GHGs), fostering further climate warming – a process known as permafrost carbon feedback. Current climate models focus on GHG release from gradual deepening of the active layer and neglect the OC turnover during lateral transport induced by thermokarst and abrupt thermo-erosion. As such, the accelerated erosion of Arctic permafrost coasts, which make up ~34 % of the global coasts, deliver vast amounts of OC into the Arctic Ocean. However, little is known about the amounts of labile and fast bioavailable dissolved OC (DOC), the impact of thermokarst on mobilized organic matter (OM) characteristics, and the release of GHGs from eroding permafrost coasts. To fill that knowledge gap, the main objectives of the thesis are to investigate (i) how much DOC is mobilized from coastal erosion, (ii) how thermokarst and -erosion alters OM characteristics upon thaw on transit to the ocean, and (iii) how much GHGs are emitted from the nearshore zones of eroding permafrost coasts. Field work and sampling took place along the Yukon coast and on Qikiqtaruk (Herschel Island) in the western Canadian Arctic. An interdisciplinary approach was used to quantify OM (OC and nitrogen) as well as to identify degradation processes. The methods used included sedimentology, geo- and hydrochemistry, remote sensing, statistical analyses, and gas chromatography. The thesis shows that considerable amounts of DOC are released from eroding permafrost coasts. Although OC fluxes into the ocean are dominated by DOC from Arctic rivers and particulate OC (POC), labile DOC derived from permafrost plays an important role as it is quickly available for biogeochemical cycling and turnover into GHGs. During transit from land to ocean OM characteristics are substantially altered by thermokarst formation and thermo-erosion. In mudpools, originating from in-situ thawed permafrost, as well as in thaw streams draining thermokarst features towards the ocean, mobilized OM issubject to dilution with melted ground ice and degradation, which result in a decrease of OM contents by more than 50 %. The turnover of OC continues in the nearshore zone. The biochemically most labile OC portions are rapidly lost within months and mineralized into GHGs. The production of GHGs in the ocean is 60 to 80 % as efficient as on land and primarily in form of carbon dioxide (CO2), due to aerobic conditions in the nearshore zone. During each open water season in the Arctic approximately 0.7 to 1.2 Tg of CO2 are emitted from the coastal fringe. The remaining OM is buried in nearshore and shelf sediments, potentially remobilized by waves, currents and ice scouring at later stages. To conclude, the thesis shows that eroding permafrost coasts release large amounts of OC, from which considerable portions are labile DOC. In the course of thermokarst formation and thermo-erosion, OM is diluted and the most labile portions subject to rapid turnover into GHGs. This shows that eroding permafrost coasts are a major yet neglected source of CO2 to the atmosphere. With increasing temperatures and longer sea ice-free conditions projected for the Arctic, the erosion of permafrost coasts accelerates. Consequently, the transfer of OC to the ocean accompanied by GHG production increases, which is expected to have drastic impacts for the climate and coastal ecosystems.

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