Abstract
Warming air and sea temperatures, longer open-water seasons and sea-level rise collectively promote the erosion of permafrost coasts in the Arctic, which profoundly impacts organic matter pathways. Although estimates on organic carbon (OC) fluxes from erosion exist for some parts of the Arctic, little is known about how much OC is transformed into greenhouse gases (GHGs). In this study we investigated two different coastal erosion scenarios on Qikiqtaruk – Herschel Island (Canada) and estimate the potential for GHG formation. We distinguished between adelayedrelease represented bymud debrisdraining a coastal thermoerosional feature and adirectrelease represented bycliff debrisat a low collapsing bluff. Carbon dioxide (CO2) production was measured during incubations at 4°C under aerobic conditions for two months and were modeled for four months and a full year. Our incubation results show thatmud debrisandcliff debrislost a considerable amount of OC as CO2(2.5 ± 0.2 and 1.6 ± 0.3% of OC, respectively). Although relative OC losses were highest in mineralmud debris, higher initial OC content and fresh organic matter incliff debrisresulted in a ∼three times higher cumulative CO2release (4.0 ± 0.9 compared to 1.4 ± 0.1 mg CO2gdw–1), which was further increased by the addition of seawater. After four months, modeled OC losses were 4.9 ± 0.1 and 3.2 ± 0.3% in set-ups without seawater and 14.3 ± 0.1 and 7.3 ± 0.8% in set-ups with seawater. The results indicate that adelayedrelease may support substantial cycling of OC at relatively low CO2production rates during long transit timesonshoreduring the Arctic warm season. By contrast,directerosion may result in a single CO2pulse and less substantial OC cyclingonshoreas transfer times are short. Once eroded sediments are deposited in thenearshore, highest OC losses can be expected. We conclude that the release of CO2from eroding permafrost coasts varies considerably between erosion types and residence timeonshore. We emphasize the importance of a more comprehensive understanding of OC degradation during the coastal erosion process to improve thawed carbon trajectories and models.
Highlights
The Arctic is outpacing the global warming trend, which has severe impacts on ecosystems and biogeochemical pathways (Ciais et al, 2013; AMAP, 2017)
The lower total organic carbon (TOC)/TNratios and δ13C values in the mud lobe in combination with lower high-molecular weight (HMW)/low-molecular weight (LMW) ratios and carbon preference index (CPI) for n-alkanes indicated slightly more matured organic matter in the mud lobe and better preserved organic matter in the cliff
Coastal morphology and erosion type play a substantial role in determining the greenhouse gas release potential during erosion along permafrost coasts in the Arctic
Summary
The Arctic is outpacing the global warming trend, which has severe impacts on ecosystems and biogeochemical pathways (Ciais et al, 2013; AMAP, 2017). Arctic river deltas, estuaries and wetlands are increasingly impacted by sea level rise and glacier melt (Bendixen et al, 2017; Ward, 2020) These systems provide important ecosystem services (Forbes, 2019) but are currently facing changes in their sedimentation dynamics, which impacts organic matter pathways in the coastal zone (Bendixen et al, 2017; Ward, 2020). The degradation of OC from thawing permafrost coastlines has barely been considered (Vonk and Gustafsson, 2013; Fritz et al, 2017)
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