Abstract
Collapse of permafrost coasts delivers large quantities of particulate organic carbon (POC) to Arctic coastal areas. With rapidly changing environmental conditions, sediment and organic carbon (OC) mobilization and transport pathways are also changing. Here, we assess the sources and sinks of POC in the highly dynamic nearshore zone of Herschel Island‐Qikiqtaruk (Yukon, Canada). Our results show that POC concentrations sharply decrease, from 15.9 to 0.3 mg L−1, within the first 100–300 m offshore. Simultaneously, radiocarbon ages of POC drop from 16,400 to 3,600 14C years, indicating rapid settling of old permafrost POC to underlying sediments. This suggests that permafrost OC is, apart from a very narrow resuspension zone (<5 m water depth), predominantly deposited in nearshore sediments. While long‐term storage of permafrost OC in marine sediments potentially limits biodegradation and its subsequent release as greenhouse gas, resuspension of fine‐grained, OC‐rich sediments in the nearshore zone potentially enhances OC turnover.
Highlights
Northern Hemisphere soils store a large amount of organic carbon (OC), the bulk of which has been frozen in permafrost for millennia (Hugelius et al, 2014; Tamocai et al, 2009; Zimov et al, 2006)
Permafrost particulate organic carbon (POC) is transported in suspension upon erosion and can subsequently be deposited in nearshore sediments, transported further offshore, or remineralized and released as greenhouse gases (GHGs) (Figure 2)
We find that old POC within the high‐energy resuspension zone originate from coastal erosion and permafrost thaw, while younger POC in surface waters further offshore is derived from marine primary production or contemporary terrestrial organic debris, originating from the active layer and modern vegetation
Summary
Northern Hemisphere soils store a large amount of organic carbon (OC), the bulk of which has been frozen in permafrost for millennia (Hugelius et al, 2014; Tamocai et al, 2009; Zimov et al, 2006). Whereas gradual deepening of the seasonally unfrozen layer on land steadily mobilizes permafrost OC, abrupt permafrost thaw and coastal erosion rapidly releases OC into aquatic systems, estimated to be up to 14 Tg OC per year on a panarctic scale (Vonk et al, 2012; Wegner et al, 2015). This flux is in the same order of magnitude as the OC flux from all major Arctic rivers combined (Fritz et al, 2017; McClelland et al, 2016). Some of the most distinct erosional features in this region are retrogressive
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