Bridging the Gap: Nearshore zones as key mediators in Arctic land-ocean carbon fluxes

The Arctic is experiencing rapid warming, leading to prolonged ice-free periods, increased storm activity, and intensified coastal erosion. These changes release organic matter-rich permafrost into the nearshore marine environment, where it either degrades to CO₂ or is transported further for potential burial on the continental shelf. However, only 5% of all sediment samples in the Arctic Ocean have been collected from the nearshore zone (depths shallower than 10 meters), suggesting that this zone has been significantly under-sampled and understudied in the global carbon cycle and along the land-ocean continuum. This study addresses this gap by investigating OC redistribution and transformation in the nearshore zone of the Canadian Beaufort Sea coast.
We collected sediment samples from five locations adjacent to eroding permafrost coasts along the Canadian Beaufort Sea coast across two shallow zones: the surf zone (0–2 m depth) and the nearshore zone (2–5 m depth). Additional samples included four shelf sediments (30–55 m depth), a sediment trap (2.2 m depth), and surface water samples. The samples were hydrodynamically fractionated (into low and high density with cutoff of 1.8 g/cm³; and subsequently size-fractionated) and analysed for their carbon (C), nitrogen (N), and δ¹³C content. We compare our results with an earlier study that characterized eroding permafrost coastal material.
                Our findings indicate that in eroding permafrost, the majority of OC is stored in the LD and HD<38 μm fractions, contributing 69±22% and 20±18% of OC, respectively. Fluvial material, as shown by sediment trap analysis, also retains most of its OC in the LD (56%) and HD<38 μm (32%) fractions. In contrast, surf zone sediments (0–2 m depth) predominantly store OC in the HD>200 μm and HD>63 μm fractions, which contribute up to 39±23% and 28±18% of total OC, respectively, while the LD fraction accounts for only 19±24%. In slightly deeper nearshore waters (2–5 m depth), OC distribution shifts, with a larger fraction in HD<38 μm (41±23%) and HD>63 μm (27±26%), and the LD fraction increasing to 28±18%. On the inner shelf, OC distribution undergoes a clear shift, with the HD<38 μm fraction becoming the dominant contributor, representing 86±2.4% of total sediment OC. Scanning Electron Microscopy (SEM) images confirm that vascular plant material can be found in the high-density (usually more mineral) fractions, particularly in the high-density (63–200 μm) fraction. These findings highlight the redistribution and transformation of vascular plant material into high-density coarse fractions, which can undergo further degradation. It further highlights the importance of shallow Arctic coastal zones in the global carbon cycle, emphasizing their role in the redistribution, transformation, and burial of terrestrial organic carbon. By focusing on these underrepresented zones, this research provides critical insights into Arctic OC dynamics under the influence of climate-driven changes.