Bridging the gap: shallow coastal zones are key mediators in Arctic land-ocean carbon fluxes

Rapid Arctic warming causes accelerated erosion of permafrost coasts rich in terrestrial organic carbon (terrOC). Upon release into the ocean this OC can either degrade or get buried in shelf sediments, yet the transport pathway and degradation potential of terrOC are still poorly understood. Here, we collected permafrost material, sediment and surface water along the Canadian Beaufort Sea coast and fractionated all material according to density (cut-off 1.8 g/cm ³ ) and size (38, 63 and 200 µm), before performing geochemical analyses and scanning electron microscopic imaging. Our results show that eroding permafrost holds approximately 43% of its terrOC in the low-density fraction, mainly as vascular plant debris. Surprisingly, we observe that this plant material is trapped within shallow (0-5 m depth) waters. This can be explained by heavy waterlogging and large particle size, converting the low-density material into high-density, coarse material with high settling velocity. Only a small fraction (<10%) of this OC-rich material is found in deeper waters (30-55 m), suggesting that the shallow coastal zone acts as a trap and biogeochemical reactor. This calls for redesigning the prevailing source-to-sink paradigm and shifting scientific focus to the overlooked and undersampled (<6% of pan-arctic shelf data) nearshore zone.