# Shallow coastal zones are key mediators in Arctic land-ocean carbon fluxes

## TL;DR

Shallow Arctic coastal waters trap most of the carbon from eroding permafrost, acting as a key biogeochemical reactor.

## Contribution

This study reveals that shallow coastal zones, not deep waters, are the main site for trapping and processing permafrost-derived organic carbon.

## Key findings

- About 43% of terrestrial organic carbon is trapped in low-density plant debris in shallow waters.
- Less than 10% of this carbon reaches deeper waters, showing shallow zones act as a trap.
- Shallow coastal zones are an undersampled but critical biogeochemical reactor for permafrost carbon.

## Abstract

Rapid Arctic warming accelerates the erosion of permafrost coasts rich in terrestrial organic carbon (terrOC). Once released into the ocean, terrOC can degrade or get buried in shelf sediments, yet its transport pathways and fate remain poorly understood. We collected permafrost material, sediment and surface water along the Canadian Beaufort Sea coast, fractionating samples by density (cut-off 1.8 g/cm3) and size (38, 63 and 200 µm) before performing geochemical and microscopic analysis. Our results show that ~43% of terrOC is trapped in low-density fractions, mainly as vascular plant debris. Surprisingly, this material is trapped within shallow (0-5 m) waters where waterlogging and large particle size increase its density and settling velocity. Less than 10% is transported to deeper waters (30-55 m), indicating that the shallow coastal zone acts as a trap and biogeochemical reactor. These findings challenge the source-to-sink paradigm and highlight the overlooked and undersampled ( < 6% of pan-arctic shelf data) nearshore zone.

The Arctic is undergoing extreme changes due to rapid warming, impacting the carbon cycle and global climate. Permafrost holds about 1,300 billion tons of carbon. Along the Arctic coasts, this terrestrial organic carbon (terrOC) is released into coastal waters upon thaw and erosion, where it can then be broken down into CO₂ or buried. Understanding its transport, redistribution, and storage is crucial for predicting its eventual climate feedback. This study focuses on the shallow coastal zones ( < 5 m depth) of the Canadian Beaufort Sea, the first receptor of terrOC. Using a transect-based sampling approach, we analysed the interplay between terrOC and the mineral matrix in sediments and surface waters to show how terrOC moves and changes from the coast to the marine shelf sea. We found that terrOC is primarily mineral-free material (plant residues) and is hydrodynamically trapped – and possibly degraded – in these shallow waters. At greater depths, only a small fraction of this material remains, implying that this region is an overlooked biogeochemical reactor. This research addresses a critical gap in data from shallow Arctic coastal zones and thereby helps us to better understand the fate of carbon from eroding permafrost coasts.

Eroding Arctic permafrost along the Canadian Beaufort Sea coasts hold large amounts of vascular plant debris, which in turn hold large amounts of terrestrial organic carbon, according to analyses of sampled permafrost material, sediment, and surface water.