Traveling Light: Arctic Coastal Erosion Releases Mostly Matrix Free, Unprotected Organic Carbon.

The Arctic rapidly warms and sea ice retreats, a large fraction of organic carbon (OC), currently stored in coastal permafrost will be released into the marine system. Once reintroduced into the active carbon cycle, this material will either be decomposed or buried on the shelf depending on its hydrodynamic and chemical properties. Currently, carbon estimates are based on bulk measurements, which does not take the hydrodynamic pathway of different fractions into account. Therefore, eight coastal permafrost locations have been sampled along the Canadian Beaufort Sea Coast, hydrodynamically fractionated and analyzed for their C, N, 13C and 14C content. We found that the matrix‐free fraction (low density <1.8 g/cm3, and high‐density >1.8 g/cm3; <38 μm) account for 77%–98% of the OC. By using a coastal classification combined with field data, our results showed that short coastal segments can become key players in delivering matrix‐free, easily degradable OC to the marine system. Plain Language Summary: Arctic coastlines consisting out of permafrost, permanently frozen ground, experience accelerated erosion due to rising global temperatures. This erosion poses a threat to local communities and releases thawed permafrost material into the Arctic Ocean. Permafrost coasts store significant amounts of organic carbon (OC), and as they thaw and erode, this carbon is released, impacting the climate through the potential release of CO2. Current estimates rely on bulk measurements, but the behavior of released OC in the ocean remains poorly understood. To address this, we used hydrodynamic fractionation on eight sampled coastal permafrost sites along the Canadian Beaufort Sea. By doing so we separate the material based on density and grain size. The results show that a significant portion of OC (77%–98%) falls into the low‐density and the high‐density fine (<38 μm) fraction, containing easily degradable plant‐rich material. Due to these two fractions short coastal segments can become key players for the delivery of degradable OC into the ocean. Key Points: Coastal types vary in size and density distribution, high‐density prevailing in weight and low‐density dominating organic carbon contentBetween 77% and 98% of eroded organic carbon resides in the matrix‐free fraction, prone to degradation and CO2 releaseHydrodynamic fractionation and site‐specific properties contribute to integration of results into land‐ocean flux models [ABSTRACT FROM AUTHOR]