Earth’s permafrost stores an estimated 20% more organic carbon than is currently in the atmosphere. Permafrost is found along 34% of the planet’s coasts, and this frozen ground erodes at a mean rate of 0.5 meters per year, releasing an estimated 14 billion kilograms of organic carbon.
Despite its major impact, precise contributions of coastal permafrost to carbon cycling remain unclear in certain parts of the Arctic, where rapid warming is likely to increase erosion rates. To address these uncertainties, Couture et al. investigated organic carbon release to the Beaufort Sea due to permafrost erosion along the Yukon Coastal Plain in northwestern Canada.
The researchers collected permafrost soil samples at multiple depths from 22 sites along the coastal bluffs, ensuring that the region’s varying terrain types were represented. They measured the organic carbon content of the samples and determined how much of the soil volume was taken up by large bodies of ground ice. They then used those measurements to calculate the amount of organic carbon stored in permafrost at each site and in areas with similar terrain. The amount of carbon varied according to bluff height and soil composition, with more than half of the soil organic carbon found below the top meter of soil.
To determine how much carbon is released by erosion, the researchers combined their soil organic carbon calculations with long-term erosion rates (the region loses a mean of 0.7 meters of coastline per year). They found that erosion of permafrost releases a total of 36 million kilograms of organic carbon from the Yukon Coastal Plain into the Beaufort Sea annually.
Analysis of carbon isotopes in seabed samples from 14 offshore sites indicated that about 13% of the carbon released by permafrost erosion ends up sequestered in nearshore sediments. The rest is either consumed by organisms in the nearshore environment or transported to the deep ocean.
This research highlights the importance of accounting for large amounts of ground ice that can make up a significant portion of total permafrost volume in some areas. Not accounting for these volumes can result in overestimating soil organic carbon by over 40% at some sites.
These findings contribute to growing knowledge of Arctic coastal dynamics, especially for the Yukon region. They could aid understanding of how carbon cycling in the region may evolve as climate change progresses and help refine predictions of future climate change. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1002/2017JG004166, 2018)
—Sarah Stanley, Freelance Writer