Release of ancient dissolved carbon by thawing submarine permafrost in the Canadian Beaufort Sea

Long-term warming of the continental shelf of the Canadian Beaufort Sea has caused decomposition of submarine permafrost. Ancient dissolved carbon preserved in submarine permafrost could be transported and released into seawater by submarine groundwater discharge derived from thawing permafrost. However, the rate and scale of such a carbon emission is currently unclear. To fill this knowledge gap, we investigate the δ¹³C, Δ¹⁴C, and composition of sediment pore fluid from samples retrieved from a shelf edge site, where rapid seafloor depressions as a result of permafrost thawing have been observed. Downcore decrease of water isotopic signatures indicate widespread meteoric freshwater seepage from the region. The Δ¹⁴C values of dissolved inorganic carbon (DIC) in pore fluids indicate an ancient source of DIC (up to 7.7 cal kyr BP). The carbon isotopic mass balance calculation with Δ¹⁴C and δ¹³C of DIC suggest an input of ancient DIC with little radiocarbon, which is not from in-situ dissolution of carbonate. In other words, mixing of DIC from in-situ degradation of local organic carbon and overlying seawater DIC cannot explain the observed Δ¹⁴C values of DIC. Based on these results from porewater profiles, we suggest a lateral discharge of low-chlorinity fluid carrying such an ancient DIC to shallow sediments as a result of the decadal degradation of submarine permafrost.