Freshwater Seepage Into Sediments of the Shelf, Shelf Edge, and Continental Slope of the Canadian Beaufort Sea

Long-term warming of the continental shelf of the Canadian Beaufort Sea caused by the transgression associated with the last deglaciation may be causing decomposition of relict offshore subsea permafrost and gas hydrates. To evaluate this possibility, pore waters from 118 sediment cores up to 7.3-m long were taken on the shelf and slope and analyzed for chloride concentrations and delta(18)0 and delta D composition. We observed downcore decreases in pore waters Cl- concentration in sediments from all sites from the inner shelf (<20-m water depth), from the shelf edge, from the outer slope (down to 1,000-m water depths), and from localized shelf features such as midshelf pingo-like features and inner shelf pockmarks. In contrast, pore water freshening is absent from all investigated cores of the Mackenzie Trough. Downcore pore waters Cl- concentration decreases indicate regional widespread freshwater seepage. Extrapolations to zero Cl- of pore water Cl- versus delta(18)0 regression lines indicate that freshwaters in these environments carry different isotope signatures and thus are sourced from different reservoirs. These isotopic signatures indicate that freshening of shelf sediments pore waters is a result of downward infiltration of Mackenzie River water, freshening of shelf edge sediments is due to relict submarine permafrost degradation or gas hydrate decomposition under the shelf, and freshening of slope sediments is consistent with regional groundwater flow and submarine groundwater discharge as far as 150 km from shore. These results confirm ongoing decomposition of offshore permafrost and suggest extensive current groundwater discharge far from the coast. Plain Language Summary The continental shelves around the Arctic Ocean were exposed to very low temperatures during the last glacial period more than 12,000 years ago. Precipitation that infiltrated these areas froze in the soils as permafrost. When climate warmed at the end of the glacial period, sea level rose and inundated the shelves warming them up. The warming may be reaching the now submarine permafrost and inducing its melting. This permafrost decomposition may be detected as freshwater seeping into the seafloor. In this study, we found evidence that in the Canadian Beaufort Sea not only permafrost is decomposing and seeping into the seafloor but also current groundwater discharge into the seafloor occurs at distances as far as 150 km from the current shore, likely routed by the permafrost presence in the shelf as a frozen lid. Active water discharge onto sediments may induce sediment instabilities that result in landslides, which can trigger tsunamis. In addition, sediment instabilities are a geohazard for sea-based infrastructure.