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

Abstract

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 δ180 and δD 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 δ180 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.

Understanding the Permafrost–Hydrate System and Associated Methane Releases in the East Siberian Arctic Shelf

“…As a result, continuing warming causes not only advanced deepening of the IBPT but also thaw-through disintegration of the permafrost body at places such as fault zones, paleo-rivers, areas affected by thaw lakes, freshwater seepages into shelf sediments, and so-called pingo-like features, which provide subsea permafrost with heat and create migration pathways for CH 4 that is released from destabilizing hydrates [72][73][74]. A pingo is a mound of sediment-covered ice.…”

A Pulse of Meteoric Subsurface Fluid Discharging Into the Chukchi Sea During the Early Holocene Thermal Maximum (EHTM)

“…More recently, Gwiazda et al. (2018) showed that the freshening in pore water from the Beaufort Slope sediments is caused by regional groundwater flow and submarine groundwater that discharges as far as 150 km from the shore. Whether a similar set of mechanisms can explain the anomalous data observed at Site ARA06C‐JPC01 is a matter of interest.…”

“…The Chukchi Borderland is an adjacent fragment of continental crust extending north into the Canada Basin of the Arctic Ocean (Dove et al., 2014). The Beaufort Shelf, located in the eastern part of the Chukchi Sea, is up to ∼150 km wide with an average bathymetric gradient of 1 m/km (Gwiazda et al., 2018) (Figure 1).…”

Influence of permafrost thaw on an extreme geologic methane seep

“…Sediment cores showing sorting and layering adjacent to pockmarks reveal information about lacustrine sedimentary processes 96 and pockmark evolution 42 . Sediment coring campaigns with a percussion corer on August26–27, 2018 and March26–28, 2019 yielded six cores of 85 cm length or less from locations near W1 and W2 (Figure 3a).…”

“…The mechanisms of pockmark formation involve upward fluid flows 20,30,40,42 including hydrodynamic blowouts, 27 submarine hydrate dissolution, 35,43 catastrophic dissolution of subterranean hydrates, 39 and explosive cryovulcanism events from compressed gas systems 44,45 . These mechanisms occur on a range of timescales spanning rapid, catastrophic explosions to gradual displacement over an undetermined length of time 27 .…”