Rapid seafloor changes associated with the degradation of Arctic submarine permafrost - PubMed (original) (raw)

Rapid seafloor changes associated with the degradation of Arctic submarine permafrost

Charles K Paull et al. Proc Natl Acad Sci U S A. 2022.

Abstract

SignificanceTemperature increases in Arctic regions have focused attention on permafrost degradation on land, whereas little is known about the dynamics of extensive glacial-age permafrost bodies now submerged under the vast Arctic Continental shelves. Repeated high-resolution bathymetric surveys show that extraordinarily rapid morphologic changes are occurring at the edge of the continental slope of the Canadian Beaufort Sea along what was once the seaward limit of relict Pleistocene permafrost. How widespread similar changes are on the Arctic shelves is unknown, as this is one of the first areas in the Arctic subjected to multiple multibeam bathymetric surveys. Rapid morphologic changes associated with active submarine permafrost thawing may be an important process in sculpturing the seafloor in other submarine permafrost settings.

Keywords: Arctic; permafrost; pingos; repeat mapping; thermokarst.

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Conflict of interest statement

The authors declare no competing interest.

Figures

Fig. 1.

Fig. 1.

Map and cross-section showing the relationship between shelf edge morphology and the subsurface thermal structure along the shelf edge in the Canadian Beaufort Sea. (A) Shows the location of the study area with respect to estimates of submarine permafrost density (see key) and thickness, modified after . Thin contours indicate permafrost thickness in meters. Thicker contour is 120-m isobath marking the shelf edge. Area of repeat mapping coverage shown in Fig. 2 is indicated with a red box. (B) Shows a schematic cross-section with contours of selected subsurface isotherms modified after along line x-x’ in A. The dotted blue line illustrates a thermal minimum (T-min) running through the relict permafrost isotherm and Beaufort Sea waters (16). Green shading indicates relict permafrost. Turquoise arrows show inferred flow of water from permafrost thawing along the base of the relict permafrost to the seafloor. The brown area indicates the zone where relict Pleistocene permafrost is predicted to have thawed with consequent movement of liberated groundwater, associated latent heat transfer and thaw consolidation causing surface settlement. Dashed brown lines define the subbottom limits for methane hydrate stability zone (MHSZ) which starts at ∼240 m below the sea surface and extends into the subsurface depending on the pressure and temperature gradient. The red box indicates the area shown in more detail in C with the same color scheme. The area of denuded seafloor in C is flanked by PLFs (dark-blue fill). Red arrows indicate the direction of heat transfer along the seaward edge of relict permafrost wedge.

Fig. 2.

Fig. 2.

(A) Shows bathymetry of a small section of the shelf edge indicated in Fig. 1_A_, with a color scale going from white (128 m) to blue (200 m) and contours at 120, 140, 170, and 200 mbsf. Outlines of areas resurveyed in 2013 (blue), 2017 (turquoise), and 2019 (green) are superimposed on the 2019 and regional 2010 survey. Colored symbols indicate locations of cores with porewater data using same key as Fig. 5_B_. The red star indicates the location of a temperature tripod deployed in the period 2015 to 2016. The location of ROV dive tracks (blue paths) are indicated. (B) Covers the same area as A with polygons identifying sites where changes were noted between surveys as follows: 2010 to 2013 (green), 2013 to 2017 (purple), 2017 to 2019 (black), and 2010 and 2019 (red). (C) Shows the same area, colored according to the difference in bathymetry between the 2019 survey and an idealized smooth surface extending between the top of the shelf edge scarp and the layered sediments occurring between the numerous PLFs. This is used to estimate the volume of material that eroded assuming the earlier Holocene seafloor corresponded with this idealized surface. Three zones of topography are labeled. Red boxes are locations of Figs. 3 and 5. (D) Shows Chirp profiles with the position of profiles shown in C and Fig. 5_A_. Light-green backdrop in X-X’ indicates possible void produced by retrogressive slide retreat used to calculated volume loss. Also indicated are TL, tilted layers; P, pingo-like-feature; and DR, diffuse reflector.

Fig. 3.

Fig. 3.

Images showing the largest observed seafloor change between the 2010 and 2019 multibeam surveys. Location of this depression in the regional map is shown in Fig. 2_C_. (A and B) Show the bathymetries measured in 2010 and 2019, respectively. (C) Shows the difference between these grids with a color scale going from 0 to −28 m. (A, B, and C) Cover the same area with polygon of change outlined in a thin red line. (D) A perspective view of the multibeam sounding data used to generate bathymetric grids in the area indicated by the red box in C with 2010 in blue and 2019 in black. Red arrow in C is the direction of the perspective view.

Fig. 4.

Fig. 4.

Selected areas of 1-m resolution bathymetry collected by the AUV in 2017 showing seafloor morphology. Locations of A to F are indicated in Fig. 5_A_. A and B have polygons showing where differences between AUV surveys (2013 to 2017) detected significant volume losses. C–F show different areas representing a progression from a nearly smooth-surfaced PLF (A) to those with increasingly large depressions (D_–_F). Black lines in B and F show the approximate ROV path on the bottom.

Fig. 5.

Fig. 5.

(A) AUV multibeam bathymetry collected during the 2017 AUV survey. Locations of bathymetry shown in Fig. 4 (red squares); cores (colored symbols); ROV dives (blue paths); position of Chirp profiles X-X’ and Y-Y’ shown in Fig. 2_C_; and morphological labels E, RT, and S are indicated. (B) Plot of porewater chloride concentrations versus subbottom depth. In the plot legend, colored symbols with lines indicate cores with statistically significant chloride gradients; gray symbols are cores without statistically significant chloride gradients. The dashed gray line is the freezing point for water with 25.3-ppt salinity at −1.5 °C (45). Data from ref. .

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