Subsea ice‐bearing permafrost on the U.S. Beaufort Margin: 2. Borehole constraints

Abstract

Borehole logging data from legacy wells directly constrain the contemporary distribution of subsea permafrost in the sedimentary section at discrete locations on the U.S. Beaufort Margin and complement recent regional analyses of exploration seismic data to delineate the permafrost's offshore extent. Most usable borehole data were acquired on a $500 km stretch of the margin and within 30 km of the contemporary coastline from north of Lake Teshekpuk to nearly the U.S.-Canada border. Relying primarily on deep resistivity logs that should be largely unaffected by drilling fluids and hole conditions, the analysis reveals the persistence of several hundred vertical meters of ice-bonded permafrost in nearshore wells near Prudhoe Bay and Foggy Island Bay, with less permafrost detected to the east and west. Permafrost is inferred beneath many barrier islands and in some nearshore and lagoonal (back-barrier) wells. The analysis of borehole logs confirms the offshore pattern of ice-bearing subsea permafrost distribution determined based on regional seismic analyses and reveals that ice content generally diminishes with distance from the coastline. Lacking better well distribution, it is not possible to determine the absolute seaward extent of ice-bearing permafrost, nor the distribution of permafrost beneath the present-day continental shelf at the end of the Pleistocene. However, the recovery of gas hydrate from an outer shelf well (Belcher) and previous delineation of a log signature possibly indicating gas hydrate in an inner shelf well (Hammerhead 2) imply that permafrost may once have extended across much of the shelf offshore Camden Bay.

Submarine Permafrost Map in the Arctic Modeled Using 1‐D Transient Heat Flux (SuPerMAP)

“…The profiles extend out to 150‐m water depth. The Harrison Bay (HB) and Camden Bay (CB) profiles transect the Alaskan Beaufort coastline, where Ruppel et al () analyze borehole records. The Mackenzie (MP) profile transects the Canadian Beaufort coastline 140 km northeast of Tuktoyaktuk and extends more than 150 km offshore, where Taylor et al () model permafrost evolution.…”

“…Geophysical borehole logs provide greater detail about the vertical distribution of permafrost‐bearing sediments but only for discrete locations. Electrical resistivity logs are the most useful for identifying and distinguishing intact permafrost, layers with thawing permafrost, and sediments lacking ice (e.g., Ruppel et al, ). Recent work using controlled source electromagnetics in shallow waters gives an indication of the thicknesses of permafrost and its distribution (Sherman et al, ).…”

“…Such alternating terrestrial and marine sediment layers are strongly suggested by the few cored and well‐described offshore cores on the Arctic shelf, which encounter alternating strata of saline and freshwater permafrost (e.g., Blasco et al, ; Ponomarev, , ; Rachold et al, ). These alternations are not generally visible in offshore permafrost temperature records, which are typically near isothermal (Lachenbruch, ) but are often suggested by sediment structure visible in geophysical records (e.g., Batchelor et al, ; Ruppel et al, ). This representation ignores possible deeper variations in salinity due to groundwater or freezing that have been assumed in other models (e.g., salinity increases to 30% at 10‐km depth in Hartikainen & Kouhia, ).…”

“…Publicly available observational data are limited to shallow boreholes drilled from ships (Kassens et al, ; Rekant et al, ) or from the sea ice (Blasco et al, ; Dallimore, ; Winterfeld et al, ), a few deeper scientific boreholes, geophysical records from industrial boreholes in the Beaufort Sea (e.g., Hu et al, ), and geophysical records (e.g., Portnov et al, ; Rekant et al, ). Data from boreholes deep enough to penetrate permafrost in the prodeltaic region of the Mackenzie River and on the Alaskan Beaufort shelf have been published and analyzed for the depth of the base of permafrost (Brothers et al, ; Hu et al, ; Issler et al, ; Ruppel et al, ). Relating geophysical observations to permafrost depth, lithology, cryostratigraphy, or sediment temperature is not trivial.…”

“…Hu et al () examine over 250 borehole records, including over 70 offshore boreholes, and find permafrost 100 to 700 m thick north of the Mackenzie Delta and eastward. Ruppel et al () and Brothers et al () analyze available borehole and seismic data from the U.S. Beaufort Sea to provide a conservative representation of permafrost extent on the shelf: it is restricted to waters less than 20 m deep and closer than 30 km from shore.…”