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

• Published in: Geochemistry, geophysics, geosystems : G3 geophysics, geosystems : G3, 2016-11, Vol.17 (11), p.4333-4353
• Main Authors: Ruppel, Carolyn D., Herman, Bruce M., Brothers, Laura L., Hart, Patrick E.
• Format: Article
• Language: English
• Subjects: Arctic Ocean; Barrier islands; borehole logging; Boreholes; climate change; Continental shelves; Distance; Distribution; Drilling; Drilling fluids; Exploration; Fluids; gas hydrate; Gas hydrates; Hydrates; Ice; Indicators; Islands; Lake ice; Lakes; Locations (working); Offshore; Permafrost; Permafrost distribution; Pleistocene; Recovery; Seismological data; Seismology; Well logging; Wells
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1002/2016GC006582

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. Key Points: Legacy borehole logs on the U.S. Beaufort margin continental shelf directly constrain the distribution of subsea permafrost The borehole data largely confirm the findings of seismic studies that show permafrost signatures confined to the inner shelf Based on gas hydrate indicators, permafrost may once have been present beneath much of the eastern U.S. Beaufort shelf Plain Language Summary Subsea permafrost forms when rising sea levels flood permanently frozen ground (permafrost) in the coastal zone at high latitudes. The ~120 m rise in sea level since the end of Earth's last major glaciation has led to the development of subsea permafrost in some locations rimming the Arctic Ocean, including in the U.S. portion of the Beaufort Sea. Using geophysical data collected in boreholes drilled offshore the Alaskan North Slo