Title:
Minimal offshore extent of ice-bearing (subsea) permafrost on the U.S. Beaufort Sea margin
Abstract:
The present-day distribution of subsea permafrost beneath high-latitude continental shelves has implications for sea level rise and climate change since the Last Glacial Maximum (~20,000 years ago). Because permafrost can be spatially associated with gas hydrate (which may be thermodynamically stable within the several hundred meters above and below the base of permafrost), the contemporary distribution of subsea permafrost also has implications for the persistence of permafrost-associated gas hydrate beneath shallow waters at high latitudes, particularly on margins that were not glaciated at the Last Glacial Maximum. On the U.S. Beaufort Sea margin offshore northern Alaska, researchers have sometimes assumed that contemporary subsea permafrost extends to the 100 meter isobath on the outer continental shelf. Using a compilation of more than 50,000 stacking velocities from ~100,000 line-km of industry-collected multichannel seismic reflection data acquired over 57,000 square kilometers of the U.S. Beaufort Sea continental shelf, we derive the average (bulk) velocity in the upper 750 milliseconds of two-way travel time (TWTT). An average velocity of 2000 meters per second (m/s) is used to delineate the offshore extent of ice-bearing permafrost that has not thawed since the end of the Last Glacial Maximum. The 2000 m/s velocity contour represented in this data release is within 37 km of the modern U.S. Beaufort shoreline and at water depths less than 25 m. The contour was determined as part of a study by Brothers, L. L., B. M. Herman, P. E. Hart, and C. D. Ruppel (2016), Subsea ice-bearing permafrost on the U.S. Beaufort Margin: 1. Minimum seaward extent defined from multichannel seismic reflection data, Geochemistry, Geophysics, Geosystems, 17, 4354–4365, doi:10.1002/2016GC006584. Direct borehole observations of ice-bearing permafrost in the same area as the 2000 m/s velocity contour from this data set are described in the associated work: Ruppel, C. D., B. M. Herman, L. L. Brothers, and P. E. Hart (2016), Subsea ice-bearing permafrost on the U.S. Beaufort Margin: 2. Borehole constraints, Geochemistry, Geophysics, Geosystems, 17, 4333–4353, doi:10.1002/2016GC006582. The placement of the 2000 m/s contour derived from seismic reflection stacking velocities is similar to, but not exactly the same as, the extent of subsea permafrost inferred based on earlier seismic refraction analyses of Brothers, L. L., P. E. Hart, and C. D. Ruppel (2012), Minimum distribution of subsea ice-bearing permafrost on the U.S. Beaufort Sea continental shelf, Geophys. Res. Lett., 39, L15501, doi:10.1029/2012GL052222.
Supplemental_Information:
The date range given for "time period information" is approximate and is meant to encompass the range of dates over which the original seismic data were acquired. In the first cross-referenced work, the original seismic data were described as being collected in the mid-1970s in the Beaufort Sea and onshore in the National Petroleum Research Reserve and the Arctic National Wildlife Refuge from 1977 to 1990. However, the permit numbers associated with the data indicate that the data were acquired only between 1977 and 1990. A subset of the data used in this study (e.g., scanned tiff images of the semblance based velocity analyses, the compiled results of the velocities picked from the semblance based analyses, processed seismic data, and navigation files) is available from the National Archive of Marine Seismic Surveys (NAMSS)
https://walrus.wr.usgs.gov/NAMSS/. All of the other velocity data used in this study are available from the Bureau of Ocean Energy Management (BOEM) Alaska Region. Interested parties may submit Freedom of Information Act requests to BOEM with the permit number in order to obtain this information. The permit numbers (NAMSS ID is in parentheses where applicable) are: 77-25 (B-25-77-AK), 80-01, 80-03 (B-03-80-AR), 80-42, 81-02 (B-02-81-AR), 81-23 (B-23-81-AR), 82-59 (B-59-82-AR), 83-28, 84-37, 85-49, 87-04, 87-15 (B-15-87-AR), 90-05, and 90-15. The original list of permit numbers is provided in the Supporting Information published with Brothers, L. L., B. M. Herman, P. E. Hart, and C. D. Ruppel (2016), Subsea ice-bearing permafrost on the U.S. Beaufort Margin: 1. Minimum seaward extent defined from multichannel seismic reflection data, Geochemistry, Geophysics, Geosystems, 17, 4354–4365, doi:10.1002/2016GC006584. The Supporting Information can be accessed at
http://agupubs.onlinelibrary.wiley.com/doi/10.1002/2016GC006584.
