Characteristics of the active layer and shallow subsea permafrost

Abstract

A seasonal active layer associated with subsea permafrost was found in the sediments near the seabed of the Beaufort Sea near Prudhoe Bay, Alaska. The active layer existed where sea ice was frozen to the seabed and also in shallow water where the under-ice seawater salinities exceeded open-water values. Initial freezing of the active layer was about coincident with the formation of new sea ice. Within 400 m of shore, it appeared to freeze to an underlying ice-bonded permafrost table (IBPT). Farther offshore, where this table is deeper, the active layer thickness decreased with distance offshore, and the layer was underlain by a talik. Relative ice contents in the active layer generally decreased with distance offshore, were a few hundred parts per thousand (ppt) in the fall, and ranged to more than 800 ppt in the spring. Seasonal changes in the bulk soil solution salinity showed that the partially frozen active layer redistributed salts during freezing, was infiltrated by concentrated brines derived from the growth of sea ice, and affected the timing of brine drainage to lower depths in the sediments. These brines provide the salts required for thawing the underlying subsea permafrost in the presence of negative sediment temperatures. The region near shore had a partially frozen transition layer just above the IBPT where the ice content increased and the brine content decreased with depth. Temperature at the IBPT is nearly constant beyond about 412 m to at least 3.5 km offshore at about -2.41øC, implying relatively constant soil solution salinities there. INTRODUCTION Subsea permafrost exists in the continental shelves of theArctic Ocean and associated seas, where it is a product of changing sea levels, shoreline erosion, and past cold climates r, ._ _, ...... ,, ....... , Lewetten, t•v•acnay, 1972; 1973]. Whea sea Icvc• mc low, cold climates cause permafrost to aggrade in those parts of the shelves where the ground surface is exposed to the atmosphere. When sea levels rise, the permafrost is covered by relatively warm and salty seawater. After submergence, this subsea permafrost degrades, thawing from the seabed downward by the influx of salt and heat as a result of the new boundary conditions, even in the presence of negative mean seabed temperatures [Harrison and Osterkamp, 1978]. Subsea permafrost also thaws from the bottom by geothermal heat flow. Northwest of Prudhoe Bay at distances up to several kilometers or more offshore, thawing rates at both the top and bottom are slow, of the order of a centimeter per year [Lachenbruch et al., 1982]. Consequently, the time after submergence required to thaw the subsea permafrost completely may approach tens of millennia. Thawing at the seabed results in a talik or thawed layer, which thickens seaward, and ice-bearing subsea permafrost which generally thins seaward [Hunter et al., 1976; Osterkamp and Harrison, 1977; Lachenbruch et al., 1982; Neave and Sellmann, 1984; Osterkamp et al., 1985]. Development of a talik below the seabed after submergence is complicated by s...

Characterisation of marine and lacustrine sediments in a drowned thermokarst embayment, Richards Island, Beaufort Sea, Canada

“…Salinities in marine cores taken in shallow water (i.e. which experience seasonal freezing) have been known to exceed 100 as a result of brine concentration during the freezing process (Osterkamp et al 1989).…”

Nearshore arctic subsea permafrost in transition

“…Sea ice and bed freezing, salt exclusion and downward brine movement created an unfrozen layer between 2 and 2.8 mbsl. Such hypersaline, partly unfrozen layers (cyropegs) within ice-bonded permafrost have also been described for the region off Prudhoe Bay (Osterkamp et al 1989). …”

“…Romanovskii 2003, Lachenbruch 1982) and geophysical data (Rekant et al 2005) but some offshore drilling results are available as well (e.g. Dallimore 1991, Osterkamp et al 1989). …”

Models for convection in thawing porous media in support for the subsea permafrost equations

“…Finally, our model is based on the position that the thawed layer consists of sediments and brine (i.e., saltwater) but that no ice is simultaneously present: The pore fluid is no mush, and neither is the layer a binary mixture of sediments, brine, and frozen brine. There are observations [Osterkamp et al, 1989] that this may locally be the case; seasonal variations are apparently the reason for the freezing of the brine just below the ocean base and, later, remelting of this ice. Accompanied this is a rejection of salt and an increase of the salinity of the pore fluid, salt fingering due to increased gravity, and a rise of the the remolten fresher water at the warmer seasons.…”