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 −2.41°C, implying relatively constant soil solution salinities there.