On the possibility of deep seabed permafrost to occupy Ca. 7 million km2 in the Arctic Ocean

Arctic Ocean (AO) shelf permafrost is the most popular and best-known type of subsea permafrost. Genetically, it is analogous to the ordinary subaerial permafrost. Today, the following types of subsea permafrost are also known: i) ice-bearing sediment layers formed from porewater freshening of terrestrial origins, ii) thermokarst lake taliks refreezing under cryotic seabed conditions, iii) bottom-fast ice preserving permafrost due to efficient heat conduction from atmosphere-ice-sediment coupling in winter, and iv) similar permafrost aggradation processes occurring beneath small islands and spits evolving with sediment dynamics. Here, we propose another type of marine permafrost: a deep seabed permafrost (DSP) underlying AO bottom sediments in a cryotic but not frozen state because of salty, supercooled seawater. Primary calculations indicate that the area of DSP deeper than 150 m may reach about 7.0 million km 2 , with the contacting seawater temperature ranging from − 2.5 °C to 0 °C. The formation and persistence of DSP or, almost equivalently, the negative-temperature regime of the AO seawater at the deeper layers may have begun at the Pliocene–Pleistocene border, ca. 3.5–2.4 Ma ago.