Stability of intra-permafrost hydrates in the Arctic shelf: Results of experimental work performed in the East Siberian Arctic Shelf

Abstract:

One possible reason for methane venting from the Arctic shelf could be destabilization of intra-permafrost hydrates due to inundation of permafrost by seawater a few thousand years ago. Formation of intra-permafrost hydrates presumably takes place when regression of the sea exposes a shelf above sea level; subsequently the sediments cool and freeze. Ice crystals growing within the pore and/or inter-pore space create high pressure; together with low temperatures (down to -22°C), this produces very specific temperature-pressure (T/P) conditions that allow the existence of hydrates at any depth below the seafloor/ground surface where such T/P conditions develop. An additional factor stabilizing this type of hydrate is the so-called self-preservation phenomenon; due to this phenomenon, it has been suggested that hydrates can remain stable during inter-glacial cycles (so-called metastable or relic hydrates). However, destabilization of intra-permafrost hydrates might occur due to significant change in the permafrost thermal regime as well as to salt penetration down the sediment core. To assess the stability of intra-permafrost hydrates, we examined sediment cores drilled from the sea ice in the near-shore zone of the East Siberian Arctic Shelf (ESAS) (Laptev Sea, Buor-Khaya Bay). There is a critical negative (below 0°C) temperature under which relic hydrates start to dissociate, causing volumetric expansion of dissociation-generated free gas. Experimental data revealed that salt penetration into frozen hydrate-containing sediments could affect the stability of intra-permafrost hydrates before heat propagation causes thawing of permafrost. Intrusion of salt ions can cause active dissociation of intra-permafrost hydrates and consequent releases of methane even from frozen hydrate-bearing sediments/ground. These findings are important in connection with methane releases observed in the ESAS underlain with subsea permafrost, which is experiencing change in its thermal regime due to inundation by seawater.