Methane-generated(?) pockmarks on young, thickly sedimented oceanic crust in the Arctic: Vestnesa ridge, Fram strait

Abstract

Acoustic backscatter imagery in the Fram strait (between Greenland and Spitzbergen) reveals a 1-3-km-wide, 50-km-long belt of ∼50 pointlike backscatter objects decorating the ∼1300-m-deep crest of Vestnesa Ridge, a 1- >2 km thick sediment drift possibly underlain by a transform-parallel oceanic basement ridge (crustal ages ∼3-14 Ma). A 3.5 kHz seismic-reflection profile indicates that at least some objects are pockmarks ∼100-200 m in diameter and 10-20 m deep. The pockmarks (possibly also mud diapirs) may have been formed by evolution of methane generated by the decomposition of marine organic matter in the Vestnesa ridge sediment drift. The ridge may be underlain by an anticlinal carapace of methane-hydrate calculated to be 200-300 m thick, comparable to the hydrate thickness measured just to the south. The rising methane would collect in the ridge-crest trap, intermittently escaping to the sea floor. This hypothesis is supported by multichannel evidence for bright spots and bottom-simulating reflectors in the area. The pockmark belt may also be located above a transcurrent fault. Sediment slumps on the flanks of Vestnesa ridge and northeast of Molloy ridge may have been triggered by plate-boundary earthquakes and facilitated by methane hydrates.