Berndt, Christian , Dumke, Ines, Feseker, Tomas, Graves, C., Franek, P., Hissmann, Karen, Hühnerbach, Veit, Krastel, Sebastian, Lieser, Kathrin, Niemann, H., Steinle, Lea and Treude, Tina (2014) Fluid dynamics and slope stability offshore W-Spitsbergen: Effect of bottom water warming on gas hydrates and slope stability - Cruise No. MSM21/4 - August 12 - September 11, 2012 - Reykjavik (Iceland) - Emden (Germany) . . MARIA S. MERIAN-Berichte, MSM21/4 . DFG-Senatskommission für Ozeanographie c/o MARUM, Bremen, 97 pp. DOI 10.2312/cr_msm21_4.

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Abstract

The main goal of MSM21/4 was the study of gas hydrate system off Svalbard. We addressed
this through a comprehensive scientific programme comprising dives with the manned
submersible JAGO, seismic and heat flow measurements, sediment coring, water column
biogeochemistry and bathymetric mapping. At the interception of the Knipovich Ridge and
the continental margin of Svalbard we collected seismic data and four heat flow
measurements. These measurements revealed that the extent of hydrates is significantly larger
than previously thought and that the gas hydrate system is influenced by heat from the oceanic
spreading centre, which may promote thermogenic methane production and thus explain the
large extent of hydrates. At the landward termination of the hydrate stability zone we
investigated the mechanisms that lead to degassing by taking sediment cores, sampling of
carbonates during dives, and measuring the methane turn-over rates in the water column. It
turned out that the observed gas seepage must have been ongoing for a long time and that
decadal scale warming is an unlikely explanation for the observed seeps. Instead seasonal
variations in water temperatures seem to control episodic hydrate formation and dissociation
explaining the location of the observed seeps. The water column above the gas flares is rich in
methane and methanotrophic microorganisms turning over most of the methane that escapes
from the sea floor. We also surveyed large, until then uncharted parts of the margin in the
northern part of the gas hydrate province. Here, we discovered an almost 40 km wide
submarine landslide complex. This slide is unusual in the sense that it is not located at the
mouth of a cross shelf trough such as other submarine landslides on the glaciated continental
margins around the North Atlantic. Thus, the most widely accepted explanation for the origin
of such slides, i.e. overpressure development due to deposition of glacial sediments on top of
water rich contourites, is not applicable. Instead we find gas-hydrate-related bottom
simulating reflectors underneath the headwalls of this slide complex, possibly indicating that
subsurface fluid migration plays a major role in its genesis.

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