Preface The expedition ARK XIX/3 with the German icebreaking RV ...

sediment water interface. A variety of redox couples interact such as methane and
sulfate, methane and oxygen, sulfide and oxygen, sulfide and nitrate. Sharp gradients
in DIC form by the fast respiration of methane. Furthermore, the gassy fluids rising
from the deep and their lateral advection leave imprints in the geochemistry of the
subsurface sediments. The formation of hydrates within the stability field at cold
seeps causes strong deviations from typical continental margin sedimentary
properties. A variety of cold seeps are currently studied around the world, but little
research has been carried out at high latitudes. This is mainly because only a few
structures have been observed so far, and much more exploration is needed to
identify active cold seeps in polar systems.
The Håkon Mosby Mud Volcano (Fig. A1-2) was discovered in 1990 by Vogt et al.
(1991) during side scan sonar mapping. An expedition in 1995 R/V "Håkon Mosby",
recovered tubeworms, indicating active chemosynthesis, measured very high
temperature gradients in the sediments, and recovered methane hydrate from 2 m
subbottom depth (Vogt et al. 1997). This was the first indication of an active cold seep
and of shallow hydrate recovered in the Nordic Seas. These discoveries led directly to
more extensive studies carried out on the Russian vessel "Professor Logachev", and
the results were published as a special volume of Geo Marine Letters (Vogt et al.
1999), with papers on hydrates, porewater chemistry, sea-floor geology, seep biota,
heat flow, and other topics.
The HMMV site is located at the Norwegian–Barents–Spitsbergen continental margin,
which is characterised by major submarine slides, large-scale mass wasting and
smaller seafloor features (Vogt et al. 1999). The HMMV is situated in a submarine
valley on the Børnøya fan, a large complex composed of glacial sediments, which
covers the entire continental slope and reaches a thickness of more than 3 km
beneath the mud volcano (Hjelstuen et al., 1999). The presence of free gas deeper
than 3 km below the HMMV is indicated by multichannel seismic data (Hjelstuen et
al., 1999). The HMMV is about 1 km in diameter and rises up to 10 m above the
seafloor, to bottom depths of 1255-1265 m (Vogt et al., 1997). The HMMV is of a
concentric structure with highly gas-saturated sediments. In the flat central a zone of
grey fluid sediments with a high geothermal gradient (Crane et al., 1997) is
surrounded by an area with bacterial mats over gas hydrates. This central region is
- 155 -