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

at a water depth of 1250 m. It has a diameter of about 2 km, with an outer rim populated
by methane-depending, chemosynthetic communities and an inner centre of about 500 m
diameter where fresh muds are expelled. Between the central plain and the outer rim, a
complex topography of hills and depressions can be observed, derived from the transport
of young sediments. Methane is rapidly oxidized with sulphate in the anaerobic sediments
at temperatures close to the freezing point (-1°C), producing a source of sulphide to the
extensive mats of giant, sufide-oxidizing bacteria surrounding the central area. Despite its
rapid turnover in the sediments, large amounts of methane dissolved in the rising mudvolcano
fluids are seeping to the hydrosphere. It is unknown how much of the methane is
removed in the aerobic bottom waters and how much escapes to the water column. The
HMMV represents an ideal model system to study methane fluxes in polar seas. So far,
only few selected key locations have been sampled with the ROV (centre of the HMMV
crater, the south and southeast of the crater and the surrounding area). For a 3-D
modelling of methane fluxes and turnover rates at the HMMV as one geological model
system representative of a focussed methane source to the sea, there is a need to
accomplish a more thorough sampling. Complementary, intense geographical surveying
of the different areas at the HMMV was planned for the coming weeks to estimate the
aerial coverage of the different chemosynthetic communities around the centre with help
of the mosaicking technology of "Victor 6000". Based on ROV video surveys and
mosaicking technique the spatial distribution of key features at the HMMV should be
identified and mass budgets about the distribution of bacterial mats could be established.
Sediment and water samples taken with the ROV were intended to be used to further
improve present knowledge on methane turnover and the distribution patterns of bacteria
and archaea, as well as the rare methanotrophic-symbiont bearing tube worms.
Measurements of natural radiotracers such as Radon, Radium and Helium can help to
elucidate the flux and the fate of methane in the bottom water column. Sampling of
carbonates was planned to investigate the contribution of methane turnover to carbonate
formation (also an interest of GEOMOUND). Measuring microbial degradation of methane
in water column and sediments using radio labelled tracers was intended for the second
cruise leg as well as sediment samples to be taken with both, the multicorer and the giant
box corer. Additionally, it was attempted to sample gas hydrates with deeper penetrating
gravity corers. Water samples taken with two types of water samplers (rosette and
horizontal water sampler) were proposed to investigate the spatial distribution of the
methane plume in the water column. In situ experiments with micro-profilers measuring
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