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

1 kg of tubeworms in wet weight. The Pogonophora manage to aerate the sediments,
hence excluding the AOM communities from the surface sediments. Our
investigations will show whether there are subsurface AOM communities in this zone
of the HMMV. Clearly, in the zones populated by the chemosynthetic communities
only very little methane escapes to the water column compared to the barren centre.
Obviously, the methane-consuming microorganisms form an effective barrier against
the greenhouse gas methane.
The relatively high biomass of methanotrophic archaea is obviously capable of
oxidising methane with sulfate in the anaerobic sediments at temperatures close to
the freezing point (-1°C), producing a source of sulfide to the extensive mats of giant,
sulfide-oxidising bacteria surrounding the central area. Despite its rapid turnover in
the sediments, large amounts of methane dissolved in the rising mud-volcano fluids
are seeping to the hydrosphere. It is yet unknown how much of the methane is
removed in the aerobic bottom waters and how much escapes to the water column.
First experiments on the aerobic oxidation of methane in the bottom waters of HMMV
show that methane turnover may be limited to the bottom water and is below
detection limit in the water column as predicted by Damm et al. (2003) from methane
stable isotope signatures. Most interestingly, methane occurs in relatively high
concentrations also in the central barren area of the HMMV but is used only by a
shallow microbial community. It is possible that in these relatively young sediments,
the population of slow growing methane oxidising archaea is too small to provide
enough sulfide for the support of sulfide dependent chemosynthetic communities.
Accordingly, the anaerobic methanotrophs were absent in the centre surface
sediments and only found deep below the Pogonophora communities populating the
outer rim of the mud volcano.
In addition to the sampling program dealing with surface sediments, we took samples
from 7 gravity cores, to investigate the presence of microbial communities down to 5
m below the seafloor. Gas hydrates occur in fine layers to bulk ice of several cm
thicknesses at the HMMV in the outer zone of the warm centre, at depths up to 25 cm
(Tab. B5.1-3). Towards the outer rim of the HMMV populated by Pogonophora, the
depth of the gas hydrate layer increases to about 2-5 m. A first result from the gravity
coring is that the subsurface sediments of the centre show very little evidence for
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