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

B. 5.3 Methane in gas hydrate bearing sediments – turnover rates and
microorganisms (MUMM)
Witte, U.
Mud volcanoes like the HMMV are present at tectonically inactive areas of continental
margins and can occur independently of volcanic sulfide and heat above gas and
petroleum reservoirs buried in the sea floor. Biogenic methane is formed in deep
sediment strata and rises to the surface of the seafloor. The gas may accumulate in
the sediments and form gas reservoirs such as gas hydrates (frozen methane). If
these gas reservoirs reach a certain pressure, they can form geological structures
called mud volcanoes. At mud volcanoes, sediment pore water, gas and mud is
expelled from deep below forming mounds and crater at the sea floor. Active mud
volcanoes are a seep for natural gas (methane) and are often densely populated by
tube worms, clams and other symbiotic organisms – just like hot vents. Hence,
chemotrophic communities, i.e. organisms which are fuelled by the chemical energy
of dissolved minerals can indicate the presence of active gas seeps. The rising
methane is often very efficiently used by a symbiosis of archaeal and bacterial
microorganisms that is able to oxidise methane with sulfate - which is abundant in
seawater. This reaction produces sulfide, which in turn fuels conspicious
chemosynthetic communities (sulfur bacteria, clams and tube worms with bacterial
symbionts).
From the HMMV, three main habitats were described from previous cruises: a central,
barren area of sediment not (yet ?) colonized by sulfur-oxidising communities, areas
covered by dense Beggiatoa mats, and Pogonophora fields. The investigations
showed that a lot of methane is emitted from the barren centre of the mud volcano. In
areas covered by chemosynthetic communities, on the other hand, only very little
methane was observed to escape to the water column. Possibly, the methaneconsuming
microorganisms that produce the sulfide fuelling these communities form
an effective barrier against the greenhouse gas methane.
In addition, an attempt was made to quantify biological turnover rates and identify key
horizons for the respective proceses involved, in order to:
a. quantify total benthic carbon turnover as well as sulfide production and
consumption rates, and finally generate a numerical transport-reaction model
coupling the processes involved,
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