Ninth international conference on - Marum

64
Abstracts of oral presentations
The gas hydrate reservoir of carbon is also characterised by some other features. It exist only in the glacial
epochs. Warm epochs are not favorable both for the formation and conservation of gas hydrates. In the glacial
epochs the zone of stability of gas hydrates (ZSGH) is widespread including the ocean floor. Hovewer, GHs
turned up to be spread extremly enevenly. Large accumulations of GHs are discrete within the limits of ZSGH
both in space and in the sediment section. The distribution of large GH accumulations is controlled by deep
tectonic (geodynamic) factors and by the localized flows of deep hydrocarbon fluids (Dmitrievsky and Valyaev,
1998, 2001; and others).
In many cases these localised flows appear to be through and are manifested on the sea bottom as hydrocarbon
discharges (mud volcanoes, seepages, etc.). In these discharge sites subbottom GH fields are formed and specific
microbial consortiums are developed as well. According to the data of deep water drilling within hydrocarbon
discharges microbial consortiums are discovered in the sediment sections of ZSCH on different depths. High
intensity of deep microbial life is discovered near the bottom of ZSGH – BSR – as well (Parkes et al, 2000; and
others). Favourable conditions for the development of deep microbial life, caused by the energy of anaerobic
oxidation of methane (hydrocarbon) are typical not only for ZSGH bottom.
As it has turned out in the formation of GH accumulations on the different depths of the section of ZSGH pore
sediments are filled with GHs only partially (up to 10-20 %, sometimes more). In that way in the medium of
pore waters, saturated by the methane, the stability of GHs is not absolute, it is relative. Typically GH
accumulations do not form the unique stratum – monolith, but they are spread in the form of numerous specific
inclusions (different volumes) in the interval of sediment sections of ZSGH favourable for the formation of gas
hudrates. Thus, the processes of the formation and the destruction of GH accumulations are accomponied by the
development of different microbial consortiums. Gas hydrate accumulations and GH reservoirs on the whole are
formed only owing to the utilization of the residial part of the ascending localized flows of hydrocarbon fluids.
The methane excreted as a result of the destruction of GHs may migrate either up into the sea water or may be
utilized by the microbial consortiums of the sediments. The important role of the GH reservoir of carbone in the
stabilization of the conditions for the existance of microbial consortiums (deep life) in the marine sediments is
evident. Gas hydrate reservoirs of carbon in the marine sediments should be considered as the most important
reservoir of the carbon for the deep biosphere.
Methane emission and associated biogeochemical consumption rates: How important are cold
seep geostructures on a local and global scale
F. Wenzhöfer 1 , J. Felden 1 , A. Lichtschlag 1 , D. deBeer 1 , T. Feseker 1,2 , J.P. Foucher 2 ,
G. Bohrmann 3 , F. Inagaki 4 , A. Boetius 1,5
1 Max Planck Institute for Marine Microbiology, Bremen, Germany
2 Centre Ifremer de Brest, Depart. Geosciences Marines, BP7, 29280 Plouzane, France
3 Research Center Ocean Margins, Bremen, Germany
4 JAMSTEC, Yokosuka, Japan
5 Jacobs University Bremen, Germany
Cold seeps (e.g. mud volcanoes, pockmarks) are very interesting ecosystems, both from the biological and
geological perspective. The rising fluid, mud and gas represent a window between the deep geosphere and the
biosphere. Cold seeps can be an important natural source of the greenhouse gas methane to the hydrosphere and
sometimes even the atmosphere. Recent investigations show that the number of active submarine cold seeps
might be much higher than anticipated, and that gas emitted from deep-sea seeps might reach the upper mixed
ocean. Unfortunately, global methane emission from active submarine seeps cannot be quantified because their
number and gas emission rates are unknown. It is also unclear how efficiently methane-oxidizing
microorganisms remove the methane. With regard to greenhouse gas effects, the study of cold seeps at
continental margins is an important contribution to our understanding and quantification of the methane sources
and sinks on earth.
Concentrated around ocean margins, the focused emission of cold and often hydrocarbon-laden fluids from
subsurface reservoirs into the oceanic hydrosphere is creating highly dynamic cold-seep ecosystems at the
seafloor. These systems are shaped by a complex interplay of biological, geochemical, and geological processes.
Associated with fluid outflow at seeps are chemosynthetic communities that utilize the chemical energy of
reduced components such as H2S, CH4, and other hydrocarbons. The production of biomass by these
communities can be several orders of magnitude greater than at non-seep sites on the nearby ocean floor. These
chemosynthetic communities are nourished by the chemical energy – mostly methane - rising from subsurface
sources and forming the basis of cold-seep ecosystems. In addition to these unique and highly specialized
ecosystems, cold seeps are recognized by several morphological features, like pockmarks, hydrocarbon seeps on
top of salt diapirs, asphalt volcanoes, brine pools, mud diapirs, and mud volcanoes. These geostructures are
characterized by specific chemical environments, e.g. oxygen depletion within the first few millimeters of the