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| Titel |
In-situ inventory of gas and gas hydrates in deposits of the Håkon Mosby mud volcano, SW Barents Sea |
| VerfasserIn |
T. Pape, T. Feseker, S. Kasten, D. Fischer, F. Abegg, H.-J. Hohnberg, G. Bohrmann |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250031633
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| Zusammenfassung |
| Submarine mud volcanoes are an important source and reservoir of methane and other
low-molecular weight hydrocarbons (LMWHC). Consequently, mud volcanoes located
within the gas hydrate stability zone (GHSZ) are known to host shallow buried gas
hydrates in high density. However, gas hydrate inventories in shallow mud volcano
deposits are strongly affected by changes in local physico-chemical conditions due to
episodic volcanic activity and response by fixation or release of abundant portions of
LMWHC.
We determined the in situ gas inventories in shallow deposits of the Håkon Mosby mud
volcano (HMMV, SW Barents Sea) in approx. 1,250 m water depth by quantitative degassing
of pressure cores recovered with our Dynamic Autoclave Piston Corer. As recognized during
previous studies of the HMMV, a concentric arrangement of geochemical parameters and
topographic features allow for the distinction of three geomorphological units (I, II, III).
During our cruise in summer 2008 we recovered pressure cores (up to 2.65 m below seafloor,
b.s.f.) from all the three units and volumetric gas–sediment (wet) ratios ranged between 2.6 in
a core taken at the northwestern outer rim (Unit III), and 25.2 obtained for a core in the
northeastern section (Unit II). Gas sub-samples collected during degassing of pressure
cores belonging to the three units showed C1/C2+ ratios >1,000 suggestive of a
predominantly microbial LMWHC origin. Hydrate stability calculations based on
LMWHC distributions, pore water salinities and bottom water temperatures suggest that
structure I hydrates are the most stable crystallographic hydrate structure at the
HMMV.
Pore water chloride and sulfate profiles combined with in situ temperature data
were used to delineate local boundaries of hydrate accumulations in each of the
three geomorphologic units. Subsequently, gas volumes in pressure cores were
referred to core segments comprising gas hydrates, and hydrate concentrations were
calculated.
Low gas hydrate densities of about 5.2% pore volume were found for a station at the active
center (Unit I), where the rapid ascent of warm fluids and mud causes high temperatures of up
to more than 20°C in near-surface sediments. Hydrate precipitation is assumed to occur in
the uppermost much colder sediments. Beyond the center, the transition between
warm mud and fluids and the cold bottom water of around –0.8°C expands due to
decreasing rates of upward fluid flow, and the GHSZ extends up to several meters b.s.f
towards the edges of the morphological unit I. At those sites the upper hydrate
limit is controlled by the anaerobic oxidation of methane (AOM), which in turn
is controlled by the penetration depth of seawater-derived sulfate. Nevertheless,
segments beyond the AOM zone of cores belonging to Unit II (n = 4) contained
highest hydrate concentrations of 24.1% of pore volume on average. In contrast,
the hydrate bearing segment of a core taken in sediments belonging to Unit III
comprised comparably low abundances of 2.9% ps. These hydrate concentrations
are taken to extrapolate the overall hydrate volumes stored in shallow deposits of
the different morphological zones and, consequently, of the entire mud volcano
structure. |
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