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| Titel |
New global estimates of marine gas hydrate accumulations based on POC degradation and reaction-transport modeling |
| VerfasserIn |
Ewa Burwicz, Lars Ruepke, Klaus Wallmann, Arne Biastoch |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250041116
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| Zusammenfassung |
| This study provides new estimates for the global methane hydrate inventory based on
reaction- transport modeling. A multi-1D model for POC degradation, gas hydrate formation
and dissolution is presented. The novel model contains an open three-phase system of two
solid (organic carbon, gas hydrates), three dissolved (methane, sulfates, inorganic carbon) and
one gaseous (free methane) compounds.
The model computes time-resolved concentration profiles for all compounds by
accounting for chemical reactions as well as diffusive and advective processes. The reaction
module builds upon a kinetic model of POC degradation based on the approach of Wallmann
K. et al., 2006. Various chemical reactions such as organic carbon decay, Anaerobic
Oxidation of Methane, methanogenesis, and sulfate reduction are resolved using time
and space dependent kinetic constants. The solid (and gaseous) phase is buried via
sediment deposition and compaction. Fluid expulsion from compacting sediments
results in a relative upward migration of dissolved species with respect to the solid
phase. Chemical species dissolved in pore waters are able to diffuse through entire
sediment profile adequate to their molecular diffusion coefficients. Gas hydrate and
free gas formation occur if the concentration of dissolved methane exceeds the
pressure, temperature, and salinity-dependent solubility limits of hydrates and/or free
gas.
Global input grids have been compiled from a variety of oceanographic, geological and
geophysical data sets. Bathymetry, bottom water temperatures and salinities are extracted
from an Ocean Circulation Model (OCM) simulation run in the ORCA_R025 configuration
(Barnier B. et al., 2006) and represent a combination of ETOPO1 and GEBCO data sets.
Geothermal gradients are based on the heat flow database provided by International Heat
Flow Commission (IHFC). Sediment thicknesses are implemented according to NOAA data.
Global TOC distribution is compiled from wide range of sediment cores data from Seiter K.
et al., 2004.
We find that the global distribution of methane hydrates does not correlate in a simple
way with the thickness of the hydrate stability zone but is a complex function of all input and
model parameters. Prominent gas hydrate provinces are found offshore Central America
where sediments are rich in organic carbon and in the Arctic Ocean where low bottom water
temperatures stabilize methane hydrates. Our new total estimates of the world’s marine
hydrate inventory formed due to POC degradation give a number of ~3x1015m3
of CH4 (at STP conditions). These findings are in good agreement with previous
studies based on direct observations (Milkov A. V., 2004) and show that numerical
modeling is a valuable tool for studying the worldwide distribution of methane
hydrates.
Barnier B. et al., 2006. Impact of partial steps and momentum advection schemes in a
global ocean circulation model at eddy-permitting resolution. Ocean Dynamics 56,
543-567.
Milkov A. V., 2004. Global estimates of hydrate-bound gas in marine sediments: how
much is really out there? Earth-Science Reviews 66, 183-197.
Seiter K. et al., 2004. Organic carbon content in surface sediments-defining regional
provinces. Deep-Sea Research I 51, 2001-2026.
Wallmann K. et al., 2006. Kinetics of organic matter degradation, microbial methane
generation, and gas hydrate formation in anoxic marine sediments. Geochimica et
Cosmochimica Acta 70, 3905-3927. |
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