HR: 1340h
AN: B43B-0148    [Abstracts]
TI: Methane Sources, its Fate, and Output From Cold Seeps in Jaco Scarp, an Embayment Caused by Seamount Subduction Offshore Costa Rica
AU: * Mau, S
EM: smau@ifm-geomar.de
AF: Sonderforschungsbereich 574, Kiel University, Wischhofstr. 1-3, Kiel, 24148 Germany
AU: Rehder, G
EM: grehder@ifm-geomar.de
AF: Sonderforschungsbereich 574, Kiel University, Wischhofstr. 1-3, Kiel, 24148 Germany
AU: Rehder, G
EM: grehder@ifm-geomar.de
AF: IFM-GEOMAR, Leibniz-Institut fr Meereswissenschaften, Wischhofstr. 1-3, Kiel, 24148 Germany
AU: Soeding, E
EM: esoeding@ifm-geomar.de
AF: Sonderforschungsbereich 574, Kiel University, Wischhofstr. 1-3, Kiel, 24148 Germany
AU: Sahling, H
EM: hsahling@uni-bremen.de
AF: DFG Forschungszentrum Ozeanr„nder, Bremen University, Am Fallturm 1, Bremen, 28359 Germany
AU: Suess, E
EM: esuess@ifm-geomar.de
AF: Sonderforschungsbereich 574, Kiel University, Wischhofstr. 1-3, Kiel, 24148 Germany
AU: Suess, E
EM: esuess@ifm-geomar.de
AF: IFM-GEOMAR, Leibniz-Institut fr Meereswissenschaften, Wischhofstr. 1-3, Kiel, 24148 Germany
AB: Methane concentrations and their stable carbon isotopic composition were measured in a scarp created by massive landslides caused by the subduction of a seamount offshore Costa Rica in order to gain more knowledge of the venting activity of these structures and to estimate its CH$_{4}$ flux into the ocean. In this scarp (Jaco Scarp) we revealed a number of vent sites. Great amounts of methane escaped from the eastern rim of the scarp and from a sedimentary sequence in the northwest corner where concentrations reached up to 1500 nmol L$^{-1}$. Repeated sampling of these sites indicates continuous venting over time even though the internal structure of the methane plume changed, which is possibly due to opening and closing of pathways of the fluids in the sedimentary sequence. Several small sources located at sites of high accumulation of organic matter e.g. above the talus were confirmed by their light isotopic ratios. The $\delta$$^{13}$C$_{CH4}$ values of all discovered sources range between -50 $\permil$ and -62 $\permil$ pointing to bacterial methane production. The vent-derived CH$_{4}$ becomes distributed and diluted in the scarp by mixing with background water. Part of the CH$_{4}$ is oxidized in the center of the main plume where concentrations of methane are high and along the walls of the scarp. This is probably due to diminished current flow and the availability of different components. The water flow is directed mainly along the isobaths towards the north-west transporting CH$_{4}$ out of the scarp. The CH$_{4}$ flux was estimated to be 58 - 65 Mg yr$^{-1}$, comparable with the flux from hydrothermal vent sites, mud extrusions, and other cold seep areas worldwide. This illustrated that scarps are important seepage sites of natural methane, because the mass wasting associated with seamount subduction can open up deep sedimentary layers from which accumulated methane can discharge. However, the extent of methane seepage from scarps on global scale remains unknown to date.
DE: 9355 Pacific Ocean
DE: 4806 Carbon cycling
DE: 4820 Gases
DE: 4870 Stable isotopes
DE: 1050 Marine geochemistry (4835, 4850)
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting