HR: 1340h
AN: V13B-1349    [Abstracts]
TI: Stability of Gas Hydrates at Mud Volcanoes and Methane Seepage Sites in the Gulf of Cadiz: Correlation with Past Oceanographic Changes
AU: * MagalhàĢes, V H
EM: vmagalhaes@uchicago.edu
AF: Department of the Geophysical Sciences, The University of Chicago, 5734 S. Ellis Avenue, Chicago, IL 60637, United States
AU: * MagalhàĢes, V H
EM: vmagalhaes@uchicago.edu
AF: Departamento de GeociàŠncias e CESAM, Universidade de Aveiro, Campus de Santiago, Aveiro, 3810-193, Portugal
AU: * MagalhàĢes, V H
EM: vmagalhaes@uchicago.edu
AF: Departamento de Geologia Marinha, INETI, Estrada da Portela, Apart. 7586, Alfragide, 2721-866, Portugal
AU: Buffett, B
EM: buffett@geosci.uchicago.edu
AF: Department of the Geophysical Sciences, The University of Chicago, 5734 S. Ellis Avenue, Chicago, IL 60637, United States
AU: Archer, D
EM: d-archer@uchicago.edu
AF: Department of the Geophysical Sciences, The University of Chicago, 5734 S. Ellis Avenue, Chicago, IL 60637, United States
AU: Pinheiro, L M
EM: lmp@geo.ua.pt
AF: Departamento de GeociàŠncias e CESAM, Universidade de Aveiro, Campus de Santiago, Aveiro, 3810-193, Portugal
AB: The Gulf of Cadiz (GC) is a highly sensitive area to paleoceanographic changes, characterized by the occurrence of mud volcanoes, diapiric ridges, pockmarks and methane seepages, both active and inactive. The high methane content in shallow sediments and the presence of gas hydrates on the most active structures indicates that these are preferential areas for the escape of fluids enriched in hydrocarbons, especially methane. Extensive fields of methane-derived authigenic carbonates (MDAC) were found along the upper and mid-continental slope, where the Mediterranean Outflow (MO) water is in direct contact with the seafloor. The estimated ages of the MDAC indicate formation over discrete episodes that correspond to periods of rapid paleoceanographic changes (such as the onsets of glacial stages terminations). In this work, the gas hydrates stability zones (GHSZ) are investigated at 7 sites where MDAC occur. Calculations of the depths of the GHSZ were done considering gas compositions, as obtained from the gas compositional values of the active mud volcanoes in the GC, ranging from two end-members: from a pure biogenic origin, with 100% of methane, to the end-member with 20% of heavier hydrocarbons, considered a mixture of biogenic and thermogenic origins. The depths of the GHSZ were calculated for different paleoceanographic scenarios: present day conditions, with variable intensities of the MO, and estimated conditions for the Last Glacial Maximum. Results indicate that the transition from glacial to interglacial conditions reduces the depth of the GHSZ by more than 46%, for pure methane gas composition; and by more than 35%, for gas composition of 80% of methane and 20% of heavier hydrocarbons. At several sites the stability zone disappears entirely for both gas compositions. The temperature increase associated with the beginning of the MO influence can reduce the depth of the GHSZ by more than 23% and 17%, considering respectively pure methane composition or a mixed gas with 20% of heavier hydrocarbons. At two of the studied sites the stability zone can be entirely extinguished. Increases in the seafloor temperature associated with these two processes can efficiently trigger episodes of dissociation of potential gas hydrates that would result in intense flux of methane rich fluids to shallow sediments or even into the seabottom.
DE: 3002 Continental shelf and slope processes (4219)
DE: 3004 Gas and hydrate systems
DE: 3653 Fluid flow
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting