HR: 11:25h
AN: OS32A-05    [Abstracts]
TI: Geophysical and Geochemical Evidence For Methane Venting at Large Gas Blowouts Along the US Mid-Atlantic Shelf Edge
AU: * Newman, K K
EM: knewman@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964 United States
AU: Cormier, M
EM: cormier@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964 United States
AU: Driscoll, N
EM: ndriscoll@ucsd.edu
AF: Scripps Institution of Oceanography, UCSD, 9500 Gillman Drive, La Jolla, CA 92093 United States
AU: Hill, J
EM: jchill@ucsd.edu
AF: Scripps Institution of Oceanography, UCSD, 9500 Gillman Drive, La Jolla, CA 92093 United States
AU: Kastner, M
EM: mkastner@ucsd.edu
AF: Scripps Institution of Oceanography, UCSD, 9500 Gillman Drive, La Jolla, CA 92093 United States
AU: Singh, H
EM: hsingh@whoi.edu
AF: Wood Hole Oceanographic Institution, MS 7, Woods Hole, MA 02543 United States
AU: Weissel, J
EM: jeffw@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964 United States
AB: Kilometer-scale, elongate gas blowouts are present at the edge of the North Carolina/Virginia continental shelf. We conducted a detailed survey in July 2004 to determine if fluids are venting at the blowouts site and to understand their origin. The Autonomous Underwater Vehicle (AUV) SeaBED collected underway data 3 m above the seafloor using a METS methane sensor, CTD and color digital camera. In addition, piston cores and hydrocasts were acquired for geochemical analysis of pore waters and the water column. Based on the AUV data, salinity and temperature exhibit a negative correlation with dissolved methane concentration. However, the raw METS measurements of dissolved methane lag behind the salinity and temperature anomalies, progressively ramping up or down compared to the impulse signal recorded for the salinity and temperature anomalies. This type of response is consistent with that expected for diffusion across a membrane, which is a characteristic of the METS sensor. Using the assumption that diffusion is responsible for the observed lag we calculated the time constant of the system to be approximately 11 minutes and used that to correct the instrument response function for the METS sensor. The corrected dissolved methane measurements show concentrations of 50-200 nM, values well above that of normal seawater (2-4 nM). Hydrocast water samples indicate methane maxima between 100 and 130 m with concentrations up to 43 nM. The positive anomalies, both from the AUV and hydrocast data, are concentrated on the upper parts of the blowout walls, extend westward onto the shelf and are observed up to 70 m depth in the water column. Methane anomalies are not generally present in the axes of the blowouts, suggesting that methane presently discharges along the blowout walls rather than through the floors. To determine if density driven stratification is present, and assuming that salinity and temperature are good proxies for methane concentration, we examined vertical CTD profiles through the water column. In some profiles we see a distinct step in the salinity and temperature data that is consistent with the (T, S) signal observed in the along-track data where methane anomalies are present. The step is absent in other profiles. Based on examination of the salinity and temperature data we propose that a stratified water mass does not exist within the blowouts and that methane is locally venting at the blowout walls and is being transported by currents. DIC carbon isotopes indicate anaerobic methane oxidation (AMO), which records intense methane flux through the sediment. AMO is the dominant reaction responsible for sulfate reduction, and is also consistent with high methane fluxes.
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1055 Organic and biogenic geochemistry
DE: 4271 Physical and chemical properties of seawater
DE: 4294 Instruments and techniques
SC: Ocean Sciences [OS]
MN: Fall Meeting 2005