A potential link between fluid expulsion and slope stability: Geochemical anomalies measured in the gas blowouts along the U.S. Atlantic margin provide new constraints on their formation

Summary

Jenna C. Hill, Neil W. Driscoll, Jeffrey K. Weissel, Miriam Kastner, Hanuman Singh, Marie-Helene Cormier, Richard Camilli, Ryan Eustice, R. Lipscomb, Neil McPhee, Kori Newman, G. Robertson, E. Solomon and K. Tomanka, A potential link between fluid expulsion and slope stability: Geochemical anomalies measured in the gas blowouts along the U.S. Atlantic margin provide new constraints on their formation. In EOS: Transactions of the American Geophysical Union Fall Meeting Supplement, 2004. (Abstract).

Abstract

Geochemical, bathymetric and AUV based surveys conducted aboard the R/V Cape Hatteras in July 2004 provided new constraints on the formation of large-scale gas blowout features located along the U.S. Atlantic margin. These features, believed to be formed by gas expulsion processes, are ~4km long, ~1km wide and up to 50m deep. The stratal geometry of these features and their location on the shelf-edge has led us to hypothesize that they may indicate incipient slope failure. Interpretation from our chirp seismic reflection data, collected in 2000, showed gas generally was trapped under a thin veneer (several tens of meters) of deltaic sediments, but may be venting along the landward wall of the blowouts. New geochemical data indicate significant methane anomalies above both the seaward and landward walls of the blowouts and reveals that these features are actively venting fluids at the seafloor. Using a METSr sensor mounted on the WHOI Seabed AUV, we observed methane concentrations ranging from 50-100nM in the water column directly above the inner and outer walls, whereas typical methane concentrations in seawater are expected to be 2-4nM. Some of these methane hot spots were also associated with salinity anomalies. Additionally, pore fluids squeezed from a series of piston cores in the blowout region show relatively high alkalinity values (>4-15mM), with a near absence of hydrogen sulfide. These initial results are particularly intriguing since high alkalinity concentrations are commonly associated with high sulfide concentrations. We speculate that there may be a flux of CO2 into the sediments that may be responsible for the high alkalinity and low sulfide. In addition to our geochemical studies, we collected a full suite of bottom photographs, gravity cores, and high resolution bathymetry. Visualization of these data in three dimensions, along with methane concentration profiles, chirp reflection, and sidescan-sonar data has enabled us to build a relatively comprehensive picture of the blowout features. There are strong spatial correlations between trapped gas and the overlying shelf-edge delta deposit, as well as with relatively high methane concentrations in the water column, and indications of inner wall venting in the chirp profiles. Nevertheless, a distinct spatial correlation between the occurrence of biological communities and the fluid expulsion sites was not observed. The active fluid expulsion we measured is consistent with our geophysical observations, and supports our hypothesis that there is a link between upslope fluid migration, downslope creep, and potential slope failure.

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