Possible relation between methane seeps at shelf-edge pockmarks and downslope methane hydrates off North Carolina and Virginia
Abstract
Water column data collected from an AUV and from shipboard hydrocasts document that methane-rich fluids are actively seeping at a series of kilometer-scale pockmarks near the shelf break offshore North Carolina and Virginia. Reprocessing of multibeam bathymetric sonar data further documents an intermittent bubble plume at the site of the highest dissolved methane concentrations. Gas plumes are also detected on the adjacent shelf from newly collected subbottom seismic profiling (CHIRP) data. However, the origin of this methane remains controversial. Although it is associated with fresher, colder water and water chemistry indicates it is biogenic in origin, these characteristics fit two models equally well. In the first one, methane is sourced from the dissociation of gas hydrates down the continental slope, triggered by post-glacial introduction of warm Gulf Stream bottom water across the top of the gas hydrate stability zone; in the second one, in-situ production of biogenic methane is derived from organic material trapped within the slope sediments. These two models have different implications for slope stability. Indeed, the dissociation of gas hydrate has been proposed to be responsible for landslides on continental slopes. The availability of a dense grid of quality multichannel seismic profiles as well as multibeam bathymetric data across an area encompassing the Late Quaternary, 10,000 km2 Currituck landslide as well as the continental slope below the shelf-edge pockmarks provide the opportunity to test these two models. We analyzed the seismic data and produced a comprehensive map of the extent of the bottom-simulating reflector (BSR), the reversed-polarity reflector marking the base of gas hydrate, as well as of landslide deposits throughout the area. In addition, the systematic analysis of the multibeam bathymetry and CHIRP data provide an inventory of active methane seeps throughout the same area. Emerging relationships will test which of the gas hydrate model or biogenic production model best explains the active methane venting at the shelf edge.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFMOS13C1549J
- Keywords:
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- 3025 MARINE GEOLOGY AND GEOPHYSICS / Marine seismics;
- 3045 MARINE GEOLOGY AND GEOPHYSICS / Seafloor morphology;
- geology;
- and geophysics;
- 3070 MARINE GEOLOGY AND GEOPHYSICS / Submarine landslides;
- 4219 OCEANOGRAPHY: GENERAL / Continental shelf and slope processes