The p in p-T is for pressure: Movement of the gas hydrate stability field during glacial sealevel lowering and its possible link to pockmark formation on the Chatham Rise, New Zealand (Invited)
Abstract
The discussion on a possible destabilization of gas hydrates caused by climate fluctuations has in recent years focused on the role of a sub-seafloor temperature increase following bottom-water warming. We here revisit the scenario that a pressure drop during glacial sealevel lowering could lead to gas hydrate dissociation. A >20,000 km2 field of seafloor depressions that we interpret as pockmarks has been identified on the southern flanks of the Chatham Rise. Three classes of pockmarks are present in two distinct water-depth ranges. The shallowest class of pockmarks with a diameter of ~150 m are present in a water-depth range of 500-700 m, close to the current top of the gas hydrate stability field. Sub-bottom profiler data show evidence for a bottom simulating reflection making it likely that gas hydrates are present beneath the seafloor. Furthermore, buried pockmarks are identified on horizons that we correlate with sealevel lowstands suggesting that pockmark formation is linked to sealevel lowering. Assuming constant bottom-water temperatures, a glacial sealevel drop by 120 m would move much of the seafloor that is covered with these pockmarks out of the gas hydrate stability field. We therefore suggest these pockmarks were formed by gas from dissociating gas hydrate due to depressurization following sealevel lowering. Two larger classes of pockmarks with diameters of 1-5 and ~10 km, respectively, are present in water depths of 800-1100 m. Here, the seafloor has probably remained within the gas hydrate stability field during sealevel lowstands. However, the associated pressure drop has moved the base of gas hydrate stability upwards by ~30 m. It is unclear whether bottom-water temperatures have changed significantly in our study area during glacial cycles - changes of 1-3° C would be required to have a similar effect on gas hydrate stability as sealevel fluctuations. The boundary between warmer subtropical and cold subantarctic waters, the subtropical front, is generally thought to be stationary through glacial cycles. Our observations suggest that on the Chatham Rise, pockmark formation and associated release of methane may be linked primarily to pressure-induced gas hydrate dissociation.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFMOS43B..01P
- Keywords:
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- 3002 MARINE GEOLOGY AND GEOPHYSICS / Continental shelf and slope processes;
- 3004 MARINE GEOLOGY AND GEOPHYSICS / Gas and hydrate systems;
- 3045 MARINE GEOLOGY AND GEOPHYSICS / Seafloor morphology;
- geology;
- and geophysics