HR: 10:40h
AN: OS32A-02 INVITED     [Abstracts]
TI: Is There Evidence of Gas Venting From Pockmark Fields?
AU: * Paull, C K
EM: paull@mbari.org
AF: MBARI, 7700 Sandholdt Road, Moss Landing, CA 95039-9644
AB: We have conducted geochemical and geologic surveys of pockmarks within three large pockmark fields: (1) Big Sur, off the California coast; (2) Belfast Bay, Maine; and (3) on the northern flank of the Storegga Slide on the Norwegian Margin. Our objective was to assess whether fluid and/or gas was actively venting from these pockmarks or had vented sometime during their development. The term pockmark has been used primarily to describe crater-like depressions in the seafloor that occur in fine-grained clastic sediments. While some use the term pockmarks to describe smaller features, we use it to describe approximately circular features at least 50 m in diameter and more than 5 m deeper than the surrounding seafloor. Interest in pockmarks has surged recently because of the increasing availability of high quality multi-beam data which demonstrate that pockmarks are common continental margin geomorphic features. Pockmarks occur on the continental slopes and rises, in shallow bays on continental shelves, and lakes as discrete features, or as coalescing structures forming extensive pockmark fields. The origin of submarine pockmarks is enigmatic. Mechanisms are required to remove sediment from pockmarks, or at least to prevent material from being deposited within them. Pockmarks are commonly attributed to some form of fluid and/or gas discharge, however the nature of the venting is poorly defined or undocumented for most pockmarks. Very little ground-truthing has been conducted to verify these assertions. Concentrations of methane, dissolved inorganic carbon, sulfate, and chloride in pore water extracted from cores collected within and around pockmarks at the three sites are generally similar in composition to seawater in the shallow subsurface and do not decrease rapidly with depth. These pore water chemical gradients indicate that neither significant accumulations of gas are likely to exist in the shallow subsurface nor is active fluid advection occurring within the sampled sediments. Water column surveys conducted at Big Sur and Belfast Bay did not detect methane anomalies indicative of active venting. ROV investigations indicated that the flanks and floor of Big Sur pockmarks are covered with a drape of sediment and are visually indistinguishable from seafloor outside the pockmarks. Lithologies of the sediments inside and outside the pockmarks are generally comparable at all three sites. In one case thin shell-bearing horizons containing Bathymodiolus mussels (known to contain methanotrophic endosymbionts) overlying lightly cemented methane-derived carbonate nodules occur 6-7 mbsf in a piston core from inside a pockmark on the Norwegian margin. This occurrence indicates that methane was previously available on the seafloor at this site. However, it does not require that gas venting excavated this pockmark. We're left wondering if there is a causal relationship between pockmark formation and gas venting. The presence of methane may be an inevitable consequence of seafloor excavation by other mechanisms. The data we have collected to date do not substantiate that active advection is occurring from any of these pockmarks. The absence or limited occurrence of either sedimentologic and/or diagenetic features indicative of the persistence of high methane concentrations makes us wonder whether these features were created or maintained via gas venting.
DE: 0448 Geomicrobiology
DE: 1051 Sedimentary geochemistry
DE: 1625 Geomorphology and weathering (0790, 1824, 1825, 1826, 1886)
DE: 3002 Continental shelf and slope processes (4219)
DE: 3004 Gas and hydrate systems
SC: Ocean Sciences [OS]
MN: Fall Meeting 2005