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Community structure comparisons of lower slope hydrocarbon seeps,
northern Gulfof Mexico

I.R. MacDonald, M.Smith, F.W.Huffer

Photographic sampling collected from ROV JASON III on randomized transects (generally 10 persite) was used to compare seven suspected hydrocarbon seeps on the lower continental slope at depths of 956 to 2330 m. The study design focused on areas containing high-amplitude reflectance patterns in seismic returns from the bottom. Animal samples nested in these areas and high-resolution photographic mosaics permitted fine scale description of the community components and their associations with each other. The transect results showed three types of chemosynthetic community: carbonateprone, brinepoolprone, and low- density. These communities are characterized by, respectively, abundant carbonate rubble and tubeworm clusters, bacterial mats and brine pools, and relatively sparse chemosynthetic fauna that tended to be over looked by the photographic survey plan. These differences were not clearly related to depth or longitude because the sites are at the opposite eastern and western extents of the project region. Photographic survey is a statistically reliable method for detecting the carbonate rubble and bacterial mat types of community, but may be unreliable for sparse or clustered fauna such as seep mussels. Photographic survey results provided relatively low taxonomic resolution, but were sufficient to distinguish statistical differences in the abundance of characteristic faunal groups, such as the Echinoidea and Gorgonacea, associated with hydrocarbon seeps. Initial studies of statistical power indicate that future photographic surveys of suspected seep communities, if conducted with an equivalent level of effort, might detect the presence of more abundant habitat, including carbonate rubble, bacterial mats, and tubeworms, but could miss less abundant indicators.

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Distinct activity phases during the recent geologic history
of a Gulf of Mexico mud volcano

I.R. MacDonald, M.B. Peccini

Laser line scan imaging and chirp sub-bottom profiling were used to detail the morphology of
a submarine mud volcano and brine-filled crater at 652 m water depth in the northern Gulf of Mexico. The mud volcano has a relief of 6 m and a basal diameter of about 80 m. The feature comprises a central, brine-filled crater (253 m2) surrounded by a continuous bed of methanotrophic mussels (Bathymodiolus childressi) covering 434 m2 and a patchy bed covering an additional 214 m2 of the periphery. The brine pool was mostly <2 m deep, but there were two holes of >28 m and 12 m deep, respectively at the northern end of the pool which emitted continual streams of small clear bubbles. Sub-bottom profiles indicated three distinct strata beneath the present surface of the mud volcano. Integration of 17 profiles shows that the mud volcano has been built in at least three successive stages: the lowest stage deposited
35,400 m3, while the middle and upper stages deposited 7700 and 20,400 m3, respectively. Piston cores were taken at the northern edge of the mussel bed and a site w100 m southwest of the pool. Mussel and lucinid shells were recovered from the closer core, lucinid shells from the distant core. A mussel shell from 3.4 m sub-bottom had a D14C age of 16.2 ka. Mixture of modern carbon with ‘‘carbon dead’’ reservoir material would produce actual ages w2 ka less than the radiocarbon ages.

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