A photographic and acoustic transect across two deep-water seafloor mounds, Mississippi Canyon, northern Gulf of Mexico

Abstract

a b s t r a c tIn the northern Gulf of Mexico, a series of seafloor mounds lie along the floor of the Mississippi Canyon in Atwater Valley lease blocks 13 and 14. The mounds, one of which was drilled by the Chevron Joint Industry Project on Methane Hydrates in 2005, are interpreted to be vent-related features that may contain significant accumulations of gas hydrate adjacent to gas and fluid migration pathways. The mounds are located w150 km south of Louisiana at w1300 m water depth.. New side-scan sonar data, multibeam bathymetry, and near-bottom photography along a 4 km northwest-southeast transect crossing two of the mounds (labeled D and F) reveal the mounds' detailed morphology and surficial characteristics. Mound D, w250 m in diameter and 7-10 m in height, has exposures of authigenic carbonates and appears to result from a seafloor vent of slow-to-moderate flux. Mound F, which is w400 m in diameter and 10-15 m high, is covered on its southwest flank by extruded mud flows, a characteristic associated with moderate-to-rapid flux. Chemosynthetic communities visible on the bottom photographs are restricted to bacterial mats on both mounds and mussels at Mound D. No indications of surficial gas hydrates are evident on the bottom photographs.Published by Elsevier Ltd.

Analysis of methane and sulfate flux in methane-charged sediments from the Mississippi Canyon, Gulf of Mexico

“…This could result from porewater advection out of the sediments exceeding downward SO 4 À2 diffusion and/or rapid SO 4 À2 reduction (organoclastic SR or AOM) occurring at or near the sediment-water column interface. Low seismic backscatter and sidescan sonar, multi-beam bathymetry, and near bottom photography by Hart et al (2008) locations active fluid migration is interpreted by the observation of distinct barite precipitation and dissolution profiles that occur in regions of active advection with a shoaling of the SMT (Castellini et al, 2006). Elevated Cl À and CH 4 concentrations that we measured in cores on the mound and at the base of the mound suggest the existence of similar patterns of fluid migration (Fig.…”

Scientific results from Gulf of Mexico Gas Hydrates Joint Industry Project Leg 1 drilling: Introduction and overview

“…Silt-and clay-rich sediments dominate at both sites and indeed throughout much of the northern Gulf of Mexico, meaning that hydrate-bearing sediment-related geohazards data acquired at the selected drill sites could be broadly applicable across a large region and even in other marine areas characterized by fine-grained sediments. Both of the selected drill sites also have seafloor features interpreted as possible gas hydrate mounds in preliminary analyses (e.g., Dai et al, Part I, 2008;Hart et al, 2008). Such features are far more prominent at AT13/14, which also has carbonate hardgrounds, chemosynthetic communities, and other indicators of rapid fluid flux and possible sediment fluidization.…”

“…At KC151, the dynamics of fluid migration are interpreted to be characterized by slow advective flux of fluids and/or free gas, resulting in the development of a BSR (e.g., Hyndman and Davis, 1992;Xu and Ruppel, 1999) and the possible formation of gas hydrate over a thick zone within the sedimentary section. In contrast, the fluid system at AT13/14 is leaky, with focused zones of enhanced fluid flow coincident with seafloor gas hydrate mounds (e.g., Hart et al, 2008) and subseafloor seismically transparent zones, gas chimneys, and an upwarped BGHS (e.g., Wood et al, 2008).…”

“…IR imagery could not be unequivocally interpreted as being consistent with gas hydrate dissociation instead of gas expansion. Seafloor morphologic data, EM data, and interpretations of seismic data sets with overlapping resolution suggest that gas hydrate might be present at some depths between the seafloor and w50 mbsf at the Atwater Valley mound sites (Dai et al, Part II, 2008;Ellis et al, 2008;Hart et al, 2008;Wood et al, 2008). Kastner et al (2008) interprets 4 ppt of pore water freshening (relative to a maximum of 55.5 ppt salinity) at ATM1 to indicate in situ gas hydrate saturation of 4-5% of pore space.…”

Electromagnetic surveying of seafloor mounds in the northern Gulf of Mexico

“…This suggests possible hard bottom conditions that can be associated with authigenic carbonates and/or hydrates. A series of bottom photographs taken during the cruise with the WHOI deep-towed camera system (Evans et al, 2004;Hart et al, 2008) does not show evidence for active fluid venting on either mound, although a community of mussels was identified on mound D. In some places, patches of white seafloor suggest the presence of bacterial mats, also an indicator of methane supply (Mills et al, 2004), and outcrops of carbonate are seen on mound D .…”