Footprint of deepwater horizon blowout impact to deep-water coral communities

On April 20, 2010, the Deepwater Horizon (DWH) blowout occurred, releasing more oil than any accidental spill in history. Oil release continued for 87 d and much of the oil and gas remained in, or returned to, the deep sea. A coral community significantly impacted by the spill was discovered in late 2010 at 1,370 m depth. Here we describe the discovery of five previously unknown coral communities near the Macondo wellhead and show that at least two additional coral communities were impacted by the spill. Although the oil-containing flocullent material that was present on corals when the first impacted community was discovered was largely gone, a characteristic patchy covering of hydrozoans on dead portions of the skeleton allowed recognition of impacted colonies at the more recently discovered sites. One of these communities was 6 km south of the Macondowellhead and over 90% of the corals present showed the characteristic signs of recent impact. The other community, 22 km southeast of the wellhead between 1,850 and 1,950 m depth, was more lightly impacted. However, the discovery of this site considerably extends the distance from Macondo and depth range of significant impact to benthic macrofaunal communities. We also show that most known deep-water coral communities in the Gulf of Mexico do not appear to have been acutely impacted by the spill, although two of the newly discovered communities near thewellhead apparently not impacted by the spill have been impacted by deep-sea fishing operations

Surficial Gas Hydrates, Part of the Fluid and Gas Expulsion Response Spectrum: Identification from 3D Seismic Data

Currently, the complex continental slope opposite Louisiana is covered with a high quality database for interpreting seafloor geology. This database consists of large, adjacent, and overlapping tracks of 3D-seismic data. Linking seismic data with field verification data derived from manned submersible observations and samples has produced a qualitative understanding of seafloor response to a spectrum of fluid and gas expulsion rates. Slow flux rates tend to produce a seafloor characterized by hard bottoms (mounds, hardgrounds, and nodular masses in unconsolidated sediment) created by precipitation of C-depleted Ca-Mg carbonates. Other precipitates such as barite have also been observed in slow-to-moderate flux settings. At the other end of the expulsion spectrum are response features derived from rapid delivery of fluids (including fluidized sediment) and gases to the seafloor. Mud-prone features such as mud volcanoes of various dimensions and thin, but widespread mud flows characterize the rapid flux part of the expulsion spectrum. Considerable heat and non-biodegraded hydrocarbons frequently accompany rapid flux of fluidized sediment. Below water depths of approximately 500 m, intermediate flux settings seem best exemplified by areas where gas hydrates occur at or very near the seafloor. These environments display considerable variability with regard to surficial geology and on a local scale have elements of both rapid and slow flux. However, this dynamic setting apparently has a constant supply of hydrocarbons to promote gas hydrate formation at the seafloor even though oceanic temperature variations cause periodic hydrate decomposition. The presence of these deposits provides the unique set of conditions necessary to sustain dense and diverse chemosynthetic communities. The cross-slope variability of seafloor response to fluid and gas expulsion is not well known. However, present data indicate that the expulsion process is highly influenced by migration pathways dictated by salt geometries that change downslope from isolated salt masses to canopy structures to nappes. 1

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Seafloor reflectivity—An important seismic property for interpreting fluid/gas expulsion geology and the presence of gas hydrate

A bottom-simulating reflection (BSR) is a seismic reflectivity phenomenon that is widely accepted as indicating the base of the gas-hydrate stability zone. The acoustic impedance difference between sediments invaded with gas hydrate above the BSR and sediments without gas hydrate, but commonly with free gas below, are accepted as the conditions that create this reflection. The relationship between BSRs and marine gas hydrate has become so well known since the 1970s that investigators, when asked to define the most important seismic attribute of marine gas-hydrate systems, usually reply, “a BSR event.” Research conducted over the last decade has focused on calibrating seafloor seismic reflectivity across the geology of the northern Gulf of Mexico (GoM) continental slope surface to the seafloor. This research indicates that the presence and character of seafloor bright spots (SBS) can be indicators of gas hydrates in surface and near-surface sediments (Figure 1). It has become apparent that SBSs on the continental slope generally are responses to fluid and gas expulsion processes. Gas-hydrate formation is, in turn, related to these processes. As gas-hydrate research expands around the world, it will be interesting to find if SBS behavior in other deepwater settings is as useful for identifying gas-hydrate sites as in the GoM

Atlas of external diseases of the eye for physicians and students /

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