Gas hydrate and chemosynthetic biota in mounded bathymetry at mid-slope hydrocarbon seeps: Northern Gulf of Mexico

Abstract

We present nested surveys of four hydrocarbon seeps that contain abundant, shallow deposits of gas hydrate. The two sites mapped in greatest detail were at ∼550 m depths, the others at ∼1050 and ∼1900 m, respectively. Formation and dissociation of shallow deposits generated variability in seafloor geology and ecology that appeared to decrease at greater depths. The deposits typically formed mounds in a size range from a few meters to hundreds of meters in diameter and centimeters to tens of meters in height. These high-flux regions are also colonized by chemosynthetic species. At the two shallower seeps, data comprised laser line-scan mosaics, analog side-scan sonar mosaics, dense bathymetric grids, quantitative video records, 2–12 kHz chirp sub-bottom profiles and surface reflectance, 3D seismic data. Additional ground-truth data were obtained from piston cores and numerous animal and sediment collections. Datasets were assembled as GIS layers and geographically co-registered. In the shallower sites, gas hydrate deposits and extensive chemosynthetic communities were concentrated adjacent to lithified fault scarps where surface reflectance data from 3D seismic records showed the lowest amplitudes. Sub-bottom profiles revealed complex and abrupt transitions from regions in which surface strata appear as distinct, closely spaced layers to areas in which the strata were blanked by continuous, moderate-intensity returns. Within the blanked zone and adjacent to the normally stratified strata, the seafloor had a distinctively undulating profile caused by irregular hydrate deposits. Hydrate occurred as layers, which were ∼10 cm to >1 m thick, and generally buried under tens of centimeters of sediment, but occasionally exposed as outcrops on slopes. Sediments over hydrate deposits were covered with bacterial mats. Larger mounds were colonized by continuous aggregations of vestimentiferans tube worms. Outcropping hydrate deposits were colonized by the polychaete Hesiocaeca methanicola and by the seep mussel Bathymodiolus childressi. Streams of oil and gas escape from vents in the seafloor or from patches of exposed gas hydrate. The tube worms were most abundant in regions that reflected less seismic energy than the surrounding seafloor. At the deeper sites, preliminary survey results found similar topography, and biological colonies that included bacterial mats and mussels, but no active gas venting and very few colonies of tube worms. Negative surface reflectance amplitudes were also absent on these mounds. This suggests that development of chemosynthetic communities changes substantially with increasing depth.