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Recent studies have reported up to 1.9 × 1029 bacterial endospores in the upper kilometre of deep subseafloor marine sediments, however, little is understood about their origin and dispersal. In cold ocean environments, the presence of thermospores (endospores produced by thermophilic bacteria) suggests that distribution is governed by passive migration from warm anoxic sources possibly facilitated by geofluid flow, such as advective hydrocarbon seepage sourced from petroleum deposits deeper in the subsurface. This study assesses this hypothesis by measuring endospore abundance and distribution across 60 sites in Eastern Gulf of Mexico (EGM) sediments using a combination of the endospore biomarker 2,6‐pyridine dicarboxylic acid or 'dipicolinic acid' (DPA), sequencing 16S rRNA genes of thermospores germinated in 50°C sediment incubations, petroleum geochemistry in the sediments and acoustic seabed data from sub‐bottom profiling. High endospore abundance is associated with geologically active conduit features (mud volcanoes, pockmarks, escarpments and fault systems), consistent with subsurface fluid flow dispersing endospores from deep warm sources up into the cold ocean. Thermospores identified at conduit sites were most closely related to bacteria associated with the deep biosphere habitats including hydrocarbon systems. The high endospore abundance at geological seep features demonstrated here suggests that recalcitrant endospores and their chemical components (such as DPA) can be used in concert with geochemical and geophysical analyses to locate discharging seafloor features. This multiproxy approach can be used to better understand patterns of advective fluid flow in regions with complex geology like the EGM basin. [ABSTRACT FROM AUTHOR]

Wave-seabed interaction is known to be a fundamental cause of sediment instability and entrainment. Observations of burial for surrogate munitions were made with a high frequency, sector scanning sonar acquiring acoustic images of the seabed every 12 min during a storm event in May 2013 in the northern Gulf of Mexico offshore of Panama City Beach, FL. Surrogate munitions burial depth was verified by divers. Driven by time series of pressure observed near the seabed, an existing poro-elastic wave-sediment interaction model in combination with a sediment failure criterion due to liquefaction (loss of vertical effective stress) was used to correctly estimate depth and timing of burial. We introduce the concept of liquefaction degree, which is defined as the portion of vertical effective stress at the sediment surface that is, counteracted by wave-induced pore pressures. The relationship between wave-induced pressure at the seabed and liquefaction degree is expressed by a complex transfer function, indicating the importance of the swell band with respect to infra-gravity and short-wave components. The liquefaction degree was used to construct and calibrate a dynamical relationship between wave action and acoustic backscatter observations near the seabed, taken as a surrogate for suspended sediment concentration. Predictions of backscatter based on liquefaction degree were accurate at time scales of 15 s-30 min during most of the time span analyzed and superior to predictions of backscatter based on shear stress. The observations and analysis provide evidence for the importance of wave-induced seepage forces in describing sediment liquefaction and entrainment processes. [ABSTRACT FROM AUTHOR]

Marine cold seeps transmit fluids between the subseafloor and seafloor biospheres through upward migration of hydrocarbons that originate in deep sediment layers. It remains unclear how geofluids influence the composition of the seabed microbiome and if they transport deep subsurface life up to the surface. Here we analyzed 172 marine surficial sediments from the deep-water Eastern Gulf of Mexico to assess whether hydrocarbon fluid migration is a mechanism for upward microbial dispersal. While 132 of these sediments contained migrated liquid hydrocarbons, evidence of continuous advective transport of thermogenic alkane gases was observed in 11 sediments. Gas seeps harbored distinct microbial communities featuring bacteria and archaea that are well-known inhabitants of deep biosphere sediments. Specifically, 25 distinct sequence variants within the uncultivated bacterial phylaAtribac-teria and Aminicenantes and the archaeal order Thermoprofundales occurred in significantly greater relative sequence abundance along with well-known seep-colonizing members of the bacterial genus Sul-furovum, in the gas-positive sediments. Metabolic predictions guided by metagenome-assembled genomes suggested these organisms areanaerobic heterotrophs capable of nonrespiratory breakdown of organic matter, likely enabling them to inhabit energy-limited deep sub-seafloor ecosystems. These results point to petroleum geofluids as avector for the advection-assisted upward dispersal of deep biospheremicrobes from subsurface to surface environments, shaping the micro-biome of cold seep sediments and providing a general mechanism for the maintenance of microbial diversity in the deep sea. [ABSTRACT FROM AUTHOR]

Polychaete worms (Annelida), the dominant macrofaunal taxon in most fine-grained marine sediments, were sampled in 1983–85 and then again in 2000–02 at nine locations at depths of 324–1454 m. on the upper continental slope of the northern Gulf of Mexico (GoM). The assemblages exhibited relative stability in abundance and diversity, but fell into six separate groups of species (>35% similarity) that were related to time-of-sampling, location, depth. This depth gradient experiences an increase in oxygen from 2.5 to 4.5 ml/L, a six degree decrease in temperature (10–12° down to 4 °C) and a decline of 30–37 mg C m −2 day −1 down to 7 mg C m −2 day −1 in estimates of the particulate organic carbon (POC) input to the sea floor, but these steep gradients had secondary effects on species turnover or depth-related zonation (Beta diversity). The species composition of four of the six groups was separated on the basis of sampling between 1983–85 and 2000–02 as opposed to depth or location. The species composition of the two groups on the eastern transect was different from the western sites and the two eastern groups differed in species composition from each other between 1983–85 and 2000–02. The two groups of species at the three deeper sites to the west (864–1410 m) were also separated on the basis of time-of-sampling but the group of species located at the three shallow locations (324–625 m) was not; it was a mixture of the two sampling periods. Significantly higher densities (p<0.05) in April 1984, on the eastern transect, suggest that seasonal recruitment may have occurred but the higher densities were attributed to only two species. [ABSTRACT FROM AUTHOR]

Highlights: [•] T-Hg, MMHg, and Hg isotopes were determined in Gulf of Mexico deep marine sediments. [•] Levels of mass independent fractionation of Hg isotopes varied from coastal sediments. [•] Levels of the MMHg were only slightly elevated at the cold seep site. [•] Cold seeps do not appear to be significant sources of MMHg to Gulf of Mexico waters. [ABSTRACT FROM AUTHOR]

Abstract: Significant horizontal stress anisotropy was encountered in three blocks in the Gulf of Mexico drilled by the Gulf of Mexico gas hydrates Joint Industry Project, namely, Atwater Valley 13, Atwater Valley 14, and Walker Ridge 313. The geological factors responsible for this state of stress and the implications for wellbore stability and drilling operations are explored. In Atwater Valley 13 and 14, stresses at the well sites were perturbed by the underlying salt. In Walker Ridge 313, a combination of thrusting by adjacent salt and material anisotropy associated with the regional minibasin structure caused the maximum horizontal stress to rotate with depth. Horizontal stress anisotropy combined with a lack of heavy mud caused breakouts to form in the well Walker Ridge 313-G (WR313-G). A simple formula to predict the depth at which wells drilled with seawater are prone to breakouts is derived. Evidence is presented that the breakouts in WR313-G produced heavy cavings which were mainly responsible for the tight hole conditions encountered while drilling this well. However, it is shown that bottom hole assembly (BHA) design and drilling practices such as backreaming may have exacerbated these problems. Modifications to drilling practices based on the experience gained from drilling WR313-G helped to improve the quality of the neighboring borehole WR313-H. [Copyright &y& Elsevier]

The microbial community structures of gas hydrate-bearing (Core 9) and non-hydrate-bearing (Core 1) marine sediments were investigated at Mississippi Canyon (MC) 118 in the Gulf of Mexico. Quantification by quantitative competitive (QC)-PCR showed that bacterial abundance was 2-3 orders of magnitude higher than archaeal abundance in these cores. Sulfate-reducing bacteria (SRB) were present at 103-104 dsrAB gene copies/g in both cores; methanogens or anaerobic methanotrophs were only present in Core 9 (102-105 mcrA gene copies/g). Denaturing gradient gel electrophoresis (DGGE) showed distinct patterns of bacterial community structure between Core 9 and Core 1 with ε-Proteobacteria predominating in the former and γ-Proteobacteria in the latter. Clone libraries were successfully constructed for both Archaea and Bacteria using functional genes (mcrA and dsrAB, respectively). The mcrA gene was present in Core 9, suggesting enhanced abundance or activity of methanogens or methane-oxidizing archaea in the hydrate-impacted sediment. The mcrA gene sequences were dominated by group c-d and group e. The majority (80%) of the dsrAB gene sequences fell into Syntrophobacteraceae-related group. This study indicates that microbial community structures are considerably different between the hydrate-bearing and non-hydrate-bearing sediment at MC 118. Our study is among the initial steps toward a comprehensive and long-term monitoring of microbial dynamics associated with gas hydrates in the Gulf of Mexico. [ABSTRACT FROM AUTHOR]

Using 2D four-component ocean-bottom-cable (2D 4-C OBC) seismic data processed in common-receiver gathers, we developed robust VP and VS interval velocities for the near-seafloor strata. A vital element of the study was to implement iterative interpretation techniques to correlate near-seafloor P-P and P-SV images. Initially, depth-equivalent P-P and P-SV layers were interpreted by visually matching similar events in both seismic modes. Complementary 1D ray-tracing analyses then determined interval values of subsea-floor VP and VS velocities across a series of earth layers extending from the seafloor to below the base of the hydrate stability zone (BHSZ) to further constrain these interpretations. Iterating interpretation of depth-equivalent horizons with velocity analyses allowed us to converge on physically reasonable velocity models. Simultaneous VP and VS velocity analysis provided additional model constraints in areas where data quality of one reflection mode (usually VP in the near-seafloor environments) would not provide adequate information to derive reliable velocity information. [ABSTRACT FROM AUTHOR]

This paper presents results of experimental deployment of a large instrumented cylinder of variable nose geometry and center of mass offset (CMO) in free-fall in realistic environment. Data on four tests series in the Gulf of Mexico are presented and analyzed statistically. The stochastic nature of the problem of the cylinder free-falling through water is outlined and described as an input to the subsequent impact burial prediction package. Significance of the CMO on the behavior of the cylinder is underlined. Influence of the release conditions on trajectory is discussed and found to affect the behavior of the cylinders only in the first 3.5 m of free-fall in water. Beyond this depth, quasi-stable (in the mean sense) conditions are achieved, Effects of three different nose shapes--blunt, hemispherical, and chamfered--on cylinder behavior are analyzed and found to have a pronounced influence on the fall trajectory. The blunt nose shape appears to be hydrodynamically most stable in free-fall. Apparent periodicity in motions of all cylinders were noted and were found to be the function of the CMO and nose shape primarily. Implications of these and other findings on modeling and impact burial predictions are discussed. [ABSTRACT FROM AUTHOR]

Submarine hydrocarbon seeps are widespread in the northwestern Gulf of Mexico and give rise to unusual seafloor features that are commonly poorly defined because water depth limits the resolution afforded by sea-surface seismic profiles. Detailed seep images are possible with side-scan sonars because such devices use beam-formed high-frequency sound to give high spatial resolution. Furthermore, side-scan sonars are useful for mapping seeps because their sound waves respond to changes in near-seafloor sediment properties caused by seep activity. To examine seep sonar signatures, we used a broad-swath, sea-surface side-scan sonar to survey three areas of the Louisiana continental slope where seeps are known to occur. In sonar mosaics from upper-slope study sites, seep features are the main anomalous seafloor response and display high backscatter of sonar waves. The mosaics show fault traces, fault scarps, authigenic carbonate mounds, irregular patches of seep-affected seafloor, mud mounds and mud volcanoes, craters, and sediment flows. Most of these features are located along faults that control seepage. Cores from seep-affected areas imply that enhanced sonar backscatter results from a combination of the perturbation of seafloor sediment physical properties and the introduction of seep-produced foreign material, including gas, oil, carbonate nodules and fragments, and chemosynthetic organism debris. In the lower slope survey area, tectonic activity and resultant mass sediment movements make identification of seeps more difficult. In this area, many sonar back-scatter anomalies appear to result from sediment flows and mass movements on the edges and bottoms of intrasalt basins. Basin walls appear as highly dissected, commonly showing crenulations and grooves, which were probably created by sediment failure and erosion by downslope sediment transport. Seeps are commonly difficult to discern because of the abundance of other anomalous... [ABSTRACT FROM AUTHOR]