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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]

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]

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]