Bernie B. Bernard

Natural gas seepage in the Gulf of Mexico

Abstract Hydrocarbon compositions and δ13C values for methane of fourteen natural seep gases and four underwater vents in the northwestern Gulf of Mexico are reported. The C1/(C2 + C3) ratios of the seep gas samples ranged from 68 to greater than 1000, whereas δPDB13C values varied from −39.9 to −65.5‰. Compositions suggest that eleven of the natural gas seeps are produced by microbial degradation whereas the remaining three have a significant thermocatalytically produced component. Contradictions in the inferences drawn from molecular and isotopic compositions make strict interpretation of the origins of a few of the samples impossible.

Methane in marine sediments

Interstitial methane profiles from six sediment cores taken on the slope and abyssal plain of the Gulf of Mexico can be explained by simple kinetic modeling. Methane is apparently produced at a constant rate and microbially consumed in the sulfate-reducing zone. Rates of production and consumption are estimated from best-fit solutions to a steady-state diagenetic equation. Production and consumption balance to form uniform concentrations of 5 to 10μ11−1 in the first few meters of slope and abyssal sediments. Effects of upward diffusion from large accumulations of methane in sulfate-free zones deeper than about 10 m are not detectable.

Biogenic hydrocarbon gases and sulfate reduction in the Orca Basin brine

The composition of light hydrocarbon gases in the Orca Basin, an anoxic, hypersaline intraslope depression on the continental slope of the northern Gulf of Mexico, indicates that both methane and ethane are biogenic in nature with a C1(C2 + C3) ratio of 730 and a δ13C of methane of −73%. relative to the PDB standard. The concentrations of methane (750 mM) and ethane (1300 mM) in the Orca Basin brine are higher than any other marine anoxic basin. These high levels result not from high rates of productivity, but from the long residence time of the brine in the basin, due to its high stability toward mixing with overlying seawater (Δσ1ΔZ = 3.2m). Both methane and ethane show well mixed distributions in the brine. These distributions probably result from convective mixing of the isohaline brine pool due to normal heat flow from the basin sediments. Methane and ethane maxima above the pycnocline at the brine/seawater interface reflect in situ production and/or consumption in the aerobic water column. Concurrent maxima in suspended particulate material distributions in this region suggest methane may be produced there in anaerobic microenvironments associated with the suspended matter. Reduced rates of anaerobic decomposition (including sulfate reduction) in the brine sediments are inferred from preserved Sargassum fronds in the sediments, vertical sulfate profiles in most cores, and the sediment organic carbon content which is two to three times higher in sediments below the high salinity brine than in the normal Gulf sediments nearby.

A carbon inventory for Orca Basin brines and sediments

Orca Basin, an intraslope depression at a depth or about 2400 m on the continental slope of the north-central Gulf of Mexico, contains an anoxic, hypersaline brine similar in composition to those reported in the Red Sea. Concentrations and stable carbon isotope compositions of various inorganic and organic carbon species have been determined on the brine and sediments in order to gain an understanding of the origin and cycling of carbon in this unique environment. ΣCO 2 in the brine (55 mg C/l) is about twice seawater with δ 13 C PDB = −16.4‰ and Δ 14 C= −501‰.CH 4 has a concentration of 12 mg C/l and δ 13 C= −73.5‰. Dissolved and particulate organic carbon concentrations are seven times higher and have δ 13 C values several permil different than the overlying seawater. ΣCO 2 and CH 4 in the interstitial waters are considerably higher in concentrations and isotopically lighter than the overlying brine. Solution of near-surface salt deposits by seawater with subsequent microbial production and consumption of methane can be used to explain most of the data.

Alvin Explores the Deep Northern Gulf of Mexico Slope

Many of the worlds productive deepwater hydrocarbon basins experience significant and ongoing vertical migration of fluids and gases to the modern seafloor. These products, which are composed of hydrocarbon gases, crude oil, formation fluids, and fluidized sediment, dramatically change the geologic character of the ocean floor, and they create sites where chemosynthetic communities supported by sulfide and hydrocarbons flourish. Unique fauna inhabit these sites, and the chemosynthetic primary production results in communities with biomass much greater than that of the surrounding seafloor.

The Nature of Gas Hydrates on the Nigerian Continental Slope

Abstract: Gas hydrates were collected in six‐meter piston cores during surface geochemical exploration (SGE) surveys in the deep and ultra deepwaters of Nigeria during 1991, 1996, and 1998. To date, gas hydrates have been collected in about 21 cores out of the more than 800 core collections on the Nigerian margin. This represents a 2.5% recovery ratio of gas hydrated cores on this margin at sites that are potential conduits for the upward migration of hydrocarbons (i.e., the core locations are sited based on two‐ and three‐dimensional seismic overfaults, mounds, acoustic wipe‐outs, etc.). Unlike the Northern Gulf of Mexico where the authors have retrieved a significant percentage of thermogenic hydrates in piston cores, all the gas hydrate collections offshore Nigeria to date are primarily biogenic in nature (methane more than 99% of the hydrocarbon gases; δ13C generally light, −60 to −117%). A few of these gas hydrated sites do contain a mixed thermogenic gas component (ethane to butane gases up to a few hundred ppm of total hydrocarbon gas) but even at these sites the primary gas in the hydrates is methane.

INPUT OF LOW-MOLECULAR-WEIGHT HYDROCARBONS FROM PETROLEUM OPERATIONS INTO THE GULF OF MEXICO

Abstract Dissolved C 1 to C 4 hydrocarbon patterns measured during the last 6 years in the Gulf of Mexico indicate that underwater venting of waste gases and brine discharges, both associated with offshore platforms, are the major sources of non-methane light hydrocarbons to upper Gulf coastal waters. These sources are apparently responsible for the two orders of magnitude increase in Louisiana Shelf waters over open ocean levels of the light hydrocarbons with average concentrations of 3100, 31, and 22 nanoliters per liter of methane, ethane, and propane, respectively. Analyses of the hydrocarbon composition of vented gases and brines and estimates of their annual discharge rates indicate that up to 450 metric tons of C 5 to C 10 hydrocarbons are being added to Louisiana Shelf waters each year. Although the C 1 to C 4 hydrocarbons per se are apparently not toxic to marine organisms, they nevertheless are proving to be highly sensitive indicators of the more toxic components of petroleum which are being introduced to the sea bv mans activities.

Potential for microbial anaerobic hydrocarbon degradation in naturally occurring petroleum-associated deep-sea sediments

Abstract The lack of microbial genomes and isolates from the deep seabed means that very little is known about the ecology of this vast habitat. Here, we investigated energy and carbon acquisition strategies of microbial communities from three deep seabed petroleum seeps (3 km water depth) in the Eastern Gulf of Mexico. Shotgun metagenomic analysis revealed that each sediment harbored diverse communities of chemoheterotrophs and chemolithotrophs. We recovered 82 metagenome-assembled genomes affiliated with 21 different archaeal and bacterial phyla. Multiple genomes encoded enzymes for anaerobic oxidation of aliphatic and aromatic compounds, including those of candidate phyla Aerophobetes, Aminicenantes, TA06 and Bathyarchaeota. Microbial interactions are predicted to be driven by acetate and molecular hydrogen. These findings are supported by sediment geochemistry, metabolomics, and thermodynamic modelling. Overall, we infer that deep-sea sediments experiencing thermogenic hydrocarbon inputs harbor phylogenetically and functionally diverse communities potentially sustained through anaerobic hydrocarbon, acetate and hydrogen metabolism.The lack of cultured isolates and microbial genomes from the deep seabed means that very little is known about the ecology of this vast habitat. Here, we investigated energy and carbon acquisition strategies of microbial communities from three deep seabed petroleum seeps (3 km water depth) in the Eastern Gulf of Mexico. Shotgun metagenomic analysis revealed that each sediment harbored diverse communities of chemoheterotrophs and chemolithotrophs. We recovered 82 metagenome-assembled genomes affiliated with 21 different archaeal and bacterial phyla. Multiple genomes encoded enzymes for acetogenic fermentation of aliphatic and aromatic compounds, specifically of candidate phyla Aerophobetes, Aminicenantes, TA06 and Bathyarchaeota. Microbial interactions in these communities are predicted to be driven by acetate and molecular hydrogen, as indicated by a high abundance of fermentation, acetogenesis, and hydrogen utilization pathways. These findings are supported by sediment geochemistry, metabolomics and thermodynamic modelling of hydrocarbon degradation. Overall, we infer that deep-sea sediments experiencing thermogenic hydrocarbon inputs harbor phylogenetically and functionally diverse communities potentially sustained through anaerobic hydrocarbon, acetate and hydrogen metabolism.