Beggiatoa in microbial mats at hydrocarbon vents in the Gulf of Mexico and Warm Mineral Springs,Florida

Microbial mats were collected from a variety of sites near hydrocarbon vents along the slope in the northern Gulf of Mexico and, for comparison, from Warm Mineral Springs, Florida, USA. A predominant microorganism in each of the mats was the giant bacterium,Beggiatoa. Diameters of the bacterial filaments ranged from about 6 µm to approximately 200 µm. The latter organisms are the largest prokaryotic organisms yet found. All filaments over about 10 µm in diameter contained a large central vacuole, producing a cell with the cytoplasm as a cylindrical tube underlying the cytoplasmic membrane. Sulfur globules were confined to this peripheral layer. Push cores often contained pyrite tubules whose appearance is suggestive of aBeggiatoa origin. Determinations of ¹³C inBeggiatoa mats from vents along the Louisiana slope yielded values in the range of –26.6 to –27.9 (PDB), suggesting an unusually high degree of isotope fractionation (–24.9) relative to the carbon source in the ambient seawater, which is typical of sulfur-oxidizing chemoautotrophs. The presence of S^O (elemental sulfur) within cells ofBeggiatoa resulting from oxidation of H₂S supports the importance of bacterial sulfate reduction processes in the underlying vents for the sustenance of theBeggiatoa mats.     

Consideration of the oasis analogy for chemosynthetic communities at Gulf of Mexico hydrocarbon vents

The analogy between desert oasis and deep-sea chemosynthetic community arose from the biomass contrast between vents and the relatively depauperate background benthic fauna. Fully developed, the analogy helps pose questions about interactions with the background fauna with respect to resources, colonization, and persistence. The chemosynthetic sites of the Gulf of Mexico provide an opportunity to consider possible interactions between vent and nonvent fauna over a 3000-m depth range. It is postulated that deep chemosynthetic communities require the operation of geochemical transporting and concentrating processes to overcome low levels of in situ methane and sulfide production. Clathrate reservoirs may serve these functions. A few chemosynthetic species at the Gulf of Mexico upper slope sites are related to shallow-water sulfide species, but it can be speculated that the dominant chemosynthetic fauna may have originated in a wide spread deep sulfide biome of the Cretaceous. Generic endemism of consumers is low in Gulf of Mexico sites, suggesting a high level of colonization from the surrounding benthos. Chemosynthetic communities may avoid excessive colonization by predators in spite of the apparent food limitation of the surrounding benthos due to toxicity or an evolutionary mechanism selecting against specialized predators. The abundance of large predators is related to the composition of the surrounding benthos and is high at the Gulf of Mexico upper slope sites. Exclusion of chemosyntheic communities from shallower depths may be due to excessive predation by generalists.     

Benthic foraminifera of bathyal hydrocarbon vents of the Gulf of Mexico: Initial report on communities and stable isotopes

Substrates associated with active hydrocarbon vents in bathyal Gulf of Mexico support numerous foraminiferal species, with a few of them showing unusually high relative abundances. In the 584- to 695-m-depth range,Bolivina ordinaria, Gavelinopsis translucens, andCassidulina neocarinata strongly dominate the vent community, whereasBolivina subaenariensis andUvigerina laevis play this role around a vent at 216 m water depth. The bathymetric imprint on the foraminiferal record is also seen in the ¹⁸O compositions of some species, includingUvigerina peregrina. The adaptation of foraminiferal communities to bacterial (Beggiatoa) mats, in which the redox boundary is very close to the sediment—water interface, and anomalous depletions of¹³C inU. peregrina (relative to the same species from nonventing sites) indicate that several species are probably facultative anaerobes and tolerant of H₂S toxicity.     

Macrobenthic infaunal communities associated with deep‐sea hydrocarbon seeps in the northern Gulf of Mexico

There are thousands of seeps in the deep ocean worldwide; however, many questions remain about their contributions to global biodiversity and the surrounding deep‐sea environment. In addition to being globally distributed, seeps provide several benefits to humans such as unique habitats, organisms with novel genes, and carbon regulation. The purpose of this study is to determine whether there are unique seep macrobenthic assemblages, by comparing seep and nonseep environments, different seep habitats, and seeps at different depths and locations. Infaunal community composition, diversity, and abundance were examined between seep and nonseep background environments and among three seep habitats (i.e., microbial mats, tubeworms, and soft‐bottom seeps). Abundances were higher at seep sites compared to background areas. Abundance and diversity also differed among microbial mat, tubeworm, and soft‐bottom seep habitats. Although seeps contained different macrobenthic assemblages than nonseep areas, infaunal communities were also generally unique for each seep. Variability was 75% greater within communities near seeps compared to communities in background areas. Thus, high variability in community structure characterized seep communities rather than specific taxa. The lack of similarity among seep sites supports the idea that there are no specific infauna that can be used as indicators of seepage throughout the northern Gulf of Mexico, at least at higher taxonomic levels.     

Subsurface gas hydrates in the northern Gulf of Mexico

The northern Gulf of Mexico (GoM) has long been a focus area for the study of gas hydrates. Throughout the 1980s and 1990s, work focused on massive gas hydrates deposits that were found to form at and near the seafloor in association with hydrocarbon seeps. However, as global scientific and industrial interest in assessment of the drilling hazards and resource implications of gas hydrate accelerated, focus shifted to understanding the nature and abundance of “buried” gas hydrates. Through 2005, despite the drilling of more than 1200 oil and gas industry wells through the gas hydrate stability zone, published evidence of significant sub-seafloor gas hydrate in the GoM was lacking. A 2005 drilling program by the GoM Gas Hydrate Joint Industry Project (the JIP) provided an initial confirmation of the occurrence of gas hydrates below the GoM seafloor. In 2006, release of data from a 2003 industry well in Alaminos Canyon 818 provided initial documentation of gas hydrate occurrence at high concentrations in sand reservoirs in the GoM. From 2006 to 2008, the JIP facilitated the integration of geophysical and geological data to identify sites prospective for gas hydrate-bearing sands, culminating in the recommendation of numerous drilling targets within four sites spanning a range of typical deepwater settings. Concurrent with, but independent of, the JIP prospecting effort, the Bureau of Ocean Energy Management (BOEM) conducted a preliminary assessment of the GoM gas hydrate petroleum system, resulting in an estimate of 607 trillion cubic meters (21,444 trillion cubic feet) gas-in-place of which roughly one-third occurs at expected high concentrations in sand reservoirs. In 2009, the JIP drilled seven wells at three sites, discovering gas hydrate at high saturation in sand reservoirs in four wells and suspected gas hydrate at low to moderate saturations in two other wells. These results provide an initial confirmation of the complex nature and occurrence of gas hydrate-bearing sands in the GoM, the efficacy of the integrated geological/geophysical prospecting approach used to identify the JIP drilling sites, and the relevance of the 2008 BOEM assessment.     

Cetacean habitat in the northern oceanic Gulf of Mexico

Cetaceans (whales and dolphins) are diverse and abundant upper trophic level predators in the Gulf of Mexico, a semi-enclosed, intercontinental sea with a total area of about 1.5 million km². The objectives of this study were to better define the habitat of cetaceans in the northern oceanic Gulf of Mexico. An integrated methodology was used that included visual surveys and hydrographic collections from ships. Near real-time sea surface altimetry from the TOPEX/POSEIDON and ERS satellites was used during ship surveys to determine the location of hydrographic features (e.g., cyclones, anticyclones and confluence zones). Archival satellite sea surface altimetry data were also used to retrospectively determine the location of hydrographic features for analysis with earlier cetacean sightings. We estimated zooplankton and micronekton biomass using both net and acoustic sampling to indicate the amount of potential food available for higher trophic level foraging by cetaceans. Nineteen cetacean species were identified during ship surveys. Cetaceans were concentrated along the continental slope in or near cyclones and the confluence of cyclone–anticyclone eddy pairs, mesoscale features with locally concentrated zooplankton and micronekton stocks that appear to develop in response to increased nutrient-rich water and primary production in the mixed layer. A significant relationship existed between integrated zooplankton biomass and integrated cephalopod paralarvae numbers, indicating that higher zooplankton and micronekton biomass may correlate with higher concentrations of cetacean prey. In the north-central Gulf, an additional factor affecting cetacean distribution may be the narrow continental shelf south of the Mississippi River delta. Low salinity, nutrient-rich water may occur over the continental slope near the mouth of the Mississippi (MOM) River or be entrained within the confluence of a cyclone–anticyclone eddy pair and transported beyond the continental slope. This creates a deep-water environment with locally enhanced primary and secondary productivity and may explain the presence of a resident, breeding population of sperm whales within 100 km of the Mississippi River delta. Overall, the results suggest that the amount of potential prey for cetaceans may be consistently greater in the cyclone, confluence areas, and south of the MOM, making them preferential areas for foraging. However, this may not be true for bottlenose dolphins, Atlantic spotted dolphins and possibly Bryde's whales, which typically occur on the continental shelf or along the shelf break outside of major influences of eddies.     

Physiographic features on the northern Gulf of Mexico continental slope

The continental slope of the northern Gulf of Mexico is diapirically controlled and is comprised of coalescing salt sheets, salt withdrawal basins, salt ridges, salt tongues and sills, and submarine canyons. Bathymetric information from single-beam data has resulted in several published maps. Many of the map areas have been remapped, using multibeam surveys, by the US National Ocean Service, and names have been given to the major physiographic features. The multibeam program was discontinued before complete coverage of the slope was accomplished. We provide charts of the remaining areas with names of features that have been accepted by the US Board of Geographic Names.     

Depositional history of the Lagniappe Delta,northern Gulf of Mexico

The northern Gulf of Mexico continental shelf is characterized by superimposing deltas. One such delta, informally named Lagniappe, extends east of the Mississippi Delta from mid-shelf to the continental slope. This late Wisconsinan delta is adjacent to, but not associated with the Mississippi Delta complex: the fluvial source was probably the ancient Pearl and/or Mobile Rivers. The fluvially dominated Lagniappe Delta is characterized by complex sigmoid-oblique seismic-reflection patterns, indicating delta switching of high-energy sand-prone facies to low-energy facies. The areal distribution and sediment thickness of the delta were partially controlled by two diapirs.     

Impact of oil and gas vents and slicks on petroleum exploration in the deepwater Gulf of Mexico

Active petroleum vents and slicks have been identified in the deep water of the northern Gulf of Mexico using numerous techniques. The occurrence and distribution of these petroleum vents are strongly influenced by the local geological framework—especially the presence of vertical migration pathways into shallow sediments. Oil and gas vents may be more useful for establishing the existence of petroleum generation on a regional scale and for evaluating the gross properties of oil migrating in the subsurface than for appraising the exploration value of individual prospects. Knowledge about petroleum venting in the deepwater Gulf of Mexico has proven to be an important element of the successful exploration activities there.     

High-resolution seismic-reflection investigation of the northern Gulf of Mexico gas-hydrate-stability zone

We recorded high-resolution seismic-reflection data in the northern Gulf of Mexico to study gas and gas-hydrate distribution and their relation to seafloor slides. Gas hydrate is widely reported near the seafloor, but is described at only one deep drill site. Our data show high-reflectivity zones (HRZs) near faults, diapirs, and gas vents and interbedded within sedimentary sections at shallow depth (<1 km). The HRZs lie below the gas-hydrate-stability zone (GHSZ) as well as within the zone (less common), and they coincide with zones of shallow water-flows. Bottom simulating reflections are rare in the Gulf, and not documented in our data.We infer HRZs result largely from free gas in sandy beds, with gas hydrate within the GHSZ. Our estimates for the base BHSZ correlate reasonably with the top of HRZs in some thick well-layered basin sections, but poorly where shallow sediments are thin and strongly deformed. The equivocal correlation results from large natural variability of parameters that are used to calculate the base of the GHSZ. The HRZs may, however, be potential indicators of nearby gas hydrate. The HRZs also lie at the base of at least two large seafloor slides (e.g. up to 250 km²) that may be actively moving along decollement faults that sole within the GHSZ or close to the estimated base of the GHSZ. We suspect that water/gas flow along these and other faults such as ‘chimney’ features provide gas to permit crystallization of gas hydrate in the GHSZ. Such flows weaken sediment that slide down salt-oversteepened slopes when triggered by earthquakes.     

Intertidal gastropod assemblages shaped by key environmental variables across the northern Persian Gulf and the Gulf of Omanss

Variations in environmental factors can alter the species distribution pattern in intertidal rocky shores. The Persian Gulf (PG) and the Gulf of Oman (GO) vary substantially with respect to environmental and oceanographic conditions. The abundance and biodiversity of intertidal rocky gastropods in five locations across the northern PG and the GO were compared, and the environmental variables underlying the distribution pattern of these organisms were investigated. A total of 67 gastropod species were identified. The largest average density (294 ind./m²) and diversity (N = 43) for gastropods occurred in the Hotel Lipar station (LIP) located in Chabahar Bay in the GO. Clypeomorus bifasciata (107.43 ind./m²) followed by Cerithium caeruleum (94.67 ind./m²) were the most abundant species. Planaxis sulcatus and Siphonaria spp. occurred in all locations during both sampling occasions. Species richness and abundance of gastropods showed significant differences between LIP and remaining locations. A significant difference was found in assemblage structure across locations. In general, the species richness and density in the locations at GO were significantly larger than those locations in the PG, suggesting that the harsh environmental condition in the PG might be the forcing factor for this diminish. Distinct grouping was observed in both assemblage structure and species composition between locations in the PG and the GO. The spatial and temporal distribution patterns of gastropods assemblages were significantly correlated with variation in salinity and substrate rugosity.     

Predictors of species richness in the deep-benthic fauna of the northern Gulf of Mexico

Species richness in macrofauna and megafauna collected with box cores and trawls from 35 standard stations over a depth range of 175–3720 m in the northern Gulf of Mexico was examined in terms of two primary questions: (1) are observed patterns random? and (2) if not, what environmental factors might account for the patterns? A null model tested whether richness vs. depth distributions were random. Groups with species that had broad vertical depth ranges fit the null model better than groups with small ranges, but for almost all phyla a non-random pattern was indicated. With randomness as a proximal explanation ruled out, further examination of the relationship between richness and environmental factors was justified. A generalized linear model (GLM) showed that a suite of 18 factors categorized as food-related, habitat-related, pollution-related and location-related were significantly related to richness patterns, but that different mixes of factors applied to different phyla. No one factor accounted for any observed patterns. Thus, each taxonomic group needs to be examined individually, and no generally applicable explanation for the causes of richness patterns may exist. Nonetheless, mapping richness itself indicates valuable areas in the Mississippi Trough that must receive special consideration and possible protection.     

The generation and preservation of multiple hurricane beds in the northern Gulf of Mexico

Cores collected from Mississippi Sound and the inner shelf of the northeast Gulf of Mexico have been examined using ²¹⁰Pb and ¹³⁷Cs geochronology, X-radiography, granulometry, and a multi-sensor core logger. The results indicate that widespread event layers were probably produced by an unnamed hurricane in 1947 and by Hurricane Camille in 1969. Physical and biological post-depositional processes have reworked the event layers, producing regional discontinuities and localized truncation, and resulting in an imperfect and biased record of sedimentary processes during the storms. The oceanographic and sedimentological processes that produced these event beds have been simulated using a suite of numerical models: (1) a parametric cyclone wind model; (2) the SWAN third-generation wave model; (3) the ADCIRC 2D finite-element hydrodynamic model; (4) the Princeton Ocean Model; (5) a coupled wave–current bottom boundary layer-sedimentation model; and (6) a model for bed preservation potential as a function of burial rate and bioturbation rate. Simulated cores from the Mississippi Sound region are consistent with the observed stratigraphy and geochronology on both the landward and seaward sides of the barrier islands.     

Trace metal concentrations in menhaden larvae Brevoortia patronus from the northern Gulf of Mexico

Whole body concentrations of Cu, Zn, Mn and Fe were measured in individual gulf menhaden larvae, Brevoortia patronus (11–18 mm standard length) from coastal waters of the northern Gulf of Mexico as part of a continuing project investigating the mechanisms of biological interaction and effect of trace metals in marine food webs. Larvae were collected at three different times between February 1981 and 1982 at two locations, offshore of Southwest Pass of the Mississippi River and offshore of Galveston, Texas.Fish at the Mississippi location had significantly (P≤0·05) greater concentrations of all metals compared with those from the Galveston location. No significant (P>0·05) differences in concentration were detected among the three sampling periods. Menhaden larvae had metal conentrations comparabe to other species of larval fish and zooplankton from the Gulf of Mexico and other coastal waters. Differences in metal concentrations in larvae from the two locations appear to be a subtle response of the fish to differences in the trace metal chemistries of the two coastal areas. Processes influencing metal concentrations are discussed.     

墨西哥湾天然气水合物矿产资源预测模型及其应用

墨西哥湾是世界上研究天然气水合物较深入的海区,调查资料丰富,已在50多处采集到天然气水合物样品,具备建立天然气水合物矿产资源预测模型的条件。选择34处已知天然气水合物矿点和34处已知无矿点作为训练区,建立矿点存在与否的预测模型。模型的相关系数值表明墨西哥湾天然气水合物与盐底辟关系密切。将该模型应用于整个墨西哥湾北部,初步获得了天然气水合物存在可能性概率分布图;概率大于0.7的预测单元包含已知矿点中的30个,利用该阈值圈定了墨西哥湾天然气水合物潜在资源分布区。     

Population structure of the mussel “Bathymodiolus” childressi from Gulf of Mexico hydrocarbon seeps

Hydrocarbon and brine seeps in the deep regions of the northern and western Gulf of Mexico often support populations of the bathymodiolin mussel, “Bathymodiolus” childressi. In this study, we use two mitochondrial and six nuclear DNA markers to investigate relationships within the metapopulation of “B.” childressi in the Gulf of Mexico from Mississippi Canyon to Alaminos Canyon over a range of 527–2222 m in depth and approximately 550 km in distance. Restriction fragment length polymorphism (RFLP) and size polymorphism analysis of the markers suggest that populations are not genetically differentiated. F_ST values were not significantly different from zero. The presence of a panmictic population of “B.” childressi over such a broad range of depth suggests that this species may be quite different from most members of the Gulf of Mexico seep chemosynthetic communities.     

Summer hypoxia in the northern Gulf of Mexico and its prediction from 1978 to 1995

An 18-year monitoring record (1978-1995) of dissolved oxygen within a region having hypoxia (dissolved oxygen less than 2 mgl(-1)) in the bottom layer was examined to describe seasonal and annual trends. The monitoring location was near or within a well-described summer hypoxic zone whose size has been up to 20,000 km(2). The monitoring data were used to hindcast the size of the hypoxic zone for before consistent shelfwide surveys started, and to predict it for 1989, when a complete shelfwide survey was not made. The concentration of total Kjeldahl nitrogen (TKN) in surface waters and concentration of bottom water oxygen were directly related, as anticipated if organic loading from surface to bottom was from in situ processes. The TKN data were used to develop a predictive relationship that suggested there was no substantial hypoxia before the 1970s, which was before nitrate flux from the Mississippi River to the Gulf of Mexico began to rise. The peak frequency in monthly hypoxic events is two to three months after both the spring maximum in discharge and nitrate loading of the Mississippi River. These results support the conclusion that persistent, large-sized summer hypoxia is a recently-developed phenomenon that began in the 1970s or early 1980s.     

Seismic expression of sedimentary volcanism on the continental slope,northern Gulf of Mexico

High-resolution geophysical data define acoustically amorphous, mounded structures on the upper, middle, and lower continental slope of the northern Gulf of Mexico. Physical samples and observations within this unique seismic facies show gassy sediments, sometimes in hydrated form and, in places, as chemosynthetic communities. The geologic setting of these mounds suggests that the process of formation falls on the continuum of mud volcanoes to mud diapirs.     

In situ hydrocarbon concentrations from pressurized cores in surface sediments, Northern Gulf of Mexico

Two newly developed coring devices, the Multi-Autoclave-Corer and the Dynamic Autoclave Piston Corer were deployed in shallow gas hydrate-bearing sediments in the northern Gulf of Mexico during research cruise SO174 (Oct–Nov 2003). For the first time, they enable the retrieval of near-surface sediment cores under ambient pressure. This enables the determination of in situ methane concentrations and amounts of gas hydrate in sediment depths where bottom water temperature and pressure changes most strongly influence gas/hydrate relationships. At seep sites of GC185 (Bush Hill) and the newly discovered sites at GC415, we determined the volume of low-weight hydrocarbons (C₁ through C₅) from nine pressurized cores via controlled degassing. The resulting in situ methane concentrations vary by two orders of magnitudes between 0.031 and 0.985 mol kg^− 1 pore water below the zone of sulfate depletion. This includes dissolved, free, and hydrate-bound CH₄. Combined with results from conventional cores, this establishes a variability of methane concentrations in close proximity to seep sites of five orders of magnitude. In total four out of nine pressure cores had CH₄ concentrations above equilibrium with gas hydrates. Two of them contain gas hydrate volumes of 15% (GC185) and 18% (GC415) of pore space. The measurements prove that the highest methane concentrations are not necessarily related to the highest advection rates. Brine advection inhibits gas hydrate stability a few centimeters below the sediment surface at the depth of anaerobic oxidation of methane and thus inhibits the storage of enhanced methane volumes. Here, computerized tomography (CT) of the pressure cores detected small amounts of free gas. This finding has major implications for methane distribution, possible consumption, and escape into the bottom water in fluid flow systems related to halokinesis.