Robert S. Carney

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.

Chemosynthetic Mussels at a Brine-Filled Pockmark in the Northern Gulf of Mexico

A large (540 square meters) bed of Bathymodiolus n. sp. (Mytilidae: Bivalvia) rings a pool of hypersaline (121.35 practical salinity units) brine at a water depth of 650 meters on the continental slope south of Louisiana. The anoxic brine (dissolved oxygen ≤0.17 milliliters per liter) contains high concentrations of methane, which nourishes methanotrophic symbionts in the mussels. The brine, which originates from a salt-cored diapir that penetrates to within 500 meters ofthe sea floor, fills a depression that was evidently excavated by escaping gas. The spatial continuity of the mussel bed indicates that the brine level has remained fairly constant; however, demographic differences between the inner and outer parts of the bed record small fluctuations.

Chemosynthetic bacterial mats at cold hydrocarbon seeps, Gulf of Mexico continental slope

Abstract White and pigmented filamentous bacterial mats dominated by several undescribed species of Beggiatoa were sampled during research submersible dives to cold hydrocarbon seep sites on the upper continental slope off Louisiana (130–550 m). Mats occur at the interface between reducing sediments and the oxygenated water column. They are localized at sea floor features related to seepage of biogenic methane and crude oil, but there is little evidence that the organisms utilize the hydrocarbons directly. Granules of elemental sulfur (S0) are visible within cells of Beggiatoa, and mat material is characterized by high contents of S0 (up to 193,940 ppm). The Beggiatoa biomass is isotopically light ( δ 13 C = −27.9‰ PDB ). Our geochemical data suggest that the Beggiatoa species are part of a complex bacterial assemblage in cold seep sediments. They oxidize H2S derived from the bacterial sulfate reduction that accompanies bacterial hydrocarbon oxidation when O2 is depleted in sediments, and fix isotopically light carbon from CO2 that is the result of bacterial hydrocarbon oxidation. Beggiatoa mats appear to retard loss of hydrocarbons to the water column by physically retaining fluids in sediments, a function that could enhance production by other bacteria of the H2S and CO2 needed by Beggiatoa.

Community structure of vestimentiferan-generated habitat islands from Gulf of Mexico cold seeps

Abstract Biologically generated structures create habitat and influence the distribution and abundance of species in many marine systems. In the rather monotonous and nutrient-poor environment of the deep-sea, cold seep environments and their associated chemosynthetic communities offer islands of primary production and habitat to a generally sparsely distributed macrofauna. In this study, we investigate the structure of macrofaunal assemblages associated with vestimentiferan aggregations on the upper Louisiana slope of the Gulf of Mexico and the relationships between assemblage composition and the size and complexity of the vestimentiferan-generated habitat. Using custom-designed and custom-built devices, we collected seven whole vestimentiferan aggregations along with their associated fauna during the summers of 1997 and 1998. Sixty-five species were found associated with the four vestimentiferan aggregations collected in 1998, more than doubling the number of species previously reported for seeps in this region. Individual aggregations contained between 23 and 44 different non-vestimentiferan species. General trends of increasing species richness with increasing habitat size and increasing faunal density with increasing habitat complexity were identified, but substantial variability suggested other factors also control the composition of faunal associates. Faunal abundances decreased with increasing aggregation age. Seep endemics dominated the communities of younger aggregations, but non-endemic species dominated communities of older aggregations. Relative dominance of the heterotrophic community by primary consumers decreased, while predatory secondary and higher-order consumers increased with increasing aggregation age. These trends are discussed in terms of successional changes in aggregation structure, habitat heterogeneity and environmental conditions.

Methane Ice Worms: Hesiocaeca methanicola Colonizing Fossil Fuel Reserves

Abstract During a research cruise in July 1997 in the Gulf of Mexico we discovered a gas hydrate approximately 1 m thick and over 2 m in diameter which had recently breached the sea floor at a depth of 540 m. The hydrate surface visible from the submarine was considerably greater than that of any other reported hydrate. Two distinct color bands of hydrate were present in the same mound, and the entire exposed surface of the hydrate was infested (2500 individuals/m2) with 2 to 4 cm-long worms, since described as a new species, Hesiocaecamethanicola, in the polychaete family Hesionidae (Desbruyères and Toulmond 1998). H.methanicola tissue stable isotope values are consistent with a chemoautotrophic food source. No evidence of chemoautotrophic symbionts was detected, but geochemical data support the presence of abundant free living bacteria on the hydrate. The activities of the polychaetes, grazing on the hydrate bacteria and supplying oxygen to their habitats, appears to contribute to the dissolution of hydrates in surface sediments.

Sea floor responses to hydrocarbon seeps, Louisiana continental slope

Observations and samples from research submersible dives confirm that brines, crude oil, fluid mud, and gases are common seep products. Through this mechanism a unique interplay of geochemical, geologic, and biological processes resulting in unusual sea floor features ranging from carbonate-rich nodular sediments to mounds with tens of meters relief. Stable carbon isotopes occluded in the carbonates provide a permanent imprint that links these authigenic carbonates to by-products of microbial breakdown of crude oil and gas. Recent DSV ALVIN dives confirm that hydrocarbon seeps and their accompanying chemosynthetic communities and authigenic carbonate mounds occur over the entire depth range of the slope.

Fine-scale distribution of methanotrophic mussels at a Louisiana cold seep

Abstract Extensive aggregations of methanotrophic mussels (Mytilidae: Bathymodiolus -like) were found in a 60×300m zone on the 640m isobath of the Louisiana slope. Within the aggregations, living mussels occurred in dense curvilinear clusters up to 5m in length. Defunct clusters, consisting of gaping and disarticulated valves, were also common. Comparison of length frequency distributions and mean densities of mussel clusters demonstrated that recruitment of juvenile mussels was ongoing in certain clusters and completely lacking in others. Surface sediments within the zone were characterized by dark patches and linear depressions, apparently associated with seeping hypersaline fluids. Pore fluids in surface sediments contained elevated salt concentrations (289% of ambient sea water), concentrations of ammonia up to 1.6mM, and up to 3mM H 2 S.

Environmental Impacts of the Deep-Water Oil and Gas Industry: A Review to Guide Management Strategies

The industrialization of the deep sea is expanding worldwide. Expanding oil and gas exploration activities in the absence of sufficient baseline data in these ecosystems has made environmental management challenging. Here, we review the types of activities that are associated with global offshore oil and gas development in water depths over 200 m, the typical impacts of these activities, some of the more extreme impacts of accidental oil and gas releases, and the current state of management in the major regions of offshore industrial activity including 18 exclusive economic zones. Direct impacts of infrastructure installation, including sediment resuspension and burial by seafloor anchors and pipelines, are typically restricted to a radius of approximately 100 m on from the installation on the seafloor. Discharges of water-based and low-toxicity oil-based drilling muds and produced water can extend over 2 km, while the ecological impacts at the population and community levels on the seafloor are most commonly on the order of 200-300 m from their source. These impacts may persist in the deep sea for many years and likely longer for its more fragile ecosystems, such as cold-water corals. This synthesis of information provides the basis for a series of recommendations for the management of offshore oil and gas development. An effective management strategy, aimed at minimizing risk of significant environmental harm, will typically encompass regulations of the activity itself (e.g. discharge practices, materials used), combined with spatial (e.g. avoidance rules and marine protected areas) and temporal measures (e.g. restricted activities during peak reproductive periods). Spatial management measures that encompass representatives of all of the regional deep-sea community types is important in this context. Implementation of these management strategies should consider minimum buffer zones to displace industrial activity beyond the range of typical impacts: at least 2 km from any discharge points and surface infrastructure and 200 m from seafloor infrastructure with no expected discharges. Although managing natural resources is, arguably, more challenging in deep-water environments, inclusion of these proven conservation tools contributes to robust environmental management strategies for oil and gas extraction in the deep sea.

Links between chemosynthetic production and mobile predators on the Louisiana continental slope: stable carbon isotopes of specific fatty acids

Abstract The Gulf of Mexico supports chemoautotrophic communities associated with hydrocarbon seeps. The chemoautotrophic symbiont-containing metazoans are dominated by mussels and tube worms that harbor bacteria which utilize reduced carbon (CH 4 ) and sulfur (H 2 S) compounds as an energy source. In the deep sea, nutritional input from photosynthetic production is scarce, and chemoautotrophic production may be a significant source of nutrients to mobile, benthic predators. Large and significant differences exist between the stable carbon isotope signatures of chemosynthetic and photosynthetic primary production. This isotopic difference makes it possible to determine the importance of each type of primary production to heterotrophs because heterotrophs incorporate the isotope signature of their food. Here, we examine the carbon isotope signatures of specific fatty acids in heterotrophic predators caught both within and approximately 2 km from known chemosynthetic communities. Heterotroph fatty acid signatures were compared to those observed in chemoautotrophic symbiont-containing fauna in order to evaluate degree of usage. Most organisms had at least a 6‰ range in their fatty acids. This reflected patterns of de novo synthesis, with essential and precursor fatty acids being 13 C enriched (in heterotrophs) and highly unsaturated fatty acids being 13 C depleted. The fatty acid δ 13 C values show that heterotrophs had a wide range in their utilization of chemosynthetic production. Bathynomus giganteus (giant isopod), captured off-site, did not utilize chemosynthetic production. Its nonessential fatty acids ranged from −26.3‰ to −18.8‰ (δ 13 C), reflecting kinetic isotope effects during de novo synthesis. Essential omega-3 fatty acids, which are directly incorporated from diet, originated from photosynthetic production (δ 13 C=−16.2‰). Sclerasterias cf. tanneri (starfish), captured on-site, relied predominantly on chemosynthetic production. Its fatty acid δ 13 C values ranged from −30.5‰ to −42.2‰, reflecting chemosynthetic production. Other predators such as Eptatretus sp. (hagfish) and Rochina crassa (spider crab) derived variable percentages of their fatty acid pool from chemosynthetic production, estimated to be 38% and 5%, respectively. Bulk analysis of δ 15 N and δ 34 S show chemosynthetic production usage patterns generally consistent with the specific fatty acid δ 13 C 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.