Surficial Hydrocarbon Seep Infauna from the Blake Ridge (Atlantic Ocean, 2150 m) and the Gulf of Mexico (690–2240 m)

Abstract.  Infauna, including foraminifera and metazoans, were enumerated and identified from five types of seep habitats and two adjacent non-seep habitats. Collections were made with the deep submergence research vessel 'Alvin' from three areas of active seepage in the Gulf of Mexico (Alaminos Canyon [2220 m], Atwater Canyon [1930 m], and Green Canyon lease block 272 [700 m]) and on the Blake Ridge Diapir [2250 m], which is located off the southeastern coast of the United States. The seep habitats sampled included four types of microbial mats ( Beggiatoa , Thioploca , thin and thick Arcobacter ) and the periphery of a large mussel bed. Sediments under large rhizopod protists, xenophyophores, were sampled adjacent to the mussel bed periphery. A non-seep site, which was >1 km away from active seeps, was also sampled for comparison. Densities of most taxa were higher in the Gulf of Mexico seeps than in Blake Ridge samples, largely because densities in the thick microbial mats of Blake Ridge were significantly lower. Diversity was higher in the Thioploca mats compared to other microbial-mat types. Within an ocean basin ( i.e. , Atlantic, Gulf of Mexico) we did not observe significant differences in meiofaunal or macrofaunal composition in Beggiatoa versus Thioploca mats or thin versus thick Arcobacter mats. Foraminifera represented up to 16% of the seep community, a proportion that is comparable to their contribution at adjacent non-seep communities. In general, the observed densities and taxonomic composition of seep sites at the genus level was consistent with previous observations from seeps ( e.g. , the foraminifers Bolivina and Fursenkoina , the dorvilleid polychaete Ophryotrocha ).     

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.     

Monterey Bay cold-seep biota: Assemblages,abundance, and ultrastructure of living foraminifera

Although there is a growing body of evidence indicating benthic foraminifera inhabit hydrocarbon and cold seep environments, biochemical and ultrastructural data on seep foraminiferal communities are not available. Therefore, sediments collected from cold seeps in Monterey Bay, CA (900–1000 m), were examined for the presence of live benthic foraminifera. Results from three independent methods (ATP assay, ultrastructural analysis, rose Bengal staining) indicate that certain species inhabit the Clam Flat and Clam Field seeps. Abundances in our seep samples were lower than in comparable non-seep sites, although not atypical for these bathyal depths. Of 38 species represented at these two seep sites by cytoplasm-containing specimens, only Spiroplectammina biformis was restricted to the seep environment. However, because S. biformis is also known from non-seep sites in other areas, it should not be considered as endemic to seeps. Ultrastructural studies show abundant peroxisomes in seep specimens, which may allow inhabitation of such environments. One specimen of Uvigerina peregrina had prokaryotes nestled in test pores, suggesting that bacteria may play a role in the survival of foraminifera in this seep environment.     

Biologic and geologic characteristics of cold seeps in Monterey Bay, California

Cold seep communities discovered at three previously unknown sites between 600 and 1000 m in Monterey Bay, California, are dominated by chemoautotrophic bacteria (Beggiatoa sp.) and vesicomyid clams (5 sp.). Other seep-associated fauna included galatheid crabs (Munidopsis sp.), vestimentiferan worms (Lamellibrachia barhami?), solemyid clams (Solemya sp.), columbellid snails (Mitrella permodesta, Amphissa sp.), and pyropeltid limpets (Pyropelta sp.). More than 50 species of regional (i.e. non-seep) benthic fauna were also observed at seeps. Ratios of stable carbon isotopes (δ¹³C) in clam tissues near 36‰ indicate sulfur-oxidizing chemosynthetic production, rather than non-seep food sources, as their principal trophic pathway. The “Mt Crushmore” cold seep site is located in a vertically faulted and fractured region of the Pliocene Purisima Formation along the walls of Monterey Canyon ( 635 m), where seepage appears to derive from sulfide-rich fluids within the Purisima Formation. The “Clam Field” cold seep site, also in Monterey Canyon ( 900 m) is located near outcrops in the hydrocarbon-bearing Monterey Formation. Chemosynthetic communities were also found at an accretionary-like prism on the continental slope near 1000 m depth (Clam Flat site). Fluid flow at the “Clam Flat” site is thought to represent dewatering of accretionary sediments by tectonic compression, or hydrocarbon formation at depth, or both. Sulfide levels in pore waters were low at Mt Crushmore (ca 0.2 mM), and high at the two deeper sites (ca 7.011.0 mM). Methane was not detected at the Mt Crushmore site, but ranged from 0.06 to 2.0 mM at the other sites.     

Deep-sea habitat heterogeneity influence on meiofaunal communities in the Gulf of Guinea

To estimate the degree of spatial heterogeneity of benthic deep-sea communities, we carried out a multiple-scale (from m's to 200 km) investigation in the Congo-Angola margins (Equatorial West African margin, 3150–4800 m) in which we examined the metazoan meiofauna at a variety of habitats along the Congo Channel system and in the associated cold seep. We investigate the structure, density, vertical distribution patterns in the sediment and biomass of meiofaunal communities in the Gulf of Guinea and how they are controlled by hydrologic and biogeochemical processes. The meiofaunal communities in the Gulf of Guinea were shaped by heterogeneous conditions on the margin, and reflect the multiple-scale spatial variability that corresponds with the different identified habitats. The two control sites, located at >100 km away from the canyon, were inhabited by very dense and the most diverse meiobenthic communities. Similar meiobenthic communities inhabited the transition zone between the canyon and the cold seep. Sites located along the Congo Channel were obviously affected by the local high-velocity bottom currents and unstable sedimentary conditions in this active submarine system. Extremely low meiobenthic densities and very low proportions in the most surficial sediment layers provided evidence for recently highly disturbed sediments at these sites. The remote operated vehicle (ROV) Victor 6000 provided images of the cold seep, showing a patchy distribution of several types of patchy distributed megafaunal communities dominated by three key symbiotic taxa (Mytilidae, Vesicomyidae and Siboglinidae). These cold seep sediments were colonised by a unique meiobenthic community, characterised by a high small-scale (m's) patchiness, low species richness and the prominent dominance of two large-sized nematode species: Sabatieria mortenseni, which is a cosmopolitan nematode known from littoral habitats, and an undescribed Desmodora species. The high individual body weight of S. mortenseni and its dominance at the cold seep site resulted in a significantly higher nematode biomass at the seep compared to the surrounding sites. In addition, the vertical nematode profiles, with maximum proportions in subsurficial layers, points to a chemosynthesis-based meiobenthic community in this cold seep, in contrast to the phytodetritus-based communities at the control sites and at the transition zone.     

Cold seeps of the deep Gulf of Mexico: Community structure and biogeographic comparisons to Atlantic equatorial belt seep communities

Quantitative collections of tubeworm- and mussel-associated communities were obtained from 3 cold seep sites in the deep Gulf of Mexico: in Atwater Valley at 1890 m depth, in Alaminos Canyon at 2200 m depth, and from the Florida Escarpment at 3300 m depth. A total of 50 taxa of macro- and megafauna were collected including 2 species of siboglinid tubeworms and 3 species of bathymodiolin mussels. In general, the highest degree of similarity was between communities collected from the same site. Most of the dominant families at the well-characterized upper Louisiana slope seep sites of the Gulf of Mexico were present at the deep sites as well; however, there was little overlap at the species level between the upper and lower slope communities. One major difference in community structure between the upper and lower slope seeps was the dominance of the ophiuroid Ophioctenella acies in the deeper communities. The transition between upper and lower slope communities appears to occur between 1300 and 1700 m based on the number of shared species with the Barbados seeps at either end of this depth range. Seep communities of the deep Gulf of Mexico were more similar to the Barbados Accretionary Prism seep communities than they were to either the upper slope Gulf of Mexico or Blake Ridge communities based on numbers of shared species and Bray–Curtis similarity values among sites. The presence of shared species among these sites suggests that there is ongoing or recent exchange among these areas. An analysis of bathymodioline mussel phylogeography that includes new collections from the west coast of Africa is presented. This analysis also suggests recent exchange across the Atlantic equatorial belt from the Gulf of Mexico to the seeps of the West Nigerian margin.     

Distribution and temporal variation of mega‐fauna at the Regab pockmark (Northern Congo Fan), based on a comparison of videomosaics and geographic information systems analyses

The Regab pockmark is a large cold seep area located 10 km north of the Congo deep sea channel at about 3160 m water depth. The associated ecosystem hosts abundant fauna, dominated by chemosynthetic species such as the mussel Bathymodiolus aff. boomerang, vestimentiferan tubeworm Escarpia southwardae, and vesicomyid clams Laubiericoncha chuni and Christineconcha regab. The pockmark was visited during the West African Cold Seeps (WACS) cruise with RV Pourquoi Pas? in February 2011, and a 14,000‐m² high‐resolution videomosaic was constructed to map the most populated area and to describe the distribution of the dominant megafauna (mussels, tubeworms and clams). The results are compared with previous published works, which also included a videomosaic in the same area of the pockmark, based on images of the BIOZAIRE cruise in 2001. The 10‐year variation of the faunal distribution is described and reveals that the visible abundance and distribution of the dominant megafaunal populations at Regab have not changed significantly, suggesting that the overall methane and sulfide fluxes that reach the faunal communities have been stable. Nevertheless, small and localized distribution changes in the clam community indicate that it is exposed to more transient fluxes than the other communities. Observations suggest that the main megafaunal aggregations at Regab are distributed around focused zones of high flux of methane‐enriched fluids likely related to distinct smaller pockmark structures that compose the larger Regab pockmark. Although most results are consistent with the existing successional models for seep communities, some observations in the distribution of the Regab mussel population do not entirely fit into these models. This is likely due to the high heterogeneity of this site formed by the coalescence of several pockmarks. We hypothesize that the mussel distribution at Regab could also be controlled by the occurrence of zones of both intense methane fluxes and reduced efficiency of the anaerobic oxidation of methane possibly limiting tubeworm colonization.     

Community structure and nutrition of deep methane-seep macrobenthos from the North Pacific (Aleutian) Margin and the Gulf of Mexico (Florida Escarpment)

Methane seeps occur at depths extending to over 7000 m along the world's continental margins, but there is little information about the infaunal communities inhabiting sediments of seeps deeper than 3000 m. Biological sampling was carried out off Unimak Island (3200–3300 m) and Kodiak Island (4500 m) on the Aleutian margin, Pacific Ocean and along the Florida Escarpment (3300 m) in the Gulf of Mexico to investigate the community structure and nutrition of macrofauna at these sites. We addressed whether there are characteristic infaunal communities common to the deep‐water seeps or to the specific habitats (clam beds, pogonophoran fields, and microbial mats) studied here, and ask how these differ from background communities or from shallow‐seep settings sampled previously. We also investigated, using stable isotopic signatures, the utilization of chemosynthetically fixed and methane‐derived organic matter by macrofauna from different regions and habitats. Within seep sites, macrofaunal densities were the greatest in the Florida microbial mats (20,961 ± 11,618 ind·m⁻²), the lowest in the Florida pogonophoran fields (926 ± 132 ind·m⁻²), and intermediate in the Unimak and Kodiak seep habitats. Seep macrofaunal densities differed from those in nearby non‐seep sediments only in Florida mat habitats, where a single, abundant species of hesionid polychaete comprised 70% of the macrofauna. Annelids were the dominant taxon (>60%) at all sites and habitats except in Florida background sediments (33%) and Unimak pogonophoran fields (27%). Macrofaunal diversity (H′) was lower at the Florida than the Alaska seeps, with a trend toward reduced richness in clam bed relative to pogonophoran field or non‐seep sediments. Community composition differences between seep and non‐seep sediments were evident in each region except for the Unimak margin, but pogonophoran and clam bed macrofaunal communities did not differ from one another in Alaska. Seep δ¹³C and δ¹⁵N signatures were lighter for seep than non‐seep macrofauna in all regions, indicating use of chemosynthetically derived carbon. The lightest δ¹³C values (average of species’ means) were observed at the Florida escarpment (−42.8‰). We estimated that on average animal tissues had up to 55% methane‐derived carbon in Florida mats, 31–44% in Florida clam beds and Kodiak clam beds and pogonophoran fields, and 9–23% in Unimak seep habitats. However, some taxa such as hesionid and capitellid polychaetes exhibited tremendous intraspecific δ¹³C variation (>30‰) between patch types. Overall we found few characteristic communities or features common to the three deep‐water seeps (>3000 m), but common properties across habitats (mat, clam bed, pogonophorans), independent of location or water depth. In general, macrofaunal densities were lower (except at Florida microbial mats), community structure was similar, and reliance on chemosynthesis was greater than observed in shallower seeps off California and Oregon.     

Community structure of infaunal macrobenthos around vestimentiferan thickets at the San Clemente cold seep,NE Pacific

The San Clemente cold seep lies within 100–200 km of other reducing habitats in the NE Pacific, offering an opportunity to compare diversity and species overlap among reducing habitats (i.e. whale‐, kelp‐, and wood‐falls) at similar depths within a single region. Video observations from the research submersible Alvin at the San Clemente seep (1800 m depth) indicated clumps (‘thickets’) of vestimentiferans distributed as meter‐scale patches interspersed with vesicomyid clam beds and black sediments. Sediment‐core samples were collected at distances of 0 to 80–200 m along randomly oriented transects radiating outward from vestimentiferan thickets to evaluate changes in macrofaunal community structure from thickets into the background community. Macrofaunal abundance was elevated at distances of 0–1 m compared to 80–200 m (i.e. the ‘background’ community). The tube‐building frenulate worms Siboglinum spp., along with peracarid crustaceans, dominated sediments within 1 m of vestimentiferan thickets. Species diversity was depressed within 1 m of thickets but with high rates of species accumulation, suggesting that seep sites greatly increase sediment heterogeneity and facilitate colonization by non‐background macrofaunal species. Stable isotope data indicate chemosynthetic nutrition for some dominant macrofaunal species within 1 m of tubeworm thickets. The macrofaunal community near vestimentiferan thickets in San Clemente seep contains intermediate levels of species richness and diversity compared to other deep‐sea seep areas in the northeast Pacific. There was low species overlap between the San Clemente seep macrofauna and communities in reducing habitats near wood‐, whale‐, and kelp‐falls at similar depths within the region, suggesting that seeps harbor a distinct infaunal community.     

Spatial distribution of diverse cold seep communities living on various diapiric structures of the southern Barbados prism

Three sectors of the south Barbados prism between 1000 and 2000 m depth were explored by the French submersible Nautile. Chemosynthesis-based benthic communities were discovered on several structures affected by diapirism, including mud volcanoes, domes and an anticlinal ridge. The communities are associated with the expulsion of methane-rich fluids which is a wide-spread process in the area. These communities are dominated by large bivalves and vestimentiferans which harbour chemoautotrophic symbiotic bacteria. The symbiotic bivalves include two species of Mytilidae and one of Vesicomyidae, with dominance of a methanotrophic mussel. Cartography of the benthic communities, interpretation of thermal measurements and observation of sedimentary patterns have been used to define the life habits of each of the three species of symbiotic bivalves. Each species has a characteristic preference for different conditions of edaphic and fluid flow: the dominant methanotrophic mussel appears to require high velocity vents and hard substratum. The vesicomyids and the other species of mussel are able to take up sulfide from the sediments, and so are associated with low seepages, but also require soft sediment. The three bivalve species are assumed successively to colonize the top of a diapiric ridge, in a succession related to the temporal evolution of fluid flow and sedimentation. The composition of the bivalve assemblages, their densities and biomasses all differ between the several mud volcanoes and domes studied, and these parameters are thought to be related to the spatial and temporal variations of fluid expulsion through the structures, and the lithification processes linked to fluid expulsion. One very active dome is at present colonized by an exceptionally large and dense population of the methanotrophic mussel. In contrast, communities in another area, on the domes and volcanoes that are currently inactive, were colonized by only a few living vesicomyids and mussels, both associated with sulfur-oxydizing bacteria, and there were numerous empty shells. The densities and biomasses of symbiotic bivalves were far greater in the area studied than in a deeper mud volcano field on the same prism that had been studied previously. This is consistent with a report that methane production is greater in the southern region of this accretionary prism than in the northern. Numerous non-symbiotic organisms were observed in and around the areas of the seeps, some are endemic to the seep communities, including some gastropods and shrimps, others are either colonists or vagrants from the surrounding deep-sea floor. Filter feeders were very abundant, and some of these, like the serpulids and large sponges, may also be dependent on the chemosynthetic production. Faunistic composition of both symbiotic and non-symbiotic taxa, of the assemblages around these cold seeps, is closely related to that reported for communities living on hydrocarbon seeps in the Gulf of Mexico.     

Mud volcanoes in the Mediterranean Sea are hot spots of exclusive meiobenthic species

Mud volcanoes are cold seeps, in which the escape of gas and fluids associated with mud creates 3-D bottom structures that enhance the spatial heterogeneity and potentially alter the functioning of the benthic ecosystems. We investigated a complex system of mud volcanoes of the Mediterranean Sea characterised by the presence of different structures (i.e., isolated domes, domes surrounded by moats, clustered domes, and ridges) displaying different levels of seepage. We hypothesize that the combined effects of seafloor heterogeneity (i.e., different 3-D structures as revealed by detailed topographic analysis), fluid emissions and trophic characteristics of these systems can influence the structural and functional biodiversity of meiofauna (with special focus on the nematodes). We found that sediments affected by intensive seepage displayed the lowest faunal abundances and number of higher taxa. However, mud volcanoes without emissions, but characterized by a high structural complexity (such as the ridges), were associated with the highest meiofaunal abundances and number of higher taxa. Mud volcanoes hosted also a remarkable abundance of rare taxa specifically associated with these structures (e.g., acarians, cumaceans, tanaids, cladocerans and hydroids) and absent in slope sediments (used as a control). Each mud-volcano area displayed a different nematode species composition. Overall 76 nematode species (from a total of 235) were exclusively associated with mud-volcano structures, whilst 29 were exclusively encountered in slope sediments. We conclude that the presence of mud volcanoes, for their contribution to increase spatial heterogeneity and for the extreme conditions associated with gas emissions, promotes higher levels of beta diversity, thus enhancing the regional (gamma) benthic diversity. These findings provide new insights on the factors controlling meiobenthic biodiversity in mud volcanoes and clues for future action of conservation of the biodiversity specifically associated with these habitats.     

Spatial scale and habitat-dependent diversity patterns in nematode communities in three seepage related sites along the Norwegian Sea margin

Assessing the relative contribution of local diversity to regional biodiversity may be the key to understanding large-scale and even global patterns in species diversity. Here, the contribution of habitat heterogeneity of cold seeps at three spatial scales [micro-scale (ms), macro-scale (10 to 100s of ms), and mega-scale (10 to 100s of km)] to the total nematode biodiversity (genus level) along the Norwegian continental margin is evaluated. Due to the development of higher resolution bathymetry and increased bottom sampling in recent years, continental margins, once regarded as monotonous landscapes, are now acknowledged to have a high degree of habitat complexity and diversity. By calculating the additive partitioning of gamma diversity in alpha and beta fractions, we examined to what extent habitat diversity of seep sites significantly increases the nematode genus composition and diversity at different spatial scales. Siboglinidae patches and control sediments yielded comparably high levels of nematode genus richness. They exhibited low turnover rates within and across the different seep sites. In contrast, the bacterial mats at Håkon Mosby Mud Volcano (HMMV) and the reduced sediments at the Nyegga pockmarks harboured genus-poor nematode communities with an equally high dominance of one or two species, which were different for each seep. Different habitats, in particular at the HMMV, contributed significantly to the seep nematode richness. This study demonstrates that the presence of distinct habitat types within multiple seep sites contributes to the high diversity of nematode communities inhabiting the seeps in the Norwegian deep sea.     

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.     

Depth-related structure and ecological significance of cold-seep communities—a case study from the Sea of Okhotsk

We discovered and investigated several cold-seep sites in four depth zones of the Sea of Okhotsk off Northeast Sakhalin: outer shelf (160–250 m), upper slope (250–450 m), intermediate slope (450–800 m), and Derugin Basin (1450–1600 m). Active seepage of free methane or methane-rich fluids was detected in each zone. However, seabed photography and sampling revealed that the number of chemoautotrophic species decreases dramatically with decreasing water depth. At greatest depths in the Derugin Basin, the seeps were inhabited by bacterial mats and bivalves of the families Vesicomyidae (Calyptogena aff. pacifica, C. rectimargo, Archivesica sp.), Solemyidae (Acharax sp.) and Thyasiridae (Conchocele bisecta). In addition, pogonophoran tubeworms of the family Sclerolinidae were found in barite edifices. At the shallowest sites, on the shelf at 160 m, the seeps lack chemoautotrophic macrofauna; their locations were indicated only by the patchy occurrence of bacterial mats.Typical seep-endemic metazoans with chemosynthetic symbionts were confined to seep sites at depths below 370 m. A comparative analysis of the structure of seep and background communities suggests that differences in predation pressure may be an important determinant of this pattern. The abundance of predators such as carnivorous brachyurans and asteroids, which can invade seeps from adjacent habitats and efficiently prey on sessile seep bivalves, decreased very pronouncedly with depth. We conclude from the obvious correlation with the conspicuous pattern in the distribution of seep assemblages that, on the shelf and at the upper slope, predator pressure may be high enough to effectively impede any successful settlement of viable populations of seep-endemic metazoans. However, there was also evidence that other depth-related factors, such as bottom-water current, sedimentary regimes, oxygen concentrations and the supply of suitable settling substrates, may additionally regulate the distribution of seep fauna in the area.As a consequence of the pronounced pattern in the distribution of seep communities, their ecological significance as food sources of surrounding background fauna increased with water depth. Isotopic analyses suggest that in the Derugin Basin seep colonists feed on chemoautotrophic seep organisms, either directly or by preying on metazoans with chemosynthetic symbionts. In contrast, seep organisms apparently do not contribute to the nutrition of the adjacent background fauna on the shelf and at the slope. In this area, elevated epifaunal abundances at seep sites were caused primarily by the availability of suitable settling substrates rather than by an enrichment of food supply.     

Higher-taxon richness as a surrogate for species richness in chemosynthetic communities

Estimations of biodiversity and species richness in deep-sea marine ecosystems are impeded by time-consuming methods of species identification. In conservation biology, in environmental monitoring, and in paleontology, a higher-taxon approach (e.g., identification to genera or families) can be used as a surrogate for species richness. We applied a higher-taxon approach to well-documented chemosynthetic communities associated with seep and vent mussel beds to test its applicability in these systems. Significant positive correlations between cumulative number of species and cumulative number of higher taxa were found at the generic, family, and order levels. The number of these higher taxa can be used to predict species richness in vent and seep mussel beds.     

Improved detection and mapping of deepwater hydrocarbon seeps: optimizing multibeam echosounder seafloor backscatter acquisition and processing techniques

Marine seep hunting surveys are a current focus of hydrocarbon exploration surveys due to recent advances in offshore geophysical surveying, geochemical sampling, and analytical technologies. Hydrocarbon seeps are ephemeral, small, discrete, and therefore difficult to sample on the deep seafloor. Multibeam echosounders are an efficient seafloor exploration tool to remotely locate and map seep features. Geophysical signatures from hydrocarbon seeps are acoustically-evident in bathymetric, seafloor backscatter, midwater backscatter datasets. Interpretation of these signatures in backscatter datasets is a fundamental component of commercial seep hunting campaigns. Degradation of backscatter datasets resulting from environmental, geometric, and system noise can interfere with the detection and delineation of seeps. We present a relative backscatter intensity normalization method and an oversampling acquisition technique that can improve the geological resolvability of hydrocarbon seeps. We use Green Canyon (GC) Block 600 in the Northern Gulf of Mexico as a seep calibration site for a Kongsberg EM302 30 kHz MBES prior to the start of the Gigante seep hunting program to analyze these techniques. At GC600, we evaluate the results of a backscatter intensity normalization, assess the effectiveness of 2X seafloor coverage in resolving seep-related features in backscatter data, and determine the off-nadir detection limits of bubble plumes using the EM302. Incorporating these techniques into seep hunting surveys can improve the detectability and sampling of seafloor seeps.     

Black spots produced by buried macroalgae in intertidal sandy sediments of the Wadden Sea: Effects on the meiobenthos

The effects of buried decaying macroalgae on meiobenthos were examined in intertidal sandy sediments of the Wadden Sea of Lower Saxony. In situ experiments confirmed that one of the principal causes of the formation of reduced surface sediments or ‘black spots’ on the tidal flats is the increasing occurrence and subsequent decomposition of filamentous green algae (Enteromorpha spp.) buried in the sediment. Five to fifteen days after algal material had been buried, the sediment surface turned black. The impact of these black spots on meiobenthos was dramatic: the changed chemical conditions in the sediment resulted in long and drastic reductions in meiofaunal abundance and number of taxa. A multi-dimensional scaling (MDS) analysis of data on meiobenthic abundances revealed that samples from black-spot areas were clearly separated from those of control and reference areas. Re-oxidized black spots showed recolonization by meiofaunal animals, with numbers of individuals and taxa similar to those of oxidized surface sediments. The use of abundances of members of higher meiobenthic taxa to monitor changes in the sediment's chemistry, especially those caused by biomass overload, is discussed.     

Interactions between sediment chemistry and frenulate pogonophores (Annelida) in the north-east Atlantic

The small frenulate pogonophores (Annelida: Pogonophora a.k.a. Siboglinidae) typically inhabit muddy sediments on the continental slope, although a few species occur near hydrothermal vents and cold seeps. We present data on the distribution and habitat characteristics of several species on the European continental shelf and slope from 48°N to 75°N and show how the animals interact with the chemistry of the sediments. The environments inhabited include: shallow (30 m), organic-rich, fjord sediments; slope sediments (1000–2200 m) and methane seeps at 330 m depth. All the species studied obtain nutrition from endosymbiotic bacteria. They take up reduced sulphur species, or in one case, methane, through the posterior parts of their tubes buried in the anoxic sediment. We conclude that most species undertake sulphide ‘mining’, a mechanism previously demonstrated in the bivalves Lucinoma borealis and Thyasira sarsi. These pogonophores participate in the sulphur cycle and effectively lower the sulphide content of the sediments. Our results show that the abundance of frenulate pogonophores increases with increasing sedimentation and with decreasing abundance of other benthos, particularly bioturbating organisms. The maximum sustainable carrying capacity of non-seep sediments for frenulate pogonophores is limited by the rate of sulphate reduction.     

Faunal communities at sites of gas- and oil-bearing fluids in Lake Baikal

Macro- and meiofaunal communities were examined at four geomorphologically distinct sites with different gas- and oil-bearing fluid characteristics in the northern, central and southern basins of Lake Baikal. All sites had elevated concentrations of bicarbonate, nitrate, sulphate and chloride ions in pore fluids, with highest values at the Frolikha vent. Elevated levels of iron ions were found in pore waters of the St. Petersburg methane seep and the Gorevoy Utes oil seep. The chemical composition of pore waters at the Malenky mud volcano was similar to that reported in earlier work. Consistent with published data, the Frolikha vent (northern basin) and the St. Petersburg methane seep (central basin) were characterised by methane of mixed genesis (thermogenic + biogenic), whereas the methane source was mainly thermogenic at the Gorevoy Utes oil seep (central basin) and biogenic at the Malenky mud volcano (southern basin). In contrast to marine seep ecosystems, the macrofauna was dominated only by amphipods, giant planarians and oligochaetes, whereas bivalves were absent; the meiofauna was similar to its marine counterpart, being dominated by nematodes, cyclops, harpacticoids and ostracods. A statistically significant positive relationship was revealed between faunal abundance and the availability of bacterial mats on seep sediments. Moreover, ANOVA tests showed significant increases in both meiozoobenthic and macrozoobenthic densities at “hot spot” vent/seep sites relative to discharge-free reference sites. The isotopic composition of carbon and nitrogen at various trophic levels of these benthic vent/seep communities was found to differ markedly from that reported by earlier studies for the pelagic and other benthic food webs in Lake Baikal. As in marine seeps, the macrofauna had variable isotopic signatures. Light δ¹³C and δ¹⁵N values suggest the utilization of chemosynthetically fixed and/or methane-derived organic matter. By contrast, the heavy δ¹³C signatures of some mobile amphipods likely reflect consumption of photosynthetically derived carbon. These findings would at least partly explain why Lake Baikal is a notable outlier in global temperature–biodiversity patterns, exhibiting the highest biodiversity of any lake worldwide but at an extremely cold average temperature.     

Recognizing and compensating for interference from the sediment’s background organic matter and biodegradation during interpretation of biomarker data from seafloor hydrocarbon seeps: An example from the Marco Polo area seeps,Gulf of Mexico,USA

A suite of seep samples from the vicinity of the Marco Polo field in the Green Canyon area of the Gulf of Mexico provides an exceptional opportunity to study the impact of interference from sediment background organic matter and alteration by biodegradation on thermogenic hydrocarbons from seafloor seeps. These seep samples contain a range in both the concentration of seeped hydrocarbons present and level of biodegradation experienced. In addition, the subsurface oil that sources the seepage was available for comparison.