Deep Sea Research Part I: Oceanographic Research Papers
Blake Ridge methane seeps: characterization of a soft-sediment, chemosynthetically based ecosystem
Introduction
A quarter of a century has passed since the first exploration of hydrothermal vents, yet the prospect remains for discovery of biogeographically and ecologically distinctive types of chemosynthetic systems in the world's oceans. Exploration and investigation of these systems will allow us to understand the diversity of habitats, species, and adaptations that can be supported by chemosynthesis. In this report, we provide preliminary characterization of a soft-sediment, chemosynthetically based ecosystem associated with a methane hydrate province on the continental margin of the eastern United States. Gas hydrates close to the sediment–seawater interface are also known from other regions, including the Gulf of Mexico (MacDonald et al., 1994), the Barbados accretionary complex (Olu et al., 1996), the Barents Sea (Egorov et al., 1999), and the Cascadia margin off Oregon (Suess et al., 1999; Sahling et al., 2002).
The focus site for this study lies near the intersection of the Carolina Rise and the Blake Ridge (Fig. 1). This area of the South Atlantic Bight has long been recognized as a major gas hydrate province within the US Exclusive Economic Zone (e.g., Markl et al., 1970; Tucholke et al., 1977; Paull and Dillon, 1981). Over most of the region, the top of the methane hydrate deposit probably lies at depths greater than 100 m below seafloor (mbsf; Paull et al., 1996). At some locations, however, gas hydrate and underlying free gas occur close to the seafloor, and interaction of the hydrate reservoir with geologic, oceanographic, and other processes leads to the development of focused seeps.
The US Atlantic continental margin south of 34°N is among the most extensively mapped gas-hydrate provinces in the world's oceans. Several generations of seismic surveys (e.g., Tucholke et al., 1977; Rowe and Gettrust, 1993; Taylor et al., 1999; Holbrook, 2000) map a regionally extensive bottom-simulating reflector (BSR) in this area. The BSR is a negative-impedance contrast reflector that marks the phase boundary between overlying gas hydrate and underlying free gas. While gas hydrates are known to occur on the Blake Ridge at locations with no BSR (e.g., Paull et al., 1996), the presence of a BSR beneath a large part of the Blake Ridge implies widespread occurrence of gas hydrates.
A line of about 20 salt diapirs begins near the intersection of the Blake Ridge with the Carolina Rise and extends northward on the eastern side of the Carolina Trough (Dillon et al., 1982). The diapirs rise to within 600 m of the seafloor and disrupt the overlying sediments. Interaction between the Blake Ridge Diapir (the southern-most diapir) and the underlying methane-hydrate reservoir was extensively investigated by Taylor et al. (2000). The high thermal conductivity of the diapir alters the local stability field for methane hydrates, causing upward warping of the BSR and shifting of the gas hydrate and free-gas system to shallower levels in the sedimentary section. At the same time, partial dissolution of the salt diapir raises local pore-water salinities, further inhibiting gas-hydrate stability and possibly contributing to the increased mobility of fluids in the sedimentary section (Taylor et al., 2000). Emplacement of the diapir has been accompanied by the development of faults that act as conduits for the transfer of free gas and waters rich in dissolved gas toward the seafloor (Paull et al., 1995).
Seismic reflection profiles across the Blake Ridge Diapir (e.g., USGS CH-06-92 Line 37) show a prominent BSR that shoals over the diapir, and a fault that extends from the BSR to nearly the seafloor (Fig. 2). Chemosynthetic communities and gas-rich plumes rising up to 320 m in the water column have been detected where the fault system intersects the seafloor (Paull et al (1995), Paull et al (1996)). Sediments consisting of hemipelagic silt and clay with 20–40% pelagic carbonate (Paull et al., 1995; Dillon and Max, 2000b) drape the diapir. These sediments, which were deposited by strong, south-flowing near-bottom currents, were accreted rapidly (up to 48 cm ka−1) during the late Pleistocene interval (Paull et al., 1996).
Leg 164 of the Ocean Drilling Program (ODP; Paull et al (1996), Paull et al (2000)) drilled 5 holes along an ∼80 m, east-west transect across the Blake Diapir. Three of the holes had total depths of 50–63 m, while two holes near the center of the diapir were drilled to less than 10 m below the seafloor. The recovered cores contained typical hemipelagic sediments along with authigenic carbonates and gas hydrates (Paull et al., 2000). Earlier research on the Blake Ridge Diapir revealed fluid-flow pathways and fluid-related features within the sedimentary section (e.g., Taylor et al., 2000). High concentrations of methane and sulfide in pore waters (1000–3400 μM CH4; 1300 μM H2S) and widespread occurrences of authigenic carbonates and gas hydrates were documented in core material, and collection of mussels at the tops of cores provided evidence for the existence of a chemosynthetic community on the crest of the diapir (Paull et al., 1996).
The geographic location of the Blake Ridge methane seep raises questions about the biogeographical affinities of its fauna. The closest known deep-sea seep sites are those of the Barbados region to the southeast (Jollivet et al., 1990; Olu et al (1996), Olu et al (1997)) and of the Florida Escarpment, on the opposite side of the Florida peninsula (Paull et al., 1984; Hecker, 1985). There is a perception that seeps support faunas that are more endemic to local regions than hydrothermal vents (Sibuet and Olu, 1998); comparisons of species lists and genetic differentiation in species from these sites can be used to test this hypothesis.
In this paper, we report new data that enhance our understanding of the geological context and ecological setting for the chemosynthetically based community on the Blake Diapir. Further accounts of methane hydrate formation, foraminiferal biology and ecology, and quantitative analyses of the invertebrate assemblage associated with Blake Ridge mussel beds will be presented elsewhere.
Section snippets
Materials and methods
Four Alvin dives were conducted at the Blake Ridge Diapir site (ODP Site 996; 32°29.623′N, 76°11.467′W; 2155 m depth) on September 25–28, 2001. A map of megafaunal distributions (mussels, clams, cake urchins) was generated from transponder navigation and digital video records from dives 3709, 3711, and 3712. Push cores were used to sample xenophyophores (P. Granuloreticulosa), bacterial mats, and sediments. Clams were collected with a suction sampler (1/4″ mesh); all other animals were collected
Submersible observations of the geological setting
Four Alvin dives targeted the crest of the Blake Ridge Diapir at ODP Site 996. The terrain observed from the submersible ranged from flat to rugged, hummocky surfaces draped by fine, readily suspended silt-clay sediment that varied in color from yellow to grey. The prominent morphologic feature on the ridge crest at 2154 m is a circular depression (50 m diameter, 4 m deep) surrounded by a steeply dipping, smooth rim. The floor of the depression is covered by beds of densely packed, live and dead
Acknowledgements
We thank Captain Silva, the crew of R/V Atlantis, Expedition Leader Dudley Foster, the pilots and technicians of Alvin, and members of the science party for their assistance at sea. A. Shepard and J. Wargo were invaluable in their support of the field effort, as was P. Keener–Chavis in undertaking the educational outreach aspects of the program. I. Macdonald provided the insulated pushcore. We are grateful to taxonomists who provided assistance with identifications of specimens: A.J. Gooday
References (66)
Benthic foraminiferal distribution and biomass related to pore-water oxygen contentCentral California continental slope and rise
Deep-Sea Research A
(1992)- et al.
Thermodynamic and pore water halogen constraints on gas hydrate distribution at ODP site 997 (Blake Ridge)
Chemical Geology
(1999) - et al.
Composition and spatial organization of a cold seep community on the South Barbados accretionary prismtectonic, geochemical and sedimentary context
Progress in Oceanography
(1990) - et al.
The ecology of xenophyophores (Protista) on eastern Pacific seamounts
Deep-Sea Research
(1988) - et al.
Spatial distribution of diverse cold seep communities living on various diapiric structures of the southern Barbados prism
Progress in Oceanography
(1996) - et al.
Cold seep communities as indicators of fluid expulsion patterns through mud volcanoes seaward of the Barbados accretionary prism
Deep-Sea Research I
(1997) - et al.
Biogeography, biodiversity and fluid dependence of deep-sea cold-seep communities at active and passive margins
Deep-Sea Research II
(1998) - et al.
Gas hydrate destabilizationenhanced dewatering, benthic material turnover and large methane plumes at the Cascadia convergent margin
Earth and Planetary Science Letters
(1999) - et al.
Gas venting and late Quaternary sedimentation in the Persian (Arabian) Gulf
Marine Geology
(1996) Geology and biology of modern and ancient submarine hydrocarbon seeps and ventsan introduction
Geo-Marine Letters
(1994)
Microbial processes and products fueled by hydrocarbons at submarine seeps
Radiometric dating of submarine hydrocarbon seeps in the Gulf of Mexico
Geological Society of America Bulletin
The influence of porewater chemistry and physiology on the distribution of vesicomyid clams at cold seeps in Monterey Bayimplications for patterns of chemosynthetic community organization
Limnology and Oceanography
Deep-ocean field test of methane hydrate formation from a remotely operated vehicle
Geology
Deep-sea hydrocarbon seep communitiesevidence for energy and nutritional carbon sources
Science
Multiple trophic resources for a chemoautotrophic community at a cold water brine seep at the base of the Florida Escarpment
Marine Biology
Microbial symbiosis: patterns of diversity in the marine environment
American Zoology
Symbiosis of methylotrophic bacteria and deep-sea mussels
Nature
Evidence for methylotrophic symbionts in a hydrothermal vent mussel (Bivalvia: Mytilidae) from the Mid-Atlantic Ridge
Applied Environmental Microbiology
Oceanic gas hydrate
The US Atlantic continental marginthe best known gas hydrate locality
Growth faulting and salt diapirism; their relationship and control in the Carolina Trough, eastern North America
Gas hydrates that outcrop the seafloorstability models
Geo-Marine Letters
Veil architecture in a sulphide-oxidizing bacterium enhances countercurrent flux
Nature
Chemoautotrophic and methanotrophic symbioses in marine invertebrates
Critical Reviews in Aquatic Sciences
δ13C measurements as indicators of carbon flow in marine and freshwater ecosystems
Contributions in Marine Science
Xenophyophores (Protista), including two new species, from two abyssal sites in the northeast Atlantic Ocean
Journal of Foraminiferal Research
A new genus and five new species of mussels (Bivalvia, Mytilidae) from deep-sea sulfide/hydrocarbon seeps in the Gulf of Mexico
Malacologia
Fauna from a cold sulfur-seep in the Gulf of Mexicocomparison with hydrothermal vent communities and evolutionary implications
Bulletin of the Biological Society of Washington
Seismic studies of the Blake Ridge; implications for hydrate distribution, methane expulsion, and free gas dynamics
Geophysical Monograph
Seabed Pockmarks and SeepagesImpact on Geology, Biology and the Marine Environment
Stable isotope partitioning in seep and vent organismschemical and ecological significance
Chemical Geology
Cited by (168)
Influence of cold-seep environments on the kinetics of methane hydrate formation
2023, Marine and Petroleum GeologyMethane seep communities on the Koryak slope in the Bering Sea
2022, Deep-Sea Research Part II: Topical Studies in OceanographyIn-situ Raman study on kinetics behaviors of hydrated bubble in thickening
2022, Science of the Total EnvironmentPhytodetritus, chemosynthesis, and the dark biosphere: Does depth influence trophic relationships at deep-sea Barbados seeps
2020, Deep-Sea Research Part I: Oceanographic Research PapersSpatial distribution of seepages and associated biological communities within Haima cold seep field, South China Sea
2020, Journal of Sea ResearchNumerical solutions of heat transfer problems in gas production from seabed gas hydrates
2020, Journal of Petroleum Science and Engineering