Blake Ridge methane seeps: characterization of a soft-sediment, chemosynthetically based ecosystem

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Abstract

Observations from the first submersible reconnaissance of the Blake Ridge Diapir provide the geological and ecological contexts for chemosynthetic communities established in close association with methane seeps. The seeps mark the loci of focused venting of methane from the gas hydrate reservoir, and, in one location (Hole 996D of the Ocean Drilling Program), methane emitted at the seafloor was observed forming gas hydrate on the underside of a carbonate overhang. Megafaunal elements of a chemosynthetically based community mapped onto dive tracks provide a preliminary overview of faunal distributions and habitat heterogeneity. Dense mussel beds were prominent and covered 20×20 m areas. The nearly non-overlapping distributions of mussels and clams indicate that there may be local (meter-scale) variations in fluid flux and chemistry within the seep site. Preliminary evidence suggests that the mussels are host to two symbiont types (sulfide-oxidizing thiotrophs and methanotrophs), while the clams derive their nutrition only from thiotrophic bacteria. Invertebrate biomass is dominated by mussels (Bathymodiolus heckerae) that reach lengths of up to 364 mm and, to a lesser extent, by small (22 mm length) vesicomyid clams (Vesicomya cf. venusta). Taking into account biomass distributions among taxa, symbiont characteristics of the bivalves, and stable-isotope analyses, the relative importance of methanotrophic vs thiotrophic bacteria in the overall nutrition of the invertebrate assemblage is on the order of 60% vs 40% (3:2).

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

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