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Blake Ridge Methane Seeps: Characterization of a Soft-Sediment, Chemosynthetically Based Ecosystem

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Blake Ridge Methane Seeps: Characterization of a Soft-Sediment, Chemosynthetically Based Ecosystem Deep-Sea Research I 50 (2003) 281–300 Blake Ridge methane seeps: characterization of a soft-sediment, chemosynthetically based ecosystem C.L. Van Dovera,*, P. Aharonb, J.M. Bernhardc, E. Caylord, M. Doerriesa, W. Flickingera, W. Gilhoolyd, S.K. Goffredie, K.E. Knicka, S.A. Mackod, S. Rapoporta, E.C. Raulfsa, C. Ruppelf, J.L. Salernoa, R.D. Seitzg, B.K. Sen Guptah, T. Shanki, M. Turnipseeda, R. Vrijenhoeke a Biology Department, College of William & Mary, Williamsburg, VA 23187, USA b Department of Geological Sciences, University of Alabama, Box 870338, Tuscaloosa, AL 35487, USA c Department of Environmental Health Sciences, University of South Carolina, Columbia, SC 29208, USA d Department of Environmental Sciences, University of Virginia, Charlottesville, VA 22903, USA e Monterey Bay Aquarium Research Institute, 7700 Sandholt Road, Moss Landing, CA 95039, USA f School of Earth & Atmospheric Sciences, Georgia Tech., Atlanta, GA 30332, USA g Virginia Institute of Marine Science, P.O. Box 1346, Gloucester Point, VA 23062, USA h Department of Geology & Geophysics, Louisiana State University, Baton Rouge, LA 70803, USA i Biology Department, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA Received 24 May 2002; received in revised form 23 October 2002; accepted 26 November 2002 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 Â 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). r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Blake Ridge; Methane hydrates; Chemosynthesis; Stable isotopes; Bathymodiolus heckerae; Seeps *Corresponding author. E-mail address: [email protected] (C.L. Van Dover). 0967-0637/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0967-0637(02)00162-0 282 C.L. Van Dover et al. / Deep-Sea Research I 50 (2003) 281–300 1. Introduction seafloor (mbsf; Paull et al., 1996). At some locations, however, gas hydrate and underlying A quarter of a century has passed since the first free gas occur close to the seafloor, and interaction exploration of hydrothermal vents, yet the pro- of the hydrate reservoir with geologic, oceano- spect remains for discovery of biogeographically graphic, and other processes leads to the develop- and ecologically distinctive types of chemosyn- ment of focused seeps. thetic systems in the world’s oceans. Exploration The US Atlantic continental margin south of and investigation of these systems will allow us to 34N is among the most extensively mapped gas- understand the diversity of habitats, species, and hydrate provinces in the world’s oceans. Several adaptations that can be supported by chemosyn- generations of seismic surveys (e.g., Tucholke et al., thesis. In this report, we provide preliminary 1977; Rowe and Gettrust, 1993; Taylor et al., characterization of a soft-sediment, chemosynthe- 1999; Holbrook, 2000) map a regionally extensive tically based ecosystem associated with a methane bottom-simulating reflector (BSR) in this area. hydrate province on the continental margin of the The BSR is a negative-impedance contrast reflec- eastern United States. Gas hydrates close to the tor that marks the phase boundary between sediment–seawater interface are also known from overlying gas hydrate and underlying free gas. other regions, including the Gulf of Mexico While gas hydrates are known to occur on the (MacDonald et al., 1994), the Barbados accre- Blake Ridge at locations with no BSR (e.g., Paull tionary complex (Olu et al., 1996), the Barents Sea et al., 1996), the presence of a BSR beneath a large (Egorov et al., 1999), and the Cascadia margin off part of the Blake Ridge implies widespread Oregon (Suess et al., 1999; Sahling et al., 2002). occurrence of gas hydrates. The focus site for this study lies near the A line of about 20 salt diapirs begins near the intersection of the Carolina Rise and the Blake intersection of the Blake Ridge with the Carolina Ridge (Fig. 1). This area of the South Atlantic Rise and extends northward on the eastern side of Bight has long been recognized as a major gas the Carolina Trough (Dillon et al., 1982). The hydrate province within the US Exclusive Eco- diapirs rise to within 600 m of the seafloor and nomic Zone (e.g., Markl et al., 1970; Tucholke disrupt the overlying sediments. Interaction be- et al., 1977; Paull and Dillon, 1981). Over most of tween the Blake Ridge Diapir (the southern-most the region, the top of the methane hydrate deposit diapir) and the underlying methane-hydrate re- probably lies at depths greater than 100 m below servoir was extensively investigated by Taylor et al. (2000). The high thermal conductivity of the diapir alters the local stability field for methane hydrates, ο ο ο ο ο causing upward warping of the BSR and shifting 82 W 80 W 78 W 76 W 74 W of the gas hydrate and free-gas system to shallower NC levels in the sedimentary section. At the same time, ο partial dissolution of the salt diapir raises local 34 N SOUTH CAROLINA pore-water salinities, further inhibiting gas-hy- 4000 drate stability and possibly contributing to the se increased mobility of fluids in the sedimentary ο section (Taylor et al., 2000). Emplacement of the 32 N GA Carolina Ri diapir has been accompanied by the development Blake of faults that act as conduits for the transfer of free Ridge Continental Shelf gas and waters rich in dissolved gas toward the 1000 2000 3000 seafloor (Paull et al., 1995). ο 30 N Seismic reflection profiles across the Blake Fig. 1. Blake Ridge Diapir study site location. Star indicates Ridge Diapir (e.g., USGS CH-06-92 Line 37) Blake Ridge Diapir; shaded off-shore area delineates region of show a prominent BSR that shoals over the diapir, gas-hydrate deposits; contour lines are in 1000-m intervals. and a fault that extends from the BSR to nearly C.L. Van Dover et al. / Deep-Sea Research I 50 (2003) 281–300 283 nates and gas hydrates were documented in core CH-95-18, Line 23 996 material, and collection of mussels at the tops of 3.0 escaping gas cores provided evidence for the existence of a chemosynthetic community on the crest of the diapir (Paull et al., 1996). BSR The geographic location of the Blake Ridge core of diapir methane seep raises questions about the biogeo- Two-Way Time (s) 3.5 graphical affinities of its fauna. The closest known 3 km deep-sea seep sites are those of the Barbados region to the southeast (Jollivet et al., 1990; Olu 200 300 400 500 et al., 1996, 1997) and of the Florida Escarpment, SW Shot Number NE on the opposite side of the Florida peninsula Fig. 2. Seismic profile across the Blake Ridge Diapir showing (Paull et al., 1984; Hecker, 1985). There is a the prominent Bottom-Simulating Reflector (BSR). Single perception that seeps support faunas that are more channel seismic data (line 23 of CH-95-18) collected by the USGS across the Blake Ridge Diapir during a cruise aboard the endemic to local regions than hydrothermal vents R/V Cape Hatteras in 1995 (Taylor et al., 1999). The raw data (Sibuet and Olu, 1998); comparisons of species have been slightly reprocessed. The seismic line crosses the lists and genetic differentiation in species from diapir from southwest to northeast. The BSR is warped upward these sites can be used to test this hypothesis. and disrupted over the core of the diapir, particularly between In this paper, we report new data that enhance shots 340 and 370. Vertically oriented features above the core of the diapir mark gas migration paths. our understanding of the geological context and ecological setting for the chemosynthetically based community on the Blake Diapir. Further accounts the seafloor (Fig. 2). Chemosynthetic communities of methane hydrate formation, foraminiferal and gas-rich plumes rising up to 320 m in the water biology and ecology, and quantitative analyses of column have been detected where the fault system the invertebrate assemblage associated with Blake intersects the seafloor (Paull et al., 1995, 1996). Ridge mussel beds will be presented elsewhere. Sediments consisting of hemipelagic silt and clay with 20–40% pelagic carbonate (Paull et al., 1995; Dillon and Max, 2000b) drape the diapir. These 2. Materials and methods sediments, which were deposited by strong, south- flowing near-bottom currents, were accreted ra- Four Alvin dives were conducted at the Blake pidly (up to 48 cm kaÀ1) during the late Pleistocene Ridge Diapir site (ODP Site 996; 3229.6230N, interval (Paull et al., 1996).
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