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In Marine Sediments Th or collective redistirbution of any portion article of any by of this or collective redistirbution SPECIAL ISSUE FEATURE articleis has been published in Oceanography GAS 19, Number journal of Th 4, a quarterly , Volume HYDRATES permitted photocopy machine, only is reposting, means or other IN MARINE SEDIMENTS LESSONS FROM SCIENTIFIC OCEAN DRILLING e Oceanography Society. 2006 by Th Copyright BY ANNE M. TRÉHU, CAROLYN RUPPEL, MELANIE HOLLAND, GERALD R. DICKENS, MARTA E. TORRES, TIMOTHY S. COLLETT, with the approval of Th DAVID GOLDBERG, MICHAEL RIEDEL, AND PETER SCHULTHEISS gran is e Oceanography All rights Society. reserved. Permission or Th e Oceanography [email protected] Send Society. to: all correspondence Certain low-molecular-weight gases, naturally in sediment beneath the Arc- Ocean drilling has proven to be an such as methane, ethane, and carbon tic permafrost and in the sediments of important tool for the study of marine dioxide, can combine with water to the continental slope. A decomposing gas hydrate systems, which have been form ice-like substances at high pres- piece of gas hydrate can be ignited and increasingly recognized as important to sure or low temperature. These com- will sustain a fl ame as the methane is society. In some places, methane hydrate pounds, commonly called gas hydrates, released, producing the phenomenon of may be concentrated enough to be an concentrate gas in solid form and occur “burning ice.” economically viable fossil fuel resource. However, geohazards may be associated ted to copy this article Repu for use copy this and research. to ted in teaching Anne M. Tréhu ([email protected]) is Professor, Marine Geology and Geophysics, with gas hydrates as well, through large- College of Oceanic and Atmospheric Science, Oregon State University, Corvallis, OR, USA. scale slope destabilization (e.g., Maslin Carolyn Ruppel is Associate Professor of Geophysics, School of Earth and Atmospheric et al., 2004) and release of methane, a e Oceanography Society, PO Box 1931, Rockville, MD 20849-1931, USA. Sciences, Georgia Tech, Atlanta, GA, USA, now at U.S. Geological Survey, Woods Hole, MA, potent greenhouse gas. Indeed, evidence USA. Melanie Holland is Research Scientist, GEOTEK, Daventry, UK. Gerald R. Dickens collected from deep-sea sediments sug- is Associate Professor, Department of Earth Science, Rice University, Houston, TX, USA. gests that major global warming episodes Marta E. Torres is Associate Professor of Chemical Oceanography, College of Oceanic and in the past were associated with massive Atmospheric Science, Oregon State University, Corvallis OR, USA. Timothy S. Collett is methane releases from gas hydrate de- Research Geologist, U.S. Geological Survey, Denver, CO, USA. David Goldberg is Doherty posits (e.g., Dickens et al., 1995; Hesslebo Senior Scientist and Director, Borehole Research Group, Lamont-Doherty Earth Observa- et al., 2000), although this hypothesis blication, systemmatic reproduction, reproduction, systemmatic blication, tory, Palisades, NY, USA. Michael Riedel is Associate Professor, Department of Earth and remains controversial (e.g., Bowen et Planetary Sciences, McGill University, Montreal, Canada. Peter Schultheiss is Managing al., 2006). Climate changes over the late Director, GEOTEK, Daventry, UK. Quaternary have also been linked to re- 124 Oceanography Vol. 19, No. 4, Dec. 2006 A. B. Ocean Surface 100 10 0 Water Ice + Gas Stability boundary Ice Hydrate Methane hydrate temperature 200 (atm) Pressure Hydrostatic 1000 Water stabilit W ater + Gas GHSZ 2000 y boundar 500 50 CO2, H2S, higher HC Depth (m) salt, Water Ice Seafloor N Geotherm 2 y + Hydrate 3000 1000 100 SeaDepth Surface Below (m) Water + Free Gas Hydrate 2000 200 4000 -10 0 10 20 30 40 -20 -10 0 10 20 30 Temperature (°C) Temperature (°C) Figure 1. (A) Th e methane hydrate stability boundary compared to water ice stability for fresh water. Th e stability boundary moves to the right if gas contains CO2, H2S, or higher-order hydrocarbons and to the left as pore-water salinity increases or if the gas contains N2. For quantitative analy- sis of the eff ects of these parameters on gas hydrate stability, see Sloan (1998). (B) Gas hydrate stability in the marine environment. Th e gas hydrate stability zone (GHSZ) extends from the depth within the ocean at which gas hydrate becomes stable (which depends on local water temperature) to a depth beneath the seafl oor that is determined by the local geothermal gradient. Much of the seafl oor is within the GHSZ. Th e thickness of the GHSZ below the seafl oor increases as water depth increases if the geothermal gradient is constant. lease of methane from submarine gas the U.S. Department of Energy (DOE)/ west coast is an accretionary complex hydrates (Kennett et al., 2003). However, ChevronTexaco Joint Industry Program drilled during ODP Legs 146 (1992) and isotopic studies of methane in ice cores (JIP). Gas-hydrate-bearing marine sedi- 204 (2002), and IODP Expedition 311 (e.g. Sowers, 2006) and budget calcula- ments were fi rst cored during Leg 11 of (2005). The Gulf of Mexico represents a tions for the global carbon cycle suggest the Deep Sea Drilling Project (DSDP) passive margin petroleum province that that the impact of gas hydrates must in 1970 at Blake Ridge (southeast U.S. has been strongly affected by salt tecton- have been minor (Maslin and Thomas, continental margin), and the fi rst deep- ics and has been the subject of IODP 2003). Methane hydrate deposits and the sea gas hydrate specimens were observed site surveys and drilling by the DOE/ associated sediments have also become in sediments recovered from the accre- ChevronTexaco JIP. an important focus for biogeochemical tionary complex of the Middle America studies of the deep biosphere to elucidate trench during DSDP Legs 66 and 67 in GAS HYDRATES IN MARINE the function and community structure 1979. Since that time, gas hydrates have SEDIMENTS of microbes that produce and consume been inferred or observed in numer- The stability of gas hydrate depends methane (e.g., Wellsbury et al., 2000). ous scientifi c boreholes. Here we focus most fundamentally on temperature, In this paper, we summarize the prin- on examples from recent expeditions pressure, gas composition and satura- cipal lessons learned about marine gas in three major gas hydrate provinces. tion, and pore-water composition (Fig- hydrate deposits through scientifi c ocean Blake Ridge is a passive margin sediment ure 1A). Gas hydrate nucleation and drilling, focusing on results of recent drift deposit located on the southeastern growth also depend on sediment grain expeditions conducted by the Ocean United States margin, which was drilled size, shape, and composition (Clennell Drilling Program (ODP), the Integrated during ODP Leg 164 (1995). The Casca- et al., 1999). These parameters, which Ocean Drilling Program (IODP), and dia maring offshore the Pacifi c North- control gas hydrate formation and per- Oceanography Vol. 19, No. 4, Dec. 2006 125 sistence, are affected by a wide range deep-ocean drilling, gas hydrate is lost as depth following the regional geothermal of processes in the ocean, leading to the core experiences decreased pressure gradient, which can be overprinted by multiple possible dynamic feedbacks and increased temperature during recov- local thermal perturbations caused by on various timescales (e.g., Kvenvolden, ery. Sampling gas hydrate, preserving it fl uid fl ow and heat focusing (e.g., near 1988; Paull et al., 1991; Dickens et al., for further study, and using observations salt diapirs). At a depth of a few to sev- 1995; Kennett et al., 2003; Buffett and in recovered cores to infer its concen- eral hundred meters below the seafl oor, Archer, 2004). tration and distribution in situ present the temperature profi le in the sediments Because methane is the dominant gas unique challenges to the ocean drill- crosses the pressure-temperature curve contained in submarine gas hydrates ing community. that defi nes methane stability conditions (Kvenvolden, 1993), we use the stabil- Temperature and pressure condi- (Figure 1B). Based on these criteria, gas ity curve calculated for pure methane tions consistent with methane hydrate hydrate could potentially form almost Structure-I hydrate in this discussion. At stability are found at the seafl oor nearly everywhere beneath the continental atmospheric pressure, methane hydrate everywhere at water depths exceeding slope and ocean basins. In most of the is stable only at temperatures below 300–800 m, depending on regional sea- deep ocean basin, however, methane ~ -80°C. Thus, when a sample contain- water temperature (Figure 1B). Temper- concentrations in pore water are below ing gas hydrate is recovered through atures below the seafl oor increase with saturation (Claypool and Kaplan, 1974). Figure 2. World map with documented and inferred gas hydrate occurrences. Inferred gas hydrate occurrences are based primarily on the presence of a seismic refl ection known as the Bottom-Simulating Refl ection (BSR), velocity amplitude peculiarities on seismic re- cords, well-log signatures indicative of the presence of gas hydrate, and freshening of pore waters in cores. Samples were recovered us- ing research submersibles, remotely operated vehicles (ROVs), grab samplers, dredges, piston coring, and coring during DSDP, ODP, and IODP operations. Data from Kvenvolden and Lorenson (2001) and updated by Milkov (2005). Reprinted from Doyle et al. (2004). 126 Oceanography Vol. 19, No. 4, Dec. 2006 This condition means that gas hydrate Short-term changes in bottom water settings. The thickness of this meth- is, generally, restricted to continental temperature and pressure due to tides, ane-depleted zone, the base of which margins and enclosed seas (Figure 2), currents, or deep eddies can also affect coincides with depletion in sulfate, is a where organic matter accumulates rap- gas hydrate deposits (e.g., Ruppel, 2000; valuable proxy for methane fl ux in diffu- idly enough to support methane produc- MacDonald et al., 1994, 2005).
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