This expedition follows the goals for gas hydrate drilling as proposed by the ODP Gas Hydrates Program Planning Group: Study the formation of natural gas hydrate in marine sediments.
The objectives of this expedition are to test gas hydrate formation models and constrain model parameters, especially models of hydrate concentration through upward fluid and methane transport. These objectives require (1) high-quality data on the vertical concentration distributions of gas hydrate and free gas and variation landward in the accretionary prism and (2) estimates of the vertical fluid and methane fluxes through the sediment section as a function of landward distance from the deformation front.
The study will concentrate on the contrast between dispersed pervasive upward flow and focused flow of fluid and methane in fault zones. The pervasive permeability may be on a grain scale, on a centimeter scale (the scaly fabric observed in previous ODP clastic accretionary prism cores), or in closely spaced faults. Strong and continuous BSRs may occur only in coarser clastic accretionary prisms (i.e., muddy silt or coarser) where grain or other small-scale permeability allows pervasive upward expulsion. In muddy, low-permeability sediments, fluid expulsion may be focused in discrete faults so hydrate does not form a continuous layer.Geochemistry
Geochemical measurements of gas hydrate, pore fluids, and sediments within and below the gas hydrate stability zone are essential to meet the outlined objectives. In addition to characterizing gas hydrate and free gas depth distribution and geochemistry, critical chemical and isotopic measurements summarized below will provide information on conditions of gas hydrate formation, relation to organic matter content and type, and the nature and temperature of fluid-rock reactions. The chemical and isotopic data are necessary for the understanding of (a) the origin of the methane and other hydrate forming gases, (b) the mode of formation of the methane hydrate, and (c) the source of the fluids carrying the gases sequestered in the gas hydrates, and if more than one source the ratio of sources, and testing which of the hypotheses of gas hydrate formation outlined in the proposal applies to this region. Representative samples will be recovered under in situ conditions with the ODP Pressure Core Sampler (PCS) and the HYACINTH pressure corer (depending on funding availability) to quantify gas concentrations in the gas-hydrate-bearing zone and below the BSR.
Based on previous geochemical studies, especially since ODP Leg 146, the most critical measurements include the following: Chloride concentrations and isotope ratios in the gas hydrates and pore fluids to determine the mode and rate of formation of gas hydrate and the fluid source from greater depths or from the in situ pore fluid (Ransom et al., 1995; Spivack et al., 2002),
Microbiologists participating in ODP for the past decade have demonstrated that more than 105 cells/cm3 consistently exist in the subsurface marine sediments even at depths of ~1000 mbsf. On the basis of these efforts, the subseafloor environment has been proposed to have the largest biomass potentials on Earth, exceeding the biomass in terrestrial and oceanic environments. However, recent data from pore water chemistry such as sulfate and methane in ODP and Deep Sea Drilling Project (DSDP) core sediments suggest that most of the microbial metabolic activity may occur in the narrow and shallow zone near the continents and that most of the deep subseafloor microorganisms are likely either inactive or adapted for extraordinarily low (or slow) metabolic activity in open ocean. Although it is currently unverified from sufficient microbiological characterizations, if it is true, the inactivity of the subseafloor microbiota is ubiquitous and is likely explained by a combination of low-temperature and low-porosity conditions coupled with the extremely slow energy and fluid fluxes conveying inorganic and organic nutrients.
Not all the deep ocean is characterized by low metabolic activity. Regions located in the subduction zones of the continental margin, often bearing a great amount of subseafloor gas hydrate deposits, are considered to be among the places for the prosperity of functionally active, metabolically diverse microbial communities. Indeed, most of the biogenic and mixed type methane hydrate is distributed around continental margins on subduction zones where the organic matter is supplied from continents and active fluid circulation is present (Cascadia margin, Nankai Trough, Black Ridge, Guatemara, Peru margin, etc.). To know which microbes specifically are responsible for producing biogenic methane and how and where they are making it is one of the most exciting challenges for subsurface microbiologists.
There have been several microbiological studies for the subsurface biosphere of biogenic methane hydratebearing sediments with culture-dependent or -independent techniques. Based on the vertical characterization of the biomass and activity of methanogens in the methane hydratebearing sediments from Blake Ridge, North Hydrate Ridge of Cascadia margin, and Nankai Trough, anomalies both in biomass and activity were observed at the methane hydratebearing layers in Blake Ridge and Cascadia margin. However, no satisfactory explanation has been offered for these microbiologic anomalies. The molecular phylogenetic analyses of deep subseafloor sediments in the presence or absence of methane hydrate deposits in Cascadia margin and Nankai Trough have demonstrated the potential microbial community structures inferred from ribosomal ribonucleic acid (rRNA) genes, but the relationship between the microbial community structure and the distribution of methane hydrate is still unclear because of the low analysis frequencies. Thus, the origin of the biogenic methane and the microbial processes in generation, transportation, and accumulation are still resolved.
Microbiological investigation will be conducted by interdisciplinary cooperation with geochemists, mineralogists, geophysicists, and geologists. Major microbiological objectives are (1) to obtain direct evidence of the existence of functionally active methanogens associated with formation of subseafloor methane hydrate deposit on the Cascadia margin and (2) to clarify the controversial hypotheses where and how the biogenic methane is originally produced: deep hot zones far below the present hydrate layers by hyperthermophilic methanogens from H2 and CO2 versus shallow cold sediments above the present hydrate layers by mesophilic methanogens from organics.
Addressing these microbiological objectives will require the following: High-resolution characterization of biomass, diversity, structure, and function of microorganisms at the community and population levels at different depths,
Almost all the microbiological characterizations will be carried out as described below. The drilling core samples are basically collected using advanced piston corer (APC) and extended core barrel (XCB) coring. In the case that APC/XCB coring yields exceptionally poor recovery, we may attempt rotary core barrel (RCB) coring. To prevent possible external microbial contamination during sampling or preservation, the samples for microbiology and fluid and gas geochemistry will be cut into whole round cores (WRCs) according to standard protocol. The procedure for subcoring should be performed in a cool environment (~4°C). Microbial contamination from drilling fluid will be evaluated by using microfluorescent beads and perfluorocarbon as described in Smith et al. (2000). The recovered WRC will be put into a sterilized anaerobic bag filled with nitrogen and stored at less than 80°C or in liquid nitrogen prior to molecular analyses. The innermost core samples will be used for preparation of a variety of slurries for cultivation, chemical fixation of the slurries for optical and electron microscopic observation, nucleic acids extraction, and lipid and biomarker extractions. The following geomicrobiological experiments are considered.Culture-Dependent Analyses
Quantitative cultivation experiments (most probable number [MPN] or serial dilution method) and enrichment analyses will be carried out to enumerate the viable population of microorganisms. Homogenized slurry samples will be inoculated into prepared media and then diluted in a series of liquid media. The results of cultivation show the potential population of viable cells that can grow in designed media for various types of chemolithoautotrophic, carboxydotrophic, methanotrophic, and chemolithoorganotrophic microorganisms under various pH and temperature conditions. The most intensively targeted phenotypes and metabolisms are hyperthermophilic or mesophilic methanogens using H2 + CO2 or organics such as acetate, methylamines, and methanols. Other phenotypes and metabolisms such as H2-oxidizing chemolithoautrophs and anoxic methane oxidiziers (AMO) will also be targeted. Growing cells will be purified by the extinction and dilution method as quantitatively dominant viable cells, and physiological characteristics will be examined in home laboratories according to previous reports.Molecular Ecological Analyses
To characterize the microbial community structure in a given habitat, a combined use of several molecular ecological techniques is the most reliable evaluation. Using nucleic acid and biomarker extracts from the innermost WRC samples, several molecular ecological analyses will be conducted. We will employ the following well-defined and reliable techniques for microbiological investigations in the proposed project: Fluorescence in situ hybridization-direct count (FISH-DC) will be performed on board or on shore to estimate the biomass of the total microbial population and targeted phylogroups of microorganisms in the core samples by fluorescence microscopy.
Gas Hydrate Concentrations and Physical Properties
Calibration of regional estimates of hydrate and free gas volumes based on remote geophysical surveying is of critical importance toward estimating the concentration of gas hydrate and evaluating its role in climate change and resource potential. Recent experience during Legs 146, 164, and 204 underlined the complexity of this issue (see also Riedel et al., in press a). During Expedition 311, we will drill through gas hydrates in a variety of sedimentological settings with different seismic characteristics and measure the physical properties of the gas hydratebearing sediments and underlying free gas zones through downhole logging and pressure coring. The downhole logging data will be complemented by visual core description and other core studies.Gas Hydrates and Slope Stability
Because gas hydrates can be destabilized by pressure and temperature changes, they are potential seafloor geohazards. The formation and dissociation of gas hydrate has a significant influence on the mechanical properties of marine sediments. Replacement of pore water by gas hydrate increases the shear strength as well as reduces the porosity and permeability of sediments (Paull et al., 2000). In turn, during gas hydrate dissociation, free gas and water are released, decreasing the shear strength and making the sediment more prone to failure. The process of gas hydrate decomposition also affects the pore pressure of the sediments (Kayen and Lee, 1993). During gas hydrate dissociation in sediments having pore fluids saturated with methane, the water and free gas released into the pore space will usually exceed the volume that was previously occupied by the hydrate. The net effect is either an increase in pressure (if the sediments are well sealed by a low-permeability cap) or an increase in volume (if the additional pressure can escape by fluid flow). Gas hydrate dissociation can occur due to changes in the pressure/temperature conditions, as outlined above, or due to continued sedimentation. The associated increase in pore pressure, expansion of sediment volume, and the development of free gas bubbles all have the potential to weaken the sediment. Failure could be triggered by gravitational loading (continued sedimentation) or seismic disturbances (earthquakes), yielding slumps, debris flows, and slides. The possible connection between gas hydrate occurrence and submarine slides was first recognized by McIver (1982). Many authors have later related major slumps on continental margins to instability associated with the breakdown of hydrates, including surficial slides and slumps on the continental slope and rise of Southwest Africa (Summerhayes et al., 1979), slumps on the U.S. Atlantic continental slope (Carpenter, 1981), large submarine slides on the Norwegian margin (Jansen et al., 1987), and massive bedding-plane slides and rotational slumps on the Alaska Beaufort Sea continental margin (Kayen and Lee, 1993). Expedition 311 will provide critical information for testing the hypothesis that the presence of gas hydrate may lead to instability of the underlying material by constraining mechanical and hydrological properties of the gas hydratebearing sediments.Cold Vents and Formation (Stability) of Near-Surface Massive Gas Hydrates
Recently, evidence for focused fluid/gas flow and gas hydrate formation has been identified on the Vancouver margin. Several seismic blank zones were observed in the seismic data (Fig. F6) over a frequency range from 20 to 4 kHz, where the degree of blanking increases with seismic frequency. The blank zones range from 80 m to several 100 m in width. The most studied site is an active cold vent field associated with near-surface hydrate close to ODP Sites 889/890 (e.g., Riedel et al., 2002). Studies include high-resolution bottom profiling, 3-D seismic surveys, piston coring, and ocean-bottom video surveying and sampling with the remotely operated vehicle ROPOS. These vents represent fault-related conduits for focused fluid and/or gas migration associated with massive hydrate formation within the fault zone and represent therefore the opposite mechanism to the widespread fluid flow.
Massive hydrate was found at several sites by piston coring within a blank zone (also referred to as Bullseye vent) at depths of 18 mbsf. Increased methane concentrations of up to 8 times the ocean background levels were measured in water samples taken above an active area (Solem et al., 2002). However, venting appears to be strongly episodic. Pore fluid alkalinity gradients from piston cores were converted to sulfate gradients, from which the amount of methane and related fluid flux were calculated. The calculated methane flux varies from 1019 mol/m2k.y. outside the vent to values 32-0 mol/m2k.y. inside the vent. Assuming full methane saturation, the maximum methane flux inside the vent corresponds to a fluid flux of ~1 mm/y.
It is thus far unknown how important these cold vents are in the total budget of fluid flow in an accretionary prism. Drilling at the vent field would help to constrain the significance of fault-related fluid and/or gas flow.
The nature of the seismic blanking is yet unresolved. Several differing models have been published (Wood et al., 2002; Riedel et al., 2002; Zühlsdorff and Spiess, 2004), but none of these models is able to explain all observations made at the vent field. Detailed coring and logging will provide the necessary data to constrain mechanisms to explain seismic blanking at the cold vents.
Two sites are considered: an active cold vent (proposed Site CAS-06A) with widespread carbonate formation, massive near-seafloor gas hydrates, and abundant chemosynthetic communities, as well as an inactive cold vent (proposed Site CAS-06B), where seismic blanking has been observed over a 500 m x 1500 m area.
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