The structural and stratigraphic setting of Hydrate Ridge contrasts with that of the adjacent slope basin to the east. Beneath the slope basin, the seismic signature of the hydrate is quite similar to that on the Blake Ridge, with an intermittent BSR and enhancement of stratigraphic reflectivity beneath the BSR (Holbrook et al., 1996). Sedimentation rate in this basin is likely very rapid, based on radiocarbon dating of a core in a neighboring basin just north of Hydrate Ridge (Karlin, 1983), which indicates a sedimentation rate of 120 cm/k.y. Sediments in that core are siliceous hemipelagic ooze with calcareous microfossils, and similarity in high-frequency energy penetration between the two basins suggests a similar sediment composition. Because of this expected high sedimentation rate and high carbon content in the sediments, we suspect that the source of methane in this setting will be dominantly local, with little or no contribution from subducted sediments. In contrast, fluids migrating upward from underthrust sediments (Fig. F2A) may be a significant source of methane for hydrate beneath Hydrate Ridge (Hyndman and Davis, 1992). Leg 204 will test the hypothesis that the distribution, texture, and chemistry of hydrate and related pore fluids beneath Hydrate Ridge differ from those of the slope basin.
The presence of massive hydrate near the seafloor is enigmatic, as most models for hydrate formation in a region of diffuse fluid flow predict a decreasing gradient in hydrate concentration above the BSR (e.g., Paull et al., 1994; Rempel and Buffett, 1998; Xu and Ruppel, 1999). Several explanations have been proposed, including formation in the past when the stability boundary was near the seafloor, formation at depth and exposure by erosion (Bohrmann et al., 1998), and transport of methane through the hydrate stability field as free gas isolated from water (Suess et al., 2001). The third explanation is most likely, given the observations of vigorous plumes of bubbles at the seafloor and in the water column where the massive hydrates are observed. The mechanisms whereby the gas is isolated from water, thus delaying hydrate formation as it passes through the stability zone, remains enigmatic. Drilling through these massive hydrate deposits will provide evidence on the extent of their distribution and texture, the composition of related pore waters, and their association with structural features.
Geochemical consequences of hydrate formation and destabilization include modification of the isotopic composition of the water in pore fluids; changes in the isotopic composition of the dissolved carbonate species, which is incorporated into carbonate phases; and sequestering of the in situgenerated H2S into the hydrate structure. Isotopic composition of carbonate cements recovered by drilling during Leg 146 were used by Sample and Kopf (1995) to infer the history of fluid flow in this margin as well as the depth of the source of the carbon reservoirs. Bohrmann et al. (1998) have suggested that the stability of the massive gas hydrate deposits on the southern summit of Hydrate Ridge has changed with time and that carbonate phases associated with the hydrates can be used to document the changes. Benthic foraminifers might also record this decrease in 13C, and thus the isotopic signal might reveal episodes of CH4 venting in the past (Wefer and al., 1994; Dickens et al., 1995, 1997; Kennett et al., 1996; Torres et al., unpubl. data). Analyses of the isotopic composition of the pore fluids in Leg 204 cores, where fluid flow rate and mechanisms are expected to vary among the sites, will provide the framework needed to unravel the history of hydrate formation and destabilization recorded in the O and C isotopes in benthic foraminifers and authigenic carbonate phases.
Better calibration of regional estimates of hydrate and free gas volumes based on geophysical mapping and modeling techniques is of critical importance toward estimating the global abundance of hydrate and evaluating its role in climate change and potential for economic exploitation. Recent experience during Legs 146 and 164 has underlined the complexity of this issue. During Leg 204, we will drill through hydrates in a variety of settings with different seismic characteristics and measure the physical properties of the hydrate stability and underlying free gas zones through downhole logging and a series of nested seismic experiments. The geophysical data will be referenced to direct observations of cores to address this fundamental objective of current hydrate studies.
The possible relationship between hydrates and slope failure is presently poorly understood. On one hand, hydrates may stabilize slopes by cementing grains. On the other hand, if hydrates impede fluid flow, they may weaken the underlying sediment by trapping fluids and free gas. There may be a feedback between these two processes such that the presence of hydrate initially delays slumping, leading to less frequent but larger episodes. Several investigators have noted the possible correlation of hydrates and slope instability (e.g., Booth et al., 1994; Trehu et al., 1995; Paull, Matsumoto, Wallace, et al.,1996) and have discussed how such slope instability might release massive amounts of methane into the ocean (Paull, Matsumoto, Wallace, et al., 1996; Nisbet and Piper, 1998). Leg 204 will provide critical information for testing the hypothesis that the presence of hydrate leads to instability of the underlying material by constraining mechanical and hydrological properties.
Microorganisms play an important role in both methane formation and oxidation and are therefore a critical component of the hydrate system. Identification of these organisms and determination of their abundances, spatial variability, and rates of activities is just beginning. Important questions to address during Leg 204 include the following: What impact do these organisms have on the volume of methane produced and oxidized beneath Hydrate Ridge? At what depths are they concentrated? What effect do they have on sediment diagenesis and the development of magnetic minerals? Does the hydrate-related biosphere differ between Hydrate Ridge and the adjacent slope basin? How do microorganisms affect sediment and pore water chemistry and texture, and vice-versa?
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