ODP Leg 164 was devoted to furthering our understanding of the in situ characteristics of gas hydrates and gas hydrate-bearing sediments and to ground-truthing the nature of BSRs. The Blake Ridge gas hydrate field was targeted for drilling because it is associated with an extensive and well-developed BSR that could be considered the archetypical section for BSRs.
During Leg 164, three closely spaced ~700-m-deep sections were drilled through the base of gas hydrate stability within the same stratigraphic unit on the Blake Ridge. This short transect extends 10 km from ridge flanks, where there is no BSR in the seismic profiles, to the ridge crest, where a very strong BSR exists in the seismic profiles.
Most of the materials recovered on Leg 164 were deposited during the Pliocene and Miocene at very rapid rates (as much as 350 m/m.y.) by the southerly flowing Western Boundary Undercurrent, that sweeps along the Atlantic Margin. The stratigraphic sequence is composed of lithologically monotonous hemipelagic clays; this allows the distribution of gas hydrate and the origin of the BSR to be studied with minimal lithologic complication. The sediments are moderately organic carbon rich (~1.5%) and, thus, have the raw material to produce quantities of biogenic gas.
The following conclusions were determined for the scientific objectives of this leg.
(1) Amounts of gas trapped in hydrate-bearing sediments
Preliminary analysis from Leg 164 drill sites on the Blake Ridge indicates that gas hydrate occupies 1% to 2% of the sediment volume in a zone that is 200-250 m thick. In fact, the site without a BSR also contained ~1% gas hydrate through a similarly thick zone. If the rest of the ~26,000-km2 region around the Blake Ridge where BSRs are present contains as much gas hydrate, rough estimates indicate that about 10 Gt of methane carbon is stored in this region. Given the number of localities worldwide in which gas hydrate occurs, the results of ODP Leg 164 provide further evidence that methane stored as gas hydrate in marine sediments represents a signficant component of the global fossil fuel carbon reservoir.
(2) Lateral variability in gas hydrate abundances
Interstitial water chemistry is very similar at the three deeply drilled (~750 mbsf) sites on the transect along the Blake Ridge (Sites 994, 995, and 997). Downhole profiles of all dissolved species show the same general trends with the same approximate depths for maxima and minima. Chloride profiles were used as a proxy indicator for the amount of in situ gas hydrate. When gas hydrate forms in sediment, water and gas are removed from the pore space, causing the residual pore waters to become saltier. Over long periods of time the local salinity anomalies produced by gas hydrate formation diffuse away. However, many of the cores that were recovered contained surprisingly fresh interstitial waters, indicating that gas hydrate had decomposed within these sediments during drilling and core recovery. A zone with anomalously low chloride values occurs at approximately the same depths (~200 to 440 mbsf) over the 10-km transect. Furthermore, the two depth intervals showing the largest chloride excursions, at approximately 250 and 400 mbsf, are also depth correlative. Thus, the similarity of the Cl- profiles at the ridge sites strongly suggests that gas hydrate is correlative over the expanse of the Blake Ridge, including Site 994, where no BSR is present in the seismic profile.
(3) Relationship between bottom-simulating reflectors and gas hydrate development
Downhole sonic log and vertical seismic profile data indicate decreasing interval velocities near the depth of the BSR. The change in sediment velocity may be related to either changes in the amount of hydrate above the BSR or the presence of gas bubbles below. Both types of sonic data indicate that gas bubbles are present at greater depths, but simple shipboard analyses of the absolute velocities do not require that the gas be present immediately below the BSR at all these sites. This issue can be resolved by combining pressure core sampling data, temperatures of cores measured after recovery, and interstitial-water chloride concentrations. Whereas the pressure core sampler data indicates that there must either be hydrate or gas bubbles present, the temperature and interstitial-water chloride data demonstrate that gas hydrate is not present below the BSR. Thus, there are gas bubbles beneath the BSR at these sites.
(4) Distribution and in situ fabric of gas hydrate within sediments
As anticipated, the recovery of gas hydrate proved to be difficult. The cores were very gassy, causing sediment to extrude from the liners as the cores arrived on deck. Some large pieces of gas hydrate were recovered. Gas hydrate recovered at Site 996 occurred as massive pieces, as veins filling vertical fractures, and as rod-shaped nodules. Fine-grained hydrate was not directly observed in sediments from any of the sites, but proxy measurements, such as the chloride concentration of interstitial pore waters, indicated that fine-grained hydrates had been present prior to core recovery. Calculations indicate that sediments in the zone from 200 to 450 mbsf had a minimum average gas hydrate content of 1% and that some individual samples contained more than 8% gas hydrate.
(5) Changes in physical properties (porosity, permeability, P-wave velocity, thermal conductivity, etc.) associated with gas hydrate formation and decomposition in continental margin sediments
Well-log measurements show the gas hydrate-bearing and free gas-bearing zones are associated with distinct characteristics, whereas shipboard lithologic and physical properties measurements did not indicate any differences in these sediments. The distinctions between shipboard and downhole measurements are believed to result from the presence of gas hydrate in situ and will be the subject of shore-based research.
(6) Source of gas contained in gas hydrate (local production or migration)
As discussed below, the interstitial-water chemical data at Site 996 suggest that methane is transported upward along faults from underlying gas hydrate-bearing sediments. This is consistent with the occurrence of hydrate as vein fillings. At Sites 994, 995, and 997, constraints on the sources of gas contained in gas hydrate must await shore-based isotopic and organic geochemical studies, as well as detailed analysis of the PCS data that will delineate the distribution of hydrate, free gas, and methane dissolved in interstitial waters.
(7) Role of gas hydrate in formation of authigenic carbonate
Fine-grained, disseminated authigenic carbonate was present in sediments at all sites. Indurated diagenetic carbonate was recovered primarily at Site 996 but also from one horizon at Site 993. At Site 996, intersititial-water chemical data suggests that carbonate precipitation is caused by intense microbial oxidation of methane, which results in high alkalinity and high bicarbonate concentrations. The methane arriving at the seafloor at this site is largely biogenic and has a composition similar to that of the gases from Sites 994, 995, and 997. Fluids and gases venting at Site 996 have probably been transported upward from the underlying gas hydrate-bearing sediment section that slopes up around the diapir.
(8) Chemical and isotopic composition, hydration number, and crystal structure of natural gas hydrate
Gas hydrate decomposition experiments indicate volumetric ratios of gas:water range from 130 to 160 and show that gas in the hydrate is ~99% methane. Numerous hydrates were sampled for storage in pressure vessels and will be used for shore-based isotopic and crystallographic studies.
(9) Role of gas hydrates in stimulating or modifying fluid circulation
An intriguing association of pore waters was documented. Pore waters are systematically fresher than seawater within the gas hydrate-bearing sections. One of the potential explanations of such patterns is that salinity changes associated with gas hydrate decomposition may be stimulating fluid circulation in these sedimentary sections. Other anomalies await shore-based isotopic and modeling studies, including variations in the amounts of free gas and gas hydrate and the sedimentary section, the occurrence of both thermogenic and biogenic hydrocarbon gases, and unexplained patterns of both the major and minor elements in dissolved pore waters.
(10) Connection between large-scale sediment failures and gas hydrate decomposition
Drilling along the top of the Cape Fear Diapir, which breeches the seafloor within the scar of the giant Cape Fear Slide, revealed soft-sediment deformation features and several biostratigraphic gaps. Both are the result of mass-transport processes associated with sediment failure and were caused by the Cape Fear Slide or emplacement of the diapir. The results of drilling did not provide any direct evidence as to whether large-scale sediment failures are related to gas hydrate decomposition.
(11) Origin of the Carolina Rise Diapirs and their influence on associated sedimentary gas hydrate
The chloride concentration of interstitial waters from sediments overlying the Cape Fear Diapir (Sites 991, 992, 993) and the Blake Ridge Diapir (Site 996) are high; the ratio of Cl- to other ions suggests nearby sources of evaporitic salts. Thus, it seems likely that both diapirs are salt cored. Presumably the salt has risen from deep within sediments of the Carolina Trough, the base of which is approximately 8 km below the seafloor at these sites.
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