Gas Hydrates of the Blake Outer Ridge Site 533, Deep Sea Drilling Project Leg 76

Kvenvolden, Keith A.; Barnard, Leo A.

doi:10.2973/dsdp.proc.76.106.1983

Cited by 74 publications

(66 citation statements)

References 19 publications

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“…The tectonically-induced dewatering of accreted sediments and the expulsion of overpressured pore fluids to the ocean floor is the mechanism for accumulation of gas hydrates in the accretionary wedge (e.g., Kvenvolden and McMenamin, 1980;Kvenvolden and Barnard, 1983). Both high amounts of free gas, and pellets and crystals of gas hydrates cause bottom-simulating reflectors (BSRs) that are believed to indicate the base of gas hydrate stability.…”

Section: Introductionmentioning

confidence: 99%

Diagenetic Carbonates from Cascadia Margin: Textures, Chemical Compositions, and Oxygen and Carbon Stable Isotope Signatures

Kopf¹,

Sample²,

Bauer³

et al. 1995

Proceedings of the Ocean Drilling Program

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Active fluid venting and carbonate formation occur in the accretionary prism in the Cascadia subduction zone off Vancouver Island and Oregon. Methane-derived authigenic carbonates are precipitated in the uppermost terrigenous sediments by subduction-induced dewatering. They can be divided into cemented silts and sands containing less than 10% carbonate cements and concretions composed partially or wholly of carbonate. Textures, chemical compositions, and carbon and oxygen isotope signatures of 64 specimens of mainly carbonate concretions from Ocean Drilling Program Sites 889/890, 891, and 892 were studied using different methods. Mineralogically, the carbonate cement consists of high-magnesian calcite, dolomite, and calcite with varying amounts of detrital constituents. Results from x-ray fluorescence, x-ray diffraction, and electron microprobe analyses suggest dominance of high magnesian calcite in most of the carbonate concretions. Complex carbonates and protodolomite to pure dolomite are found in minor amounts. Cemented sediments and concretions taken in the vicinity of a prominent fault zone at Site 892 show an almost pure calcitic composition and the most intense deformation of all material drilled in the Cascadia Margin. Concretion fragmentation is believed to be caused by both brecciation and hydrofracture from mechanical stress and lowered lithostatic load during upward migration of rocks and fluids along the fault plane. Strain evaluation based on distortions of initially homogeneous marker particle distributions (the Fry technique) obtained aspect ratios (R f ) between 1.2 and 1.71 reflecting uniaxial shortening in the sediment as a response to compaction prior to carbonate formation. Compactional strains (e z , elongation parallel to core axis) are found to range from -0.17 to -0.42 and give evidence for significant settling of the sediment prior to concretion formation. Evidence for post-formational compaction, displacement of carbonate, or recrystallization incipient diagenesis could not be observed. Stable carbon and oxygen isotope signatures identify distinct groups of methane-derived carbonate concretions. Almost half of the examined concretions show variable carbon isotope signatures, ranging from -12%c to -52%c PDB (Peedee belemnite) with no systematic trend downhole. These carbonate concretions evidently reflect precipitation from thermogenic methane, probably with contributions of biogenic methane in minor amounts. A second set of samples is characterized by δ 13 C values between -6% 0 to +24%o PDB. Values between -4%o and 0%o reflect marine carbonate precipitated from seawater. The substantial enrichment in 13 C is most likely caused by supply of CO 2 evolved from fermentation in the sediment pile. Oxygen isotope ratios largely scatter around δ 18 θ values for modern ocean water (-l%c to +6%c PDB). Exceptional "light" δ 18 θ signatures with values ranging from -ll%o to -17%e PDB were only found in the vicinity of a prominent fault zone at Site 892. That depletion in 18 O may be expla...

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Section: Introductionmentioning

confidence: 99%

Diagenetic Carbonates from Cascadia Margin: Textures, Chemical Compositions, and Oxygen and Carbon Stable Isotope Signatures

Kopf¹,

Sample²,

Bauer³

et al. 1995

Proceedings of the Ocean Drilling Program

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“…In addition, one site was selected for the specific study of gas hydrates, with specialized equipment to be deployed. This is the second site selected for intensive study of gas hydrates, the first being Site 533 on Leg 76 (Kvenvolden and Barnard, 1983).…”

Section: Introductionmentioning

confidence: 99%

Isotopic Composition of Interstitial Fluids and Origin of Methane in Slope Sediment of the Middle America Trench, Deep Sea Drilling Project Leg 84

Claypool¹,

Threlkeld²,

Mankiewicz³

et al. 1985

Initial Reports of the Deep Sea Drilling Project

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CH 4 and CO 2 species in pore fluids from slope sediments off Guatemala show extreme 13 C-enrichment (δ 13 C of -41 and +38‰, respectively) compared with the typical degree of 13 C-enrichment in pore fluids of DSDP sediments (δ 13 C of -60 and + lO‰). These unusual isotopic compositions are believed to result from microbial decomposition of organic matter, and possibly from additional isotopic fractionation associated with the formation of gas hydrates. In addition to the isotopic fractionation displayed by CH 4 and CO 2 , the pore water exhibits a systematic increase in δ 18 θ with decrease in chlorinity. As against seawater δ 18 θ values of 0 and chlorinity of 19‰, the water collected from decomposed gas hydrate from Hole 570 had a δ 18 θ of + 3.0‰ and chlorinity of 9.5‰. The isotopic compositions of pore-fluid constituents change gradually with depth in Hole 568 and discontinuously with depth in Hole 570.

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“…The "BGH" indicates the calculated base of stable gas hydrate at Site 987, which is estimated based on the pressure-temperature stability field of gas hydrates (cf. Kvenvolden and Barnard, 1983). >^BGH maximum recorded temperature at 490 mbsf in Hole 987E was 18°C.…”

Section: Downhole Temperature Measurementsmentioning

confidence: 90%

“…16), based on the pressure-temperature stability field of gas hydrates (cf. Kvenvolden and Barnard, 1983), a geothermal temperature gradient of about 104°C/ km (see "Physical Properties" section, this chapter), a water depth of 1684 m, and a bottom-water temperature of about 0°C. Therefore, the high concentration of methane between 30 and 180 mbsf may potentially be related to the presence of methane hydrates.…”

mentioning

confidence: 99%

Site 987

1996

Proceedings of the Ocean Drilling Program

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Coring totals:Type HOLE 987C'Jansen, E., Raymo, M.E., Blum, P., et al., 1996. Proc. ODP, Ink. Repts., 162: College Station, TX (Ocean Drilling Program). Coring totals:Type Formation:Unit II: 363.3-369.2 mbsf; late Pliocene; silty clay with sand and gravel (debris flows) Unit III: 369.2-575.5 mbsf; late to early Pliocene; silty clay, clayey silt with sand, tilted/contorted beds, dropstones Unit IV: 575.5-657.6 mbsf; early Pliocene; silty clay with sand and gravel (debris flows) Unit V: 657.6-859.4 mbsf; early Pliocene to Miocene; silty clay, very indurated, dropstones, dipping/contorted beds Principal results: Five holes were drilled at Site 987 (EGM-4), with a maximum penetration of 859.4 mbsf, estimated to be within a few meters of oceanic basement at this site. All major seismic sequence boundaries were penetrated with overall good recovery. Despite gas expansion problems in the upper few hundred meters of the section, offsets between holes could be determined for the upper -180 mbsf, and a continuous spliced section was produced for the upper 100 mbsf (approximately the last 1 Ma). Paleomagnetic data provided time control, enabling detailed documentation of the glacial history of the Greenland Ice Sheet back to the late Miocene. Biostratigraphic age information is scarce due to predominance of fossilbarren intervals. The recovered sediments are mostly fine-to coarse-grained siliciclastics. Evidence for glacial depositional environments prevails throughout the recovered section. Five lithostratigraphic units were defined.Unit I (0-305.6 mbsf; Pleistocene to late Pliocene) consists of silty clays interbedded with silt and silt with sand and clay. Thick sandy turbidites with sharp erosional lower contacts and graded upper contacts are common. Turbidites are thicker and most common toward the top of the unit. Dropstones were identified throughout this unit. Sediments containing up to 35% carbonate occur in inorganic calcite-rich bands, but the average for the unit is only 4.9%. Unit I is characterized by low magnetic susceptibility.Unit II (305.6-369.2 mbsf; late Pliocene) consists of predominantly silty clay with sand, silty clay with sand and gravel, sand-silt-clay, and clayey silt with sand and gravel. Carbonate content is very low, but some inorganic calcite layers occur at the base of the unit. The unit contains numerous clasts >l.O cm (up to 91 clasts in Core 162-987E-40X) and randomly oriented gravel, interpreted as components of debris-flow deposits rather than as dropstones. Crystalline rocks are the dominant rock type of these clasts. The unit is primarily defined by the presence of numerous isolated clasts. The top of Unit II is marked by a sharp increase in magnetic susceptibility and sand-sized fraction, as well as a decrease in the claysized content.Unit III (369.2-575.5 mbsf; late to early Pliocene) is composed of silty clay with sand, silty clay with sand and gravel, silty clay with gravel, and silty clay. It is defined by a downcore decrease in magnetic susceptibility and in sand-sized particl...

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Gas Hydrates of the Blake Outer Ridge Site 533, Deep Sea Drilling Project Leg 76

Cited by 74 publications

References 19 publications

Diagenetic Carbonates from Cascadia Margin: Textures, Chemical Compositions, and Oxygen and Carbon Stable Isotope Signatures

Diagenetic Carbonates from Cascadia Margin: Textures, Chemical Compositions, and Oxygen and Carbon Stable Isotope Signatures

Isotopic Composition of Interstitial Fluids and Origin of Methane in Slope Sediment of the Middle America Trench, Deep Sea Drilling Project Leg 84

Site 987

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