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Microbial methane generation and gas transport in shallow sediments of an accretionary complex, southern hydrate ridge (ODP Leg 204), offshore Oregon, USA
Sediments at the southern summit of Hydrate Ridge display two distinct modes of gas hydrate occurrence. The dominant mode is associated with active venting of gas exsolved from the accretionary prism and leads to high concentrations (15%â40% of pore space) of gas hydrate in seafloor or near-surface sediments at and around the topographic summit of southern Hydrate Ridge. These near-surface gas hydrates are mainly composed of previously buried microbial methane but also contain a significant (10%â15%) component of thermogenic hydrocarbons and are overprinted with microbial methane currently being generated in shallow sediments. Focused migration pathways with high gas saturation (>65%) abutting the base of gas hydrate stability create phase equilibrium conditions that permit the flow of a gas phase through the gas hydrate stability zone. Gas seepage at the summit supports rapid growth of gas hydrates and vigorous anaerobic methane oxidation. The other mode of gas hydrate occurs in slope basins and on the saddle north of the southern summit and consists of lower average concentrations (0.5%â5%) at greater depths (30â200 meters below seafloor [mbsf]) resulting from the buildup of in situâgenerated dissolved microbial methane that reaches saturation levels with respect to gas hydrate stability at 30â50 mbsf. Net rates of sulfate reduction in the slope basin and ridge saddle sites estimated from curve fitting of concentration gradients are 2â4 mmol/mÂł/yr, and integrated net rates are 20â50 mmol/ m²/yr. Modeled microbial methane production rates are initially 1.5 mmol/mÂł/yr in sediments just beneath the sulfate reduction zone but rapidly decrease to rates of 100 mbsf. Integrated net rates of methane production in sediments away from the southern summit of Hydrate Ridge are 25â80 mmol/m²/yr. Anaerobic methane oxidation is minor or absent in cored sediments away from the summit of southern Hydrate Ridge. Ethane-enriched Structure I gas hydrate solids are buried more rapidly than ethane-depleted dissolved gas in the pore water because of advection from compaction. With subsidence beneath the gas hydrate stability zone, the ethane (mainly of low-temperature thermogenic origin) is released back to the dissolved gas-free gas phases and produces a discontinuous decrease in the Câ/Câ vs. depth trend. These ethane fractionation effects may be useful to recognize and estimate levels of gas hydrate occurrence in marine sediments
Global and local controlson continental margin stratigraphy
Integrated Ocean Drilling Program (IODP) Expedition 317 was devoted to understanding
the relative importance of global sea level (eustasy) versus local tectonic and sedimentary processes in controlling continental margin sedimentary cycles. The expedition
recovered sediments from the Eocene to recent period, with a particular focus
on the sequence stratigraphy of the late Miocene to recent, when global sea level change was dominated by glacioeustasy. Drilling in the Canterbury Basin, on the eastern
margin of the South Island of New Zealand, takes advantage of high rates of Neogene
sediment supply, which preserves a high-frequency (0.1â0.5 m.y.) record of depositional cyclicity. The Canterbury Basin provides an opportunity to study the complex interactions between processes responsible for the preserved stratigraphic record
of sequences because of the proximity of an uplifting mountain chain, the Southern Alps, and strong ocean currents. Currents have locally built large, elongate sediment drifts within the prograding Neogene section. Expedition 317 did not drill into one of these elongate drifts, but currents are inferred to have strongly influenced deposition across the basin, including in locations lacking prominent mounded drifts.
Upper Miocene to recent sedimentary sequences were cored in a transect of three sites on the continental shelf (landward to basinward, Sites U1353, U1354, and U1351) and one on the continental slope (Site U1352). The transect provides a stratigraphic record of depositional cycles across the shallow-water environment most directly affected
by relative sea level change. Lithologic boundaries, provisionally correlative with seismic sequence boundaries, have been identified in cores from each site and provide insights into the origins of seismically resolvable sequences. This record will be used to estimate the timing and amplitude of global sea level change and to document
the sedimentary processes that operate during sequence formation. Sites U1353 and U1354 provide significant, double-cored, high-recovery sections through the Holocene
and late Quaternary for high-resolution study of recent glacial cycles in a continental
shelf setting.
Continental slope Site U1352 represents a complete section from modern slope terrigenous
sediment to hard Eocene limestone, with all the associated lithologic, biostratigraphic,
physical, geochemical, and microbiological transitions. The site also provides a record of ocean circulation and fronts during the last ~35 m.y. The early Oligocene (~30 Ma) Marshall Paraconformity was the deepest drilling target of Expedition
317 and is hypothesized to represent intensified current erosion or nondeposition associated with the initiation of thermohaline circulation following the separation of Australian and Antarctica.
Expedition 317 set a number of scientific ocean drilling records: (1) deepest hole drilled in a single expedition and second deepest hole in the history of scientific ocean drilling (Hole U1352C, 1927 m); (2) deepest hole and second deepest hole drilled by the R/V JOIDES Resolution on a continental shelf (Hole U1351B, 1030 m; Hole U1353B, 614 m); (3) shallowest water depth for a site drilled by the JOIDES Resolution
for scientific purposes (Site U1353, 84.7 m water depth); and (4) deepest sample
taken by scientific ocean drilling for microbiological studies (1925 m, Site U1352).
Expedition 317 supplements previous drilling of sedimentary sequences for sequence stratigraphic and sea level objectives, particularly drilling on the New Jersey margin (Ocean Drilling Program [ODP] Legs 150, 150X, 174A, and 174AX and IODP Expedition
313) and in the Bahamas (ODP Leg 166), but includes an expanded Pliocene section.
Completion of at least one transect across a geographically and tectonically distinct siliciclastic margin was the necessary next step in deciphering continental margin stratigraphy. Expedition 317 also complements ODP Leg 181, which focused on drift development in more distal parts of the Eastern New Zealand Oceanic Sedimentary
System (ENZOSS).Integrated Ocean Drilling Program Management InternationalPublished2.2. Laboratorio di paleomagnetismorestricte
Organic matter, isotopic composition of calcite veins in basement basalt and pore water at DSDP Leg 78A Holes
Sediments of the Barbados Ridge complex, cored on DSDP Leg 78A, contain low concentrations of acid-insoluble carbon (0.05-0.25%) and nitrogen (C/N 1.5-5) and dispersed C1-C6 hydrocarbons (100-800 ppb). The concentrations of organic carbon and 13C in organic carbon decrease with depth, whereas the concentration of dispersed hydrocarbons increases slightly with depth. These trends may reflect the slow oxidation of organic matter, with selective removal of 13C and slow conversion of the residual organic matter to hydrocarbons. Very minor indications of nitrogen gas were observed at about 250 meters sub-bottom at two of the drilling sites. Basement basalts have calcite veins with d13C values in the range of 2.0 to 3.2 per mil and d18O-SMOW values ranging from 28.5 to +30.6 per mil. Interstitial waters have d18O-SMOW of 0.2 to -3.5 per mil and dD-SMOW of -2 to -15 per mil. The oxygen isotopic composition of the calcite veins in the basement basalts gives estimated equilibrium fractionation temperatures in the range of 11 to 24°C, assuming precipitation from water with d18O-SMOW in the range of +0.1 to -1.0 per mil. This suggests that basalt alteration and precipitation of vein calcite occurred in contact either with warmer Campanian seawater or, later, with pore water, after burial to depths of 200- 300 meters. Pore waters from all three sites are depleted in deuterium and 18O, and dissolved sulfate is enriched in 34S at Sites 541 and 542, but not at Site 543
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