(Table 1) Stable isotope data of chemoherm carbonates at Hydrate Ridge on the Cascadia continental margin

Two active chemoherm build-ups growing freely up into the oceanic water column, the Pinnacle and the South East-Knoll Chemoherms, have been discovered at Hydrate Ridge on the Cascadia continental margin. These microbially-mediated carbonate formations rise above the seafloor by several tens of meters and display a pinnacle-shaped morphology with steep flanks. The recovered rocks are pure carbonates dominated by aragonite. Based on fabric and mineralogic composition different varieties of authigenic aragonite can be distinguished. Detailed visual and petrographic investigations unambiguously reveal the involvement of microbes during the formation of the carbonates. The fabric of the cryptocrystalline and fibrous aragonite can be described as thrombolitic. Fossilized microbial filaments in the microcrystalline aragonite indicate the intimate relationship between microbes and carbonates. The strongly 13C-depleted carbon isotope values of the samples (as low as -48.1 per mill PDB) are characteristic of methane as the major carbon source for the carbonate formation. The methane-rich fluids from which the carbonates are precipitated originate most probably from a gas reservoir below the bottom-simulating reflector (BSR) and rise through fault systems. The d18O values of the aragonitic chemoherm carbonates are substantially higher (as high as 5.0 per mill PDB) than the expected equilibrium value for an aragonite forming from ambient seawater (3.5 per mill PDB). As a first approximation this indicates formation from glacial ocean water but other factors are considered as well. A conceptual model is presented for the precipitation of these chemoherm carbonates based on in situ observations and the detailed petrographic investigation of the carbonates. This model explains the function of the consortium of archaea and sulfate-reducing bacteria that grows on the carbonates performing anaerobic oxidation of methane (AOM) and enabling the precipitation of the chemoherms above the seafloor surrounded by oxic seawater. Beggiatoa mats growing on the surface of the chemoherms oxidize the sulfide provided by sulfate-dependent anaerobic oxidation of methane within an oxic environment. The contact between Beggiatoa and the underlying microbial consortium represents the interface between the overlying oxic water column and an anoxic micro-environment where carbonate formation takes place.

Isotopic composition of interstitial fluids in sediment at DSDP Sites 87-582 and 87-583

The isotopic compositions of dissolved CO2 and CH4 in sediments of the Nankai Trough indicate that CH4 is formed during early diagenesis by microbial reduction of CO2. At the shallowest sampled depths, the CO2 dissolved in the pore water is unusually enriched in 12C (d13C = -35.2 per mil), indicating contribution of CO2 from oxidation of CH4. The most intense microbiological activity appears to be confined to the uppermost 50 m of sediment, based on relative lack of change in the isotopic compositions below this depth. Gas hydrate probably is not present at these localities (Sites 582, 583) because of CH4 concentrations that are insufficient to saturate the pore water with respect to gas hydrate stability.

Geochemical and isotopic composition of sediments and pore waters from ODP Leg 164 sites

Authigenic carbonates were recovered from several horizons between 0 and 52 mbsf in sediments that overlay the Blake Ridge Diapir on the Carolina Rise (Ocean Drilling Program [ODP] Site 996). Active chemosynthetic communities at this site are apparently fed by fluid conduits extending beneath a bottom-simulating reflector (BSR). Gas hydrates occur at several depth intervals in these near-surface sediments. The carbonate nodules are composed of rounded to subangular intraclasts and carbonate cemented mussel shell fragments. Electron microprobe and X-ray diffraction (XRD) investigations show that aragonite is the dominant authigenic carbonate. Authigenic aragonite occurs both as microcrystalline, interstitial cement, and as cavity-filling radial fibrous crystals. The d13C values of the authigenic aragonite vary between -48.4 per mil and -30.5 per mil (Peedee belemnite [PDB]), indicating that carbon derived from 13C-depleted methane is incorporated into these carbonates. The d13C of pore water sum CO2 values are most negative in the upper 10 mbsf, near the sediment/water interface (-38 per mil ± 5 per mil), but noticeably more positive below 25 mbsf (+5 per mil ± 6 per mil). Because carbonates derive their carbon from HCO3-, dissimilarities between the d13C values of carbonate precipitates recovered from greater than 10 mbsf and d13C values of the associated pore fluids suggests that these carbonates formed near the seafloor. Differences of about 1 per mil in the oxygen isotopic composition of carbonate precipitates from different depths are possibly related to changes in bottom-water conditions during glacial and interglacial time periods. Measurements of the strontium isotopic composition on 13 carbonate samples show 87Sr/86Sr values between 0.709125 and 0.709206 with a mean of 0.709165, consistent with the approximate age of their host sediment. Furthermore, the 87Sr/86Sr values of six pore-water samples from Site 996 vary between 0.709130 and 0.709204. The similarity of these values to seawater (87Sr/86Sr = 0.709175), and to 87Sr/86Sr values of pore water from similar sample depths elsewhere on the Blake Ridge (Sites 994, 995, and 997), indicates a shallow Sr source. The 87Sr/86Sr values of the authigenic carbonates at Site 996 are not consistent with the Sr isotopic values predicted for carbonates precipitated from fluids transported upward along fault conduits extending through the base of the gas hydrate-stability zone. Based on our data, we see no evidence of continuing carbonate diagenesis with depth. Therefore, with the exception of their seafloor expression as carbonate crusts, fossil vent sites will not be preserved. Because these authigenic features apparently form only at the seafloor, their vertical distribution and sediment age imply that seepage has been going on in this area for at least 600,000 yr.

Characterization of diagenetic fluids at ODP Site 103-639

Site 639, drilled during Leg 103 of the Ocean Drilling Program, penetrated an Upper Jurassic to Lower Cretaceous carbonate platform on a tilted fault block along the Galicia margin off the northwest Iberian Peninsula. The carbonate platform is composed primarily of a sequence of dolomite overlying limestone. Samples were analyzed for mineral chemistry, stable isotope geochemistry, fluid inclusion microthermometry, and volatile contents and by dolomite pyrolysis mass spectrometry for trace sulfate minerals. The dolomite recovered from the Galicia margin at Site 639 formed during shallow burial from sulfate-bearing, hypersaline brines at slightly elevated temperatures. The light oxygen isotopic signatures of the dolomite are interpreted as the result of the evaporative loop and slightly elevated temperatures during dolomite formation or from reequilibration at higher temperatures during deeper burial. The hypersalinity is interpreted to be associated with a nearby, shallow restricted basin that formed during rifting of the Iberian margin from Newfoundland. The dolomitization of the platform is therefore a by-product of the rifting.

Isotopic composition of interstitial fluids and origin of methane at DSDP Leg 84 Holes

CH4 and CO2 species in pore fluids from slope sediments off Guatemala show extreme 13C-enrichment (d13C of -41 and +38 per mil, respectively) compared with the typical degree of 13C-enrichment in pore fluids of DSDP sediments (d13C of - 60 and + 10 per mil). 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 CH4 and CO2, the pore water exhibits a systematic increase in d18O with decrease in chlorinity. As against seawater d18O values of 0 and chlorinity of 19 per mil, the water collected from decomposed gas hydrate from Hole 570 had a d18O of + 3.0 per mil and chlorinity of 9.5 per mil. The isotopic compositions of pore-fluid constituents change gradually with depth in Hole 568 and discontinuously with depth in Hole 570.

Chemistry and age determination of basement fluids from Juan de Fuca Ridge

The geochemical implications of thermally driven flow of seawater through oceanic crust on the mid-ocean ridge flank have been examined on a well-studied 80 km transect across the eastern flank of the Juan de Fuca Ridge at 48°N, using porewater and basement fluid samples obtained on ODP Leg 168. Fluid flow is recognised by near-basement reversals in porewater concentration gradients from altered values in the sediment section to seawater-like values in basaltic basement. In general, the basement fluids become more geochemically evolved with distance from the ridge and broadly follow basement temperature which ranges from not, vert, similar16° to 63°C. Although thermal effects of advective heat exchange are only seen within 20 km east of where basement is exposed near the ridge crest, chemical reactivity extends to all sites. Seawater passing through oceanic crust has reacted with basement rocks leading to increases in Ca2+ and decreases in alkalinity, Mg2+, Na+, K+, SO42- and delta18O. Sr isotope exchange between seawater and oceanic crust off axis is unequivocally demonstrated with endmember 87Sr/86Sr ~ 0.707. Evidence of more evolved fluids is seen at sites where rapid upwelling of fluids through sediments occurs. Chlorinities of the basement fluids are consistent with post-glacial seawater and thus a short residence time in the crust. Rates of lateral flow have been by estimated by modelling porewater sulphate gradients, using Cl as a glacial chronometer, and from radiocarbon dating of basal fluids. All three methods reveal fluid flow with 14C ages less than 10,000 yr and particle velocities of ~1-5 m/yr, in agreement with thermally constrained volumetric flow rates through a ~600 m thick permeable layer of ~10% porosity. Delta(element)/Delta(heat) extraction ratios are similar to values for ridge-crest hydrothermal systems.

Pore water acetate and hydrogen, helium and methane gas concentrations in ODP Leg 204 sediments

Acetate and hydrogen concentrations in pore fluids were measured in samples taken at seven sites from southern Hydrate Ridge (SHR) offshore Oregon, USA. Acetate concentrations ranged from 3.17 to 2515 µM. The maximum acetate concentrations occurred at Site 1251, which was drilled on a slope basin to the east of SHR at depths just above the bottom-simulating reflector (BSR) that marks the boundary of gas hydrate stability. Acetate maxima and localized high acetate concentrations occurred at the BSR at all sites and frequently corresponded with areas of gas hydrate accumulation, suggesting an empirical relationship. Acetate concentrations were typically at a minimum near the seafloor and above the sulfate/methane interface, where sulfate-reducing bacteria may consume acetate. Hydrogen concentrations in pressure core samples ranged from 16.45 to 1036 parts per million by volume (ppmv). In some cases, hydrogen and acetate concentrations were elevated concurrently, suggesting a positive correlation. However, sampling of hydrogen was limited in comparison to acetate, so any relationships between the two analytes, if present, were difficult to discern.

Geochemical and isotopic composition of pore fluids of ODP Hole 131-808C

At the Western Nankai Trough subduction zone at ODP Site 808, chemical concentration and isotopic ratio depth profiles of D, O, Sr, and He do not support fluid flow along the décollement nor at the frontal thrust. They do, however, support continuous or periodic lateral fluid flow: (1) at the base of the Shikoku Basin volcanic-rich sediment member, situated ~140 m above the décollement, and particularly (2) below the décollement. The latter must have been rather vigorous, as it was capable of transporting clay minerals over great distances. The fluid at ~140 m above the décollement is characterized by lower than seawater concentrations of Cl- (>=18% seawater dilution). It is 18O-rich and D-poor and has a non-radiogenic, oceanic, or volcanic arc Sr isotopic signature. It originates from "volcanic" clay diagenesis. The fluid below the décollement has also less Cl- than seawater (>20% dilution), is more enriched in 18O and depleted in D than fluid, but its Sr isotopic signature is radiogenic, continentalterrigenous. The source of this fluid is located arcward, is deep-seated, where illitization of the subducted clay minerals, a mixture of terrigenous and volcanic clays, occurs. The 3He/4He ratio below the décollement points to an ~25% mantle contribution. The nature of the physical and chemical discontinuities across the décollement suggests it is overpressured and is forming a leaky "dynamic seal" for fluid flow. In contrast with the situation at Barbados and Peru, where the major tectonic features are mineralized, here, although the complex is extremely fractured and faulted, mineralized macroscopic veins, fractures, and faults are absent. Instead, mineralized microstructures are widespread, indicating a diffuse mode of dewatering.

Chemical and isotopic composition of interstitial waters and hydrocarbon composition at ODP Site 184-1146

Bottom-simulating reflectors were observed beneath the southeastern slope of the Dongsha Islands in the South China Sea, raising the potential for the presence of gas hydrate in the area. We have analyzed the chemical and isotopic compositions of interstitial water, headspace gas, and authigenic siderite concretions from Site 1146. Geochemical anomalies, including a slight decrease of chlorine concentration in interstitial water, substantial increase of methane concentration in headspace gas, and 18O enrichment in the authigenic siderite concretion below 400 meters below seafloor are probably caused by the decomposition of gas hydrate. The low-chlorine pore fluids contain higher molecular-weight hydrocarbons and probably migrate to Site 1146 along faults or bedded planes.