Isotopic ratios of methane, ethane, carbon dioxide, inorganic carbon and organic matter from ODP Leg 164 sites

The isotopic characteristics of CH4 (d13C values range from -101.3 per mil to -61.1 per mil PDB, and dD values range from -256 per mil to -136 per mil SMOW) collected during Ocean Drilling Program (ODP) Leg 164 indicate that the CH4 was produced by microbial CO2 reduction and that there is not a significant contribution of thermogenic CH4 to the sampled sediment gas from the Blake Ridge. The isotopic values of CO2 (d13C range -20.6 per mil to +1.24 per mil PDB) and dissolved inorganic carbon (DIC; d13C range -37.7 per mil to +10.8 per mil PDB) have parallel profiles with depth, but with an offset of 12.5 per mil. Distinct downhole variations in the carbon isotopic composition of CH4 and CO2 cannot be explained by closed-system fractionation where the CO2 is solely derived from the locally available sedimentary organic matter (d13C -2.0 per mil ± 1.4 per mil PDB) and the CH4 is derived from CO2 reduction. The observed isotopic profiles reflect the combined effects of upwards gas migration and decreased microbial activity with depth.

Organic-geochemical and petrographical composition of sediments from ODP Leg 164 sites

Sediments from Holes 994C, 995A, 997A, and 997B have been investigated for "combined" gases (adsorbed gas and that portion of free gas that has not escaped from the pore volume during core recovery and sample collection and storage), solvent-extractable organic compounds, and microscopically identifiable organic matter. The soluble materials mainly consist of polar compounds. The saturated hydrocarbons are dominated by n-alkanes with a pronounced odd-even predominance pattern that is derived from higher plant remains. Unsaturated triterpenoids and 17ß, 21ß-pentacyclic triterpenoids are characteristic for a low maturity stage of the organic matter. The low maturity is confirmed by vitrinite reflectance values of 0.3%. The proportion of terrestrial remains (vitrinite) increases with sub-bottom depth. Within the liptinite fraction, marine algae plays a major role in the sections below 180 mbsf, whereas above this depth sporinites and pollen from conifers are dominant. These facies changes are confirmed by the downhole variations of isoprenoid and triterpenoid ratios in the soluble organic matter. The combined gases contain methane, ethane, and propane, which is a mixture of microbial methane and thermal hydrocarbon gases. The variations in the gas ratios C1/(C2+C3) reflect the depth range of the hydrate stability zone. The carbon isotopic contents of ethane and propane indicate an origin from marine organic matter that is in the maturity stage of the oil window.

Major element oxides in ODP Site 164-997 sediments

Since being first discovered in the Blake-Bahama region of the west Atlantic in the 1970s (Hollister, Ewing, et al., 1972, doi:10.2973/dsdp.proc.11.1972), submarine gas hydrates have been identified in the continental margin worldwide. Ocean Drilling Program (ODP) Leg 164 was the first drilling designated to study the occurrence and distribution of natural gas hydrates in Blake Ridge where a well developed, distinct BSR (Bottom Simulating Reflector) has been identified (Paull, Matsumoto, Wallace, et al., 1996, doi:10.2973/odp.proc.ir.164.1996). It has been reported there is a prominent discrepancy between the BSR and the base of gas hydrate stability (Paull, Matsumoto, Wallace, et al., 1996, doi:10.2973/odp.proc.ir.164.1996; Ruppel, 1997, doi:10.1130/0091-7613(1997)025<0699:ACTOAT>2.3.CO;2), though theoretically they should be at the same depth. Natural gas hydrate in marine sediments coexists with sediment particles, so detailed delineation of sediment geochemistry will be of benefit to solve this apparent discrepancy. The main objectives of this study are to supply background data of the major chemical compositions of sediments from a hydrated sediment section.

Pore water methane characteristics and sulphate reduction rates of ODP Leg 164 sediments

Anaerobic methane oxidation (AMO) was characterized in sediment cores from the Blake Ridge collected during Ocean Drilling Program (ODP) Leg 164. Three independent lines of evidence support the occurrence and scale of AMO at Sites 994 and 995. First, concentration depth profiles of methane from Hole 995B exhibit a region of upward concavity suggestive of methane consumption. Diagenetic modeling of the concentration profile indicates a 1.85-m-thick zone of AMO centered at 21.22 mbsf, with a peak rate of 12.4 nM/d. Second, subsurface maxima in tracer-based sulfate reduction rates from Holes 994B and 995B were observed at depths that coincide with the model-predicted AMO zone. The subsurface zone of sulfate reduction was 2 m thick and had a depth integrated rate that compared favorably to that of AMO (1.3 vs. 1.1 nmol/cm**2/d, respectively). These features suggest close coupling of AMO and sulfate reduction in the Blake Ridge sediments. Third, measured d13CH4 values are lightest at the point of peak model-predicted methane oxidation and become increasingly 13C-enriched with decreasing sediment depth, consistent with kinetic isotope fractionation during bacterially mediated methane oxidation. The isotopic data predict a somewhat (60 cm) shallower maximum depth of methane oxidation than do the model and sulfate reduction data.