(Table 1) Porosity, particle diameter, ATP content and carbon and nitrogen composition of ODP Site 164-994 sediments

Sediment samples were obtained for detailed Adenosine 5'-Triphosphate (ATP) analysis down to 57.8 m below the seafloor (mbsf). The samples were also analyzed for particle-size distribution, calcium carbonate (CaCO3), organic carbon, and total nitrogen. The concentrations of ATP ranged between 360 and 7050 pg/g (dry weight sediment), which agree well with a limited number of direct bacteria counts. Principal component analyses show that 63% of the total variance can be accounted for by the first two principal components. The concentration of ATP (bacterial numbers by inference) is virtually independent of the concentration of sedimentary organic carbon, but correlates with CaCO3 and coarse particles.

Hydrocarbon gas concentrations in sediments from ODP Leg 164 sites

Residual concentrations and distributions of hydrocarbon gases from methane to n-heptane were measured in sediments at seven sites on Ocean Drilling Program (ODP) Leg 164. Three sites were drilled at the Cape Fear Diapir of the Carolina Rise, and one site was drilled on the Blake Ridge Diapir. Methane concentrations at these sites result from microbial generation which is influenced by the amount of pore-water sulfate and possible methane oxidation. Methane hydrate was found at the Blake Ridge Diapir site. The other hydrocarbon gases at these sites are likely the product of early microbial processes. Three sites were drilled on a transect of holes across the crest of the Blake Ridge. The base of the zone of gas-hydrate occurrence was penetrated at all three sites. Trends in hydrocarbon gas distributions suggest that methane is microbial in origin and that the hydrocarbon gas mixture is affected by diagenesis, outgassing, and, near the surface, by microbial oxidation. Methane hydrate was recovered at two of these three sites, although gas hydrate is likely present at all three sites. The method used here for determining amounts of residual hydrocarbon gases has its limitations and provides poor assessment of gas distributions, particularly in the stratigraphic interval below about ~100 mbsf. One advantage of the method, however, is that it yields sufficient quantities of gas for other studies such as isotopic determinations.

(Table 3) Noble gas composition of dissociated gas hydrate specimens from ODP Leg 164 sites

Fractionation of the noble gases should occur during formation of a Structure I gas hydrate from water and CH4 such that CH4 hydrate is greatly enriched in Xenon. Noble gas concentrations and fractionation factors (F[4He], F[22Ne], F[86Kr], and F[132Xe] as well as R/Ra) were determined for eight gas hydrate specimens collected on Leg 164 to evaluate this theoretical possibility and to assess whether sufficient quantities of Xe are hosted in oceanic CH4 hydrate to account for Xe "missing" from the atmosphere. The simplest explanation for our results is that samples contain mixtures of air and two end-member gases. One of the end-member gases is depleted in Ne, but significantly enriched in Kr and Xe, as anticipated if the source of this gas involves fractionation during Structure I gas hydrate formation. However, although oceanic CH4 hydrate may be greatly enriched in Xe, simple mass balance calculations indicate that oceanic CH4 hydrate probably represents only a minor reservoir of terrestrial Xe. Noble gas analyses may play an important role in understanding the dynamics of gas hydrate reservoirs, but significantly more work is needed than presented here.

(Table 1) Deuterium ratios, chloride content and hydrate volume of pore water and hydrate meltwater from ODP Site 164-996

Site 996 is located above the Blake Diapir where numerous indications of vertical fluid migration and the presence of hydrate existed prior to Ocean Drilling Program (ODP) Leg 164. Direct sampling of hydrates and visual observations of hydrate-filled veins that could be traced 30-40 cm along cores suggest a connection between fluid migration and hydrate formation. The composition of pore water squeezed from sediment cores showed large variations due to melting of hydrate during core recovery and influence of saline water from the evaporitic diapir below. Analysis of water released during hydrate decomposition experiments showed that the recovered hydrates contained significant amounts of pore water. Solutions of the transport equations for deuterium (d2H) and chloride (Cl-) were used to determine maximum (d2H) and minimum (Cl-) in situ concentrations of these species. Minimum in situ concentrations of hydrate were estimated by combining these results with Cl- and d2H values measured on hydrate meltwaters and pore waters obtained by squeezing of sediments, by the means of a method based on analysis of distances in the two-dimensional Cl- d2H space. The computed Cl- and d2H distribution indicates that the minimum hydrate amount solutions are representative of the actual hydrate amount. The highest and mean hydrate concentrations estimates from our model are 31% and 10% of the pore space, respectively. These concentrations agree well with visual core observations, supporting the validity of the model assumptions. The minimum in situ Cl- concentrations were used to constrain the rates of upward fluid migration. Simulation of all available data gave a mean flow rate of 0.35 m/k.y. (range: 0.125-0.5 m/k.y.).

(Table 1) Relative abundance of planktonic foraminifers in ODP Hole 164-997A sediments

Hole 997A was drilled during Leg 164 of the Ocean Drilling Program at a depth of 2770 m on the topographic crest of the Blake Ridge in the western Atlantic Ocean. We report here an analysis of the faunal assemblages of planktonic foraminifers in a total of 91 samples (0.39-91.89 mbsf interval) spanning the last 2.15 m.y., latest Pliocene to Holocene. The abundant species, Globigerinoides ruber, Globigerinoides sacculifer, Neogloboquadrina dutertrei, Globorotalia inflata, and Globigerinita glutinata together exceed over ~70% of the total fauna. Each species exhibits fluctuations with amplitudes of 10%-20% or more. Despite their generally low abundance, the distinct presence/absence behavior of the Globorotalia menardii group is almost synchronous with glacial-interglacial climate cycles during the upper part of Brunhes Chron. The quantitative study and factor analysis of planktonic foraminiferal assemblages shows that the planktonic foraminiferal fauna in Hole 997A consists of four groups: warm water, subtropical gyre (mixed-layer species), gyre margin (thermocline/upwelling species), and subpolar assemblages. The subtropical gyre assemblage dominates throughout the studied section, whereas the abundance of gyre margin taxa strongly control the overall variability in faunal abundance at Site 997. In sediments older than the Olduvai Subchron, the planktonic foraminiferal faunas are characterized by fluctuations in both the subtropical gyre and gyre margin assemblages, similar to those in the Brunhes Chron. The upwelling/gyre margin fauna increased in abundance just before the Jaramillo Subchron and was dominant between 0.7 and 1.07 Ma. The transition from this gyre margin-dominated assemblage to an increase in abundance of the subtropical gyre and gyre margin species occurred around 0.7 Ma, near the Brunhes/Matuyama boundary. The presence of low-oxygen-tolerant benthic foraminifers, pyrite tubes, and abundant diatoms below the Brunhes/Matuyama boundary suggests decreased oxygenation of intermediate waters and more upwelling over the Blake-Bahama Outer Ridge, perhaps because of weaker Upper North Atlantic Deep Water ventilation. The changes in the relative composition of foraminifer assemblages took place at least twice, around 700 and 1000 ka, close to the ~930-ka switch from obliquity-forced climate variation to the 100-k.y. eccentricity cycle. The climate shift at 700 ka suggests a transition from relatively warmer conditions in the early Pleistocene to warm-cool oscillations in the Brunhes Chron.

(Table 1) Stable nitrogen isotopes of ammonium in interstitial waters from ODP Site 164-997

Ammonium (NH4+) concentration profiles in piston-core sediments of the Carolina Rise and Blake Ridge generally have linear concentration profiles within the sulfate reduction zone (Borowski, 1998). Deep Sea Drilling Project (DSDP) Site 533, located on the Blake Ridge, also displayed a linear ammonium concentration profile through the sulfate reduction zone and the profile linearity continues into the upper methanogenic zone to a depth of ~200 meters below seafloor (mbsf), where the first methane gas hydrates probably occur (Jenden and Gieskes, 1983, doi:10.2973/dsdp.proc.76.114.1983; Kvenvolden and Barnard, 1983, doi:10.2973/dsdp.proc.76.106.1983). Sediments from the Ocean Drilling Program (ODP) Leg 164 deep holes (Sites 994, 995, and 997) also exhibit linear ammonium profiles above the top of the gas hydrate zone (~200 mbsf) (Paull, Matsumoto, Wallace, et al., 1996, doi:10.2973/odp.proc.ir.164.1996). We hypothesized that a possible cause of linear ammonium profiles was diffusion of ammonium from a concentrated ammonium source at depth. We further reasoned that if this ammonium were produced by microbial fermentation reactions at depth, that a comparison of the nitrogen isotopic composition of sedimentary organic nitrogen and the nitrogen with pore-water ammonium would test this hypothesis. Convergence with depth of d15N values of the nitrogen source (sedimentary organic matter) and the nitrogen product (dissolved NH4+) would strongly suggest that ammonium was produced within a particular depth zone by microbial fermentation reactions. Here, we report d15N values of pore-water ammonium from selected interstitial water (IW) samples from Site 997, sampled during ODP Leg 164.