Description: Twenty-two trace elements in 355 sediment samples from Site 997 on the Blake Ridge were examined by inductively coupled plasma-optical emission spectrometry and inductively coupled plasma-mass spectrometry, for respective fractions of acid-soluble and insoluble compositions. Downhole profiles of these elements exhibit complicated fluctuations throughout late Miocene to Pleistocene, principally due to the variations in the acid-soluble fraction. Noncarbonate composition is given from the acid-insoluble residues, which permits us to recognize secular feature of selected element variance for four intervals. These intervals (I: 0-183 mbsf; II: 183- 440 mbsf; III: 440-618 mbsf; and IV: 618-750 mbsf) are interpreted to have originated from changes in the suite of sediments of particular sources and chemical composition, sedimentation rate, dilution of biogenic carbonate abundance, and possibly the current system that controlled deposition and reworking of the terrigenous materials.

Description: 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.

Description: 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.

Description: The d18O values of interstitial waters from Site 994 and Site 997 sediments, Blake Ridge, western Atlantic, tend to decrease with depth from 0.3 per mil to -0.5 per mil Standard Mean Ocean Water in the upper 200 mbsf, then fluctuate with significant positive spikes of Delta = 0.2 per mil - 0.5 per mil in the gas hydrate zone (200 to 450 mbsf), and finally increase from -0.4 per mil to -0.2 per mil toward 700 mbsf. Positive shifts of d18O IW in the gas hydrate zone are probably caused by the dissociation of gas hydrates originally contained in sediment cores. Gas hydrates recovered from the sites are enriched in 18O, d18O ranging between 2.7 per mil and 3.5 per mil. d18O values of gas hydrates and ambient interstitial waters give an oxygen isotopic fractionation factor of 1.0034-1.0040 at 12°-16°C and ~31 MPa (3 km below sea level). Based on this fractionation and observed isotopic anomalies in the gas hydrate zone, gas hydrates occupy 6% to 12% of pore-space volume within Blake Ridge sediments.

Description: Stable isotope studies of Cl, O, and H carried out in addition to routine shipboard chemical analyses performed during Ocean Drilling Program (ODP) Leg 164 aid in constraining estimates of hydrate concentration and mechanisms of hydrate formation and dissociation in the submarine hydrate zone of the Blake Ridge in the West Atlantic. Chlorine isotope ratios show a steady downward decrease from a shallow maximum of about 0 per mil near the roof of the hydrate zone (postulated to occur at 24 mbsf) to a d37Cl value of -3.68 per mil near the bottom of Site 997, one of the more negative Cl-isotope values measured in marine pore waters. The downward depletion in the heavy isotope is coupled with a chlorinity decrease, determined by the shipboard measurements. Modeling of the chlorine isotopic data provides an independent test of the hydrochemical model developed by Egeberg and Dickens to explain the chlorinity profile of Site 997. According to that model, the downward chlorinity decrease is largely due to advection of low-chlorinity water from below the drilled section, the difference to the shallow maximum being bridged by diffusion. Using the Egeberg and Dickens advection-diffusion model for the Cl-isotope data gives the best fit between measured trend and simulation for an upward advection rate of 0.18 mm/yr and an assumed ratio of 1.0023 for the diffusion coefficients of the light and heavy Cl isotopes. The low-chlorinity water advected from below the drilled section carries the 37Cl-depleted isotopic signature, but the source of this water is unknown, as are the mechanisms for the heavy isotope depletion. Fractionation of the chlorine isotopes due to different mobilities during downward diffusion from the chloride maximum can only explain a minor portion of the 37Cl depletion. Positive d18O and dD spikes in the upper part of the section drilled at Site 997 are within the range of isotopic excursions during the Quaternary glaciations and can be explained by inheritance from buried connate waters, like the chloride maximum in the upper 24 mbsf, whereas the effects of hydrate formation (salt exclusion and preferential uptake of the heavy isotopes) are minor due to the low hydrate concentrations at the site. Negative d18O values 〈= 0.6 per mil in the middle of the section between 159 and 532 mbsf largely reflect authigenic carbonate formation, whereas the solid reaction partners that cause negative dD spikes below 200 mbsf remain unknown. Heavy oxygen and hydrogen isotope spikes between 200 mbsf and the base of the hydrate zone at 452 mbsf correspond to low-chlorinity spikes, indicating hydrate dissociation in local, hydrate-rich sediment layers. Overall, oxygen isotope ratios increase below 300 mbsf, whereas hydrogen isotope ratios show a general downward decrease for the site.