A total of eleven gas hydrate samples were recovered and measured from Sites 994, 996, and 997 (Table 1). Water depths at the sites ranged from 2170 m at Site 996 to 2798 m at Site 994. Nine gas hydrate samples were analyzed from Site 996, two of these in duplicate, from depths ranging from 0.2 to 58.6 meters below seafloor (mbsf). Two gas hydrate samples were analyzed, one each from Sites 994 and 997, from depths of 259.9 to 331.0 mbsf, respectively. A 1-cm-diameter wafer of gas hydrate was recovered from Section 164-995A-11X-3 (414 mbsf) but proved too small and decomposed on which to perform any reliable analyses.
Site 996 was drilled directly over the Blake Ridge Diapir where fault conduits intersect the seafloor, resulting in an area of active gas venting, pockmarks, and chemosynthetic communities (Paull et al., 1995). Nodular (Fig. 3) and vertical veins of gas hydrate were recovered from less than 1 mbsf and intermittently through the maximum cored depth of 62 mbsf.
Sites 994, 995, and 997 on a transect across the Blake Ridge (Fig. 1) were selected because geophysical indicators of gas hydrate occurrence were particularly distinct. This information includes (1) a well-developed bottom-simulating reflector (BSR) that marks the base of gas hydrate stability on marine seismic records and is present at Sites 995 and 996 but not at Site 994, and (2) the occurrence of seismic blanking, a region of low impedance contrast on marine seismic records above the BSR and a possible indicator of gas hydrate (Lee et al., 1994). Holes were drilled to maximum depths of 750 mbsf, exceeding that of the BSR at ~450 mbsf.
Downhole logging data, in concert with geochemical and core temperature profiles, indicate that gas hydrate at Sites 994, 995, and 997 occurs from about 180 to 450 mbsf and is disseminated in sediment as 5- to 30-m-thick zones that contain up to 15% bulk volume gas hydrate (Collett and Ladd, Chap. 19, this volume). Massive gas hydrate samples were recovered from depths of 260 mbsf at Site 994, and one 30-cm-long piece was recovered from 331 mbsf at Site 997. Occurrences of massive gas hydrate were rare and are probably fault filling and/or stratigraphically controlled.
Shipboard measurements of dissociated gas hydrate are reported as the volumetric ratio of gas volume to water at STP. Blake Ridge gas hydrate gas to water volumetric ratios have a wide range of values from 29 to 154 (Table 1, Table 2). The same is true of previous measurements, which range from 4 to 177 listed in Table 2. Calculations of maximum expected ratios in theory and in nature are tabulated in Table 3.
Comparisons of Blake Ridge gas hydrate gas to water volumetric ratios were made to all known measurements worldwide (Fig. 1, Table 1, Table 2). The measurement is rare, mainly because gas hydrate samples are rare, and because specialized equipment and procedures needed to be in place when gas hydrates were recovered.
The following hydrocarbon gases in gas hydrate and surrounding sediment are reported in Table 1: methane (C1), ethane (C2), and the sum C3+ composed of propane (C3), isobutane (i-C4), and n-butane (n-C4). In addition, carbon dioxide concentrations and the ratio C1/(C2+C3) are given. Other hydrocarbon gases are present in Blake Ridge sediments including: neopentane (neoC5), isopentane (i-C5), n-pentane (n-C5), cyclopentane (cycloC5), neohexane (neoC6), isohexane (i-C6), n-hexane (n-C6), isoheptane (i-C7), and methylcyclohexane + n-heptane (methylcycloC6+n-C7). They are discussed in more detail in Kvenvolden and Lorenson (Chap. 3, this volume).
Gas hydrate median composition was 99.5% C1, ranging from 98.4% to 99.9%; 0.4% CO2, ranging from 0.0% to 1.6%; 750 ppmv C2, ranging from 86 to 1010 ppmv; and 10.4 ppmv C3+, ranging from 0.5 to 23.8 ppmv. C3+ compounds in dissociated Structure I gas hydrate gas are likely contaminants. Adjacent sediment gas (within 11 m of gas hydrate occurrences) median composition was 98.1% C1, ranging from 73.9% to 99.8%; 1.9% CO2, ranging from 0.2% to 26.1%; 629 ppmv C2, ranging from 125 to 776 ppmv; and 27.6 ppmv C3+, ranging from 2.1 to 52.1 ppmv (Fig. 4).
Hydrogen sulfide (H2S) has been found previously in near-surface gas hydrate (Kastner et al., 1998). H2S was also detected and measured in sediments and gas hydrate at Site 996. Because H2S is very soluble in water and the gas hydrate dissociation measurements are always contaminated with some sediment and pore water, we are doubtful the H2S was within the gas hydrate structure.
Gas hydrate methane carbon
isotopic values range from -70.7
to -65.8 with
a median of -67.0,
whereas hydrogen values range from -199.7
to -175.0
with a median value of -194.6.
Gas hydrate CO2 carbon isotopic values range from -26.9
to -5.7 with
a median of -21.9.
Adjacent sediment gas methane carbon isotopic values range from -68.2
to -65.0 with
a median of -65.0,
whereas just one hydrogen value of -195.0
was measured. Sediment gas CO2 carbon isotopic values range from 11.7
to -4.4 with
a median of -6.5.
