Solid pieces of gas hydrate were recovered from ~82 and ~148 mbsf at Site 1230. Indirect evidence for gas hydrate also exists in several cores below 70 mbsf. Collectively, physical and chemical data suggest that small quantities of gas hydrate are present, at least intermittently, from ~70 mbsf to the base of the recovered interval at 278.3 mbsf.
The dissociation of gas hydrate is an endothermic reaction that produces gas and decreases the temperature of surrounding sediment (Sloan, 1990). All cores from Site 1230 were inspected immediately after retrieval for indications of gas hydrate, including white nodules, fizzing, or unusually cold spots. As soon as the core liner had been placed on the catwalk, the IR camera was run along the core to identify unusually cold core liner temperatures. Intervals that potentially contained gas hydrate were immediately cut out of the core and split. The split core surfaces were then inspected for hydrate or fizzing.
Small pieces of dirty white gas hydrate were recovered from sediments of lithostratigraphic Subunit IB at ~82 mbsf in Core 201-1230B-12H and the top of Subunit ID at ~148 mbsf in Core 201-1230A-19H. A 5-cm3 piece of hydrate from Core 201-1230B-12H was scraped with a spatula to remove surrounding sediment and placed into a syringe to collect gas and water for geochemical analyses. The remaining scraped sediment was then collected for microbiological analyses. One fizzing and anomalously cold 15-cm whole-round section (Sample 201-1230A-19H-1, 135-150 cm) was squeezed for interstitial water. At least three other sections contained disseminated gas hydrate based on observed fizzing and low temperatures (see Table T14), although discrete hydrate pieces were not recovered. Table T14 summarizes the six sections with cold spots and observed hydrate or fizzing sediment.
IR thermal imaging was useful for identifying zones of potential gas hydrate in sediment cores immediately after retrieval at Site 1230 (Fig. F32). For example, temperature varied <1°C down Core 201-1230A-6H. This core displayed no IR evidence for hydrate. In contrast, temperature changes by nearly 8°C down Core 201-1230A-19H. These intervals are anomalously cold compared with surrounding sediment and voids, the latter being anomalously warm (Fig. F30B). This core contained definite gas hydrate at 148.3 mbsf and likely at ~149.7 mbsf (Table T14). Several other cores exhibited temperature excursions of about -5°C, indicating that they may have contained hydrate. The shallowest of these significant temperature anomalies was present at ~71 mbsf in Core 201-1230A-10H (Fig. F14).
Cores 201-1230B-11H and 12H (73.5-90.5 mbsf) that surround and contain the upper interval with hydrate samples, consist of dark gray to olive clay-bearing diatom silt and diatom-rich nannofossil silt. High-angle normal faults with offsets of several centimeters are present in Sections 201-1230B-11H-4, 11H-5, and 12H-6. The sediments are characterized by pervasive cleavage with both low-angle and horizontal attitude. The recovered hydrate sample from Core 201-1230B-12H consisted of several vertical to subvertical wavy veins of white gas hydrate, up to 3 mm thick, separated by dark gray sediment (Fig. F33). Cores 201-1230A-18H and 19H (138.8-156.8 mbsf) that surround and contain the lower interval of hydrates consist of dark gray to black quartz-bearing clay-rich diatom ooze. The sediments have a stiff and highly fractured appearance, and recovery was generally low throughout the subunit, possibly due to a combination of fracturing and high gas concentrations (see "Biogeochemistry"). Horizontal to low-angle foliation is common in most cores of Subunit ID. In addition, Section 201-1230A-19H-2 was characterized by a ~30° cleavage direction. The co-occurrence of steeply dipping gas hydrate veins, pervasive cleavage, and high-angle normal faults, at least in the upper part of the hydrate-bearing interval, suggests structural control on gas hydrate precipitation at Site 1230. Gas hydrate distribution at other locations appears to be structurally controlled because faults and fractures can provide conduits for transporting methane (e.g., Ginsburg and Solovieu, 1977; Wood and Ruppel, 2000).
Sediment cores recovered from sequences containing gas hydrate are typically disturbed (Paul, Matsumoto, and Wallace, et al., 1996; Westbrook et al., 1994). Extensive core disturbance was noted in most of the cores below Core 201-1230A-3H. The following types of disturbance, ranging from a few centimeters to tens of centimeters in vertical extent, were observed:
The depth interval and types of sediment disturbance are similar to those found during Leg 112 at Site 685 (Suess, von Huene, et al., 1988). This disturbance alone does not indicate the presence of hydrate. However, it does suggest depth intervals where methane concentrations are sufficiently high that gas rapidly escapes from sediment during core recovery. Such concentrations are necessary to form gas hydrate.
High-resolution profiles of interstitial water chemistry, particularly of conservative solutes such as chloride, can be used to quantify the amount and distribution of gas hydrate in drill holes (e.g., Hesse et al., 1985; Egeberg and Dickens, 1999). Gas hydrate formation excludes dissolved ions, which, over time, advect or diffuse away from zones of gas hydrate. During core recovery and processing, gas hydrates can rapidly dissociate, releasing their freshwater into the pore space. Consequently, deviations in interstitial water composition toward freshwater can signify dilution by dissociated gas hydrate.
Freshening occurs throughout the cored sediment column below ~20 mbsf at Site 1230 (Fig. F35). As observed in the high-resolution dissolved lithium profile (Fig. F3), it is most extreme in two zones centered at ~80 and ~150 mbsf. The hydrate sample from Core 201-1230B-12H (~80 mbsf) was analyzed for dissolved barium, chloride, iron, lithium, manganese, strontium, acetate, and formate. Although the sample has measurable quantities of all these species (Table T15), the water is much fresher than surrounding interstitial water. It is also fresher than most previous water samples collected from dissociated gas hydrate specimens in deep boreholes (e.g., Kvenvolden and McDonald, 1985; Paull, Matsumoto, Wallace, et al., 1996). The sample is probably contaminated by ~3% interstitial water, based on its ionic content, although the lattice water may contain trace amounts of dissolved species.
An interval fizzing on the catwalk and suspected of containing gas hydrate (Sample 201-1230A-19H-1, 135-150 cm; 149.65 mbsf) was collected and processed as an IW sample. This sample had concentrations of dissolved species significantly below those found in IW samples with minimal or no evidence for hydrate (Table T15). In order to account for the low chloride and lithium concentrations of this sample relative to the general interstitial water profiles of these elements (Fig. F3) (see "Interstitial Water" in "Biogeochemistry"), the pore space must have contained ~20% hydrate.
The abundance of gas hydrate in sediment can be determined from the in situ gas concentrations (i.e. the concentrations before degassing during conventional core recovery [Dickens et al., 1997]). The PCS was deployed 10 times at Site 1230 between 22 and 277 mbsf in order to construct an in situ gas concentration profile (see Dickens et al., this volume). Preliminary estimates suggest that in situ methane concentration at Site 1230 range from 13 mM (Core 201-1230B-4P) to 400 mM (Core 201-1230A-20P). The latter value greatly exceeds methane solubility with respect to the dissolved methane-methane-hydrate partial saturation curve (Handa, 1990). It is consistent with the presence of several percent gas hydrate in pore space of sediment at ~150 mbsf.
Site 1230 provided an opportunity to examine the links between hydrate presence and physical properties in downhole logs. Wireline logging measurements most sensitive to hydrate presence are resistivity and sonic velocity. Resistivity increases because of the insulating properties of ice and because of the drilling-induced freshening of interstitial water. Sonic velocity increases because of the solidifying effect of solid ice in the pore space (Collett, 1998). The presence of hydrate has been shown to increase sonic attenuation and decrease the amplitude of sonic logging waveforms (Guerin et al., 1999). These features are visible in the logging data from Site 1230, and there are four broad intervals where logging data may indicate the presence of gas hydrate in Hole 1230A (Fig. F36). The extent of these intervals is mostly defined by areas where dipole waveforms, and to a lesser extent monopole waveforms, have lower amplitudes. Guerin et al. (1999) observed that at low hydrate concentration, dipole waveforms are more sensitive to the presence of hydrate, which could explain the stronger amplitude contrasts in the dipole waveforms in Hole 1230A. The FMS images indicate fine features, such as those shown in Figure F30B and F30C. This suggests that any hydrate in these intervals is likely disseminated rather than in massive accumulations. The density log helps distinguish intervals where increased resistivity cannot be attributed to hydrate because hydrate has a slightly lower density than water. Resistivity highs between 216 and 220 and between 224 and 232 mbsf are probably not caused by hydrate but rather by cemented layers because they are associated with increases in density. In comparison, resistivity highs at 128, 140, and 241 mbsf could indicate gas hydrate (Fig. F36). Because of limitations imposed by the resolution of the wireline logs (see "Downhole Logging" in the "Explanatory Notes" chapter) and depth matching between separate log runs, reprocessing of sonic data will be required to confidently correlate individual peaks in resistivity and sonic velocity.
Previous work at Site 685 suggested that gas hydrates were present below 40 mbsf but were concentrated in two intervals at ~107 and ~165 mbsf (Suess, von Huene, et al., 1988). Information collected at Site 1230 supplements these findings. Two intervals of concentrated gas hydrate were found at Site 1230, but they are offset from those of Site 685 by ~20 m. One of the gas hydrate pieces recovered at Site 1230 was composed of several-millimeter-thick layers oriented at a high angle to bedding surfaces. We suspect that this gas hydrate was associated with a fault plane because small high-angle faults were common at Site 1230 and because previous work on hydrates in other regions has shown a link between faults and gas hydrate presence (Ginsburg and Solovieu, 1997; Wood and Ruppel, 2000). The depth offset between zones of concentrated hydrate may reflect preferential emplacement of hydrate along high-angle faults.
At other ODP sites where gas hydrate is present, much of the hydrate is inferred to be disseminated in pore space and most does not survive the wireline trip (e.g., the Blake Ridge, Paull, Matsumoto, Wallace, et al., 1996). This may also be the case at Site 1230. In addition to the zones of concentrated hydrate, temperature observations, core disturbance, interstitial water chloride concentrations, and velocity and electrical resistivity log data suggest that small amounts of disseminated gas hydrate may be present throughout the recovered sedimentary section at Site 1230, starting below ~70 mbsf or even shallower (Fig. F37).