Site 1246 (proposed Site HR1b) is located in 848 m of water near the crest of Hydrate Ridge, ~3 km north of the southern summit (Fig. F1). The 3-D seismic data show that the BSR is at a depth of ~114 mbsf at this site. This site also samples a pair of bright regional seismic reflectors, referred to as Horizons B and B', at depths of ~60 and 100 mbsf, respectively. At this site, the temperature and pressure at the seafloor are well within the GHSZ, indicating that gas hydrates can exist within the entire stratigraphic section above the BSR if hydrate-forming gases are available in concentrations that exceed their in situ solubility.
The primary objective at Site 1246 was to sample Horizons B and B' where they are within the GHSZ. By comparing this site to Site 1244, where Horizons B and B' are below the GHSZ, we hope to constrain lithologic and hydrologic explanations for the strong reflectivity of these seismic horizons, obtain insights into the processes that transport fluids into and through the GHSZ, and develop more effective strategies to predict hydrate presence from seismic and other remote sensing data.
Hole 1246A was drilled to a depth of 180 mbsf without coring to obtain the initial LWD data for this site. In Hole 1246B, we obtained 16 APC cores, sampling to a depth of 136.7 mbsf with 99% recovery. The APCT was run five times; no other special tools were deployed. These cores were sampled for headspace and void gas analyses, chemical analysis of interstitial water (two per core), basic physical property measurements, biostratigraphic analysis, and lithostratigraphic description.
The precruise 3-D seismic reflection site survey and the LWD data obtained in Hole 1246A provided a roadmap that was used to guide the sampling and analysis strategy at this site. As mentioned above, the seismic data define the link between Sites 1244 and 1246 and suggest approximate depths to Horizons B and B' and the BSR of ~60, 100, and 114 mbsf, respectively. The LWD data were processed and available for interpretation for 3 weeks prior to coring. These logs provided a first look at the probable distribution of hydrate within the GHSZ. Based on the Archie's Law relationships between electrical resistivity and porosity, LWD data suggest that hydrate should be present intermittently between 53 and 109 mbsf (Fig. F13). The LWD data also indicate that Horizon B is characterized by relatively high density and resistivity, as was found at Site 1244. Unlike Site 1244, no prominent low-density anomaly is found associated with Horizon B' at Site 1246.
Use of the IR camera continued both to rapidly identify gas hydrate through the core liner and to investigate the distribution and texture of hydrate in the cores and visualize the process of dissociation. Temperature anomalies in the IR thermal images suggest the intermittent presence of hydrate from ~15 to 117 mbsf (Fig. F13), a somewhat more extensive depth range than is indicated by other hydrate proxies (see discussion of interstitial waters and LWD). In addition, several of the IR thermal anomalies are correlated with changes in the lithologic and physical properties of the sediments and with anomalies in chemistry of the pore waters. These changes are in turn correlated with seismic Horizon B. Whole rounds of core containing particularly strong IR temperature anomalies, and therefore suspected of containing gas hydrate veins or nodules, were recovered from 66.5, 96.6, and 105.0 mbsf and preserved in liquid nitrogen for detailed shore-based studies. A gas hydrate sample recovered from 109.5 mbsf was divided into two pieces; one piece was allowed to dissociate on board for chemical analysis (discussed below), and the other was preserved.
On the basis of visual sediment descriptions, examination of smear slides, and correlation with physical property data (especially MS), the sedimentary sequence can be divided into two primary lithostratigraphic units (Fig. F10). Lithostratigraphic Unit I (from the seafloor to 21.7 mbsf) is a late Pleistocene–Holocene unit characterized by dark greenish gray diatom and nannofossil-bearing hemipelagic clay. Lithostratigraphic Unit II, which extends to the base of the Hole 1246B, is defined by the onset of graded silt and sand layers, which represents a series of turbidites of varying thicknesses bounded by erosional contacts and separated by periods of bioturbated hemipelagic sedimentation. Layers composed of >50% sand are found at 62, 71, and 136 mbsf, respectively. The sandy layer at 62 mbsf (found in Core 204-1246B-8H) can be correlated with the base of seismic Horizon B. (Note that the thickness and base of this layer are not defined because whole rounds were taken for microbiology and interstitial water analyses prior to core description). The overlying graded sequence is ~2 m thick and gray colored, in sharp contrast to most of the sediment cored during this leg. The gray color results from a high percentage of quartz grains. This zone contains fewer biogenic components than adjacent strata, suggesting rapid deposition. A similar sequence is found at 56 mbsf in Core 204-1246B-7H, although the base of this upper turbidite sequence contains less sand.
Detailed analysis of physical property data reveals that the two turbidite sequences that comprise Horizon B are characterized by high density and high MS anomalies and that each layer is ~2.5 m thick (Fig. F17). Preliminary synthetic seismograms, calculated based on the density log, confirm that the resulting double-peaked density anomaly extending from 54 to 67 mbsf explains the complicated waveform of seismic Horizon B at this site. The relatively high density probably results from the grain size and packing of the sediments. The source of the high MS has not yet been identified. A thin (<50 cm thick) high-resistivity anomaly also appears near the base of each of these turbidites, suggesting the presence of gas hydrate. Low temperatures measured on the catwalk with the IR camera are additional indirect indicators for the presence of gas hydrate in the lowermost coarse-grained portion of each turbidite sequence. A sample thought to contain hydrate was recovered from the base of the lower turbidite (Sample 204-1246B-8H-4, 25–30 cm) and was preserved in liquid nitrogen for postcruise analysis. Samples of this coarse-grained layer were also taken for interstitial water and microbiology studies.
The correlation between physical properties, lithology, and seismic Horizon B' at Site 1246, where it is present a few meters above the BSR, is not clear. A thin (<1 cm thick) volcanic ash–rich layer is observed at 96 cm depth in Section 204-1246B-11H-4, 96 cm (~95 mbsf). Strong, narrow (<1 m), high-resistivity, and thermal anomalies indicative of the presence of gas hydrate are observed at 96–97 mbsf in the LWD and IR data, respectively. A sample thought to contain hydrate was recovered in Core 204-1246B-11H-5 at this depth. Recovery in Core 204-1246B-11H was 80%, and the primary lithologic source of Horizon B' may not have been recovered. Shore-based studies are planned to determine the age and provenance of this ash and to compare it to the ash recovered from Site 1244. The ash data will complement the biostratigraphic ages, which suggest an age of ~0.3 Ma at Horizon B'. Seismic modeling will also provide constraints on the nature of fluids in this horizon. At Site 1244, where Horizon B' is found at 216 mbsf (well below the GHSZ), it is associated with a 60-cm-thick layer that is rich in detrital shards of volcanic glass and corresponds to a distinctive low-density anomaly in the LWD and shipboard physical property data. We will test the model that Horizon B' contains free gas at Site 1244 and that the disappearance of the density anomaly at Site 1246 results from incorporation of free gas into gas hydrate.
Geochemical analyses of interstitial waters made during Leg 204 revealed variations in the concentration of several different chemical species with depth that correlate with the GHSZ. At Site 1246, the most direct correlation is based on chloride concentrations, which show a pattern similar to those observed at most other sites, with the exception of those at the southern summit of Hydrate Ridge (Fig. F14). From the seafloor to ~40 mbsf, the chloride concentrations is similar to that of seawater, suggesting that no hydrate is present. Between 40 mbsf and the BSR at ~114 mbsf, there are numerous low-chloride spikes that likely reflect the dilution of pore water by water from dissociated hydrate. The lowest chloride value at this site (~430 mM) is from a sample that fortuitously coincided with the coarse-grained basal zone of the Horizon B turbidite discussed above. Assuming that an in situ no hydrate background concentration of chloride (as was estimated for Site 1244), the amplitude of this chloride anomaly suggests that 23% of the pore space in this layer is filled by hydrate. Ba, Li, and Na are depleted in this interval, and Ca, Mg, and Sr are depleted in the overlying sample located ~3 m higher in the section. The correlation of interstitial water chemistry with specific horizons defined by lithologic and physical properties suggests that modeling of these chemical data may provide constraints on the origin, evolution, and flow rate of fluids that transport methane into and through the GHSZ.
Analyses of the organic chemistry of gases at Site 1246 indicate that processes here are similar to those at Site 1244. The ratio of methane to ethane (C1/C2) shows a steplike decrease at the BSR, which reflects an increase in C2 beneath the BSR rather than a change in methane concentration. The presence of propane (C3) below the BSR suggests the upward migration of higher hydrocarbons from below, and the absence of C3 within the GHSZ (with one exception at ~22 mbsf) indicates that gas above solubility in this zone should be in the form of Structure I rather than Structure II hydrate. The apparent fractionation of C2 into gas hydrate, which was reported at Site 1244, is not apparent here. However, this may be an artifact of sampling because only one hydrate sample was available for gas analysis at Site 1246. Because no PCS runs were made at this site, no in situ gas concentration estimates are possible.
The methane/sulfate boundary at Site 1246 falls between 4 and 7 mbsf but is poorly defined as a result of sparse sampling; it is slightly shallower than at Site 1244, where it is identified at 9 mbsf (Fig. F18). Variations in the depth of the methane/sulfate boundary among Leg 204 sites will be compared to variations in depth to the first occurrence of hydrate as part of a postcruise study.
Five downhole temperature measurements were made at Site 1246 (Fig. F18). The apparent temperature at the seafloor appeared to change abruptly by –2°C between the first and second runs, suggesting a problem with the instrument. This was confirmed when the probe indicated a temperature of –2.53°C after being placed in an ice-water bath. The APCT tool that was used showed a short offset of –2°C between the first and second runs. Correcting for this offset, the data suggest a temperature gradient of 0.049°C/m, which is lower than the gradient of 0.057°C/m measured at Sites 1244 and 1251. This temperature gradient predicts that the BSR should be present at 150 mbsf, considerably deeper than the observed depth of ~114 mbsf. This observation is consistent with results from other sites near the crest of Hydrate Ridge, whereas sites located away from the crest show much less difference between measured and predicted temperatures at the BSR.
There are multiple correlations between geological and geophysical parameters and the presence of gas hydrate at Site 1246. The primary preliminary result is that seismic Horizon B is caused by a pair of high-MS, high-density, and low-porosity layers ~2.5 m thick and spaced 10 m apart. Sedimentological analysis indicates that each layer is formed by a turbidite sequence with a complicated internal structure indicating deposition, erosion, and redeposition. Electrical resistivity, IR temperature, and geochemical anomalies are associated with the basal coarser-grained layers of each of the two turbidites that constitute Horizon B, indicating that hydrates preferentially form here. Indirect and direct indicators of hydrate were also found associated with Horizon B'. Postcruise work is planned to determine the source of the MS anomaly and to correlate lithologic and physical properties of these horizons between Site 1244 and Site 1246. Additional efforts will be focused on modeling the seismic response of this horizon as it changes from a fluid-rich layer beneath the BSR to a hydrate-bearing layer above it and to constrain the source and evolution of the fluids using the geochemical data.