Site 1248 (proposed Site HR6) was drilled in a 832 depth of water, ~300 m northwest of the southern Hydrate Ridge summit (Fig. F1). This site is located in the middle of a small (~150 m diameter) high-reflectivity spot on the seafloor imaged by a deep-towed side-scan sonar survey (Fig. F7). The small spot is located 300 m north of a larger circular high-reflectivity patch around the Pinnacle, a well-known active carbonate chemoherm. These are the only two locations on southern Hydrate Ridge where high backscatter reflectivity is observed and is interpreted as seafloor manifestations of fluid venting. Television-sled surveys revealed some evidence for the presence of scattered authigenic carbonate fragments within the small high-reflectivity patch, which might be responsible for the higher backscatter signal observed in the side-scan sonar data. The 3-D seismic data show attenuation of the underlying stratigraphic reflectivity, similar to what is observed beneath the Pinnacle (Fig. F7). Both areas of high backscatter overlie the intersection of seismic Horizon A and the BSR.
The principal objectives at Site 1248 were to (1) investigate whether the sediments below the high-reflectivity seafloor spot contain evidence of active fluid advection and (2) if these fluids are supplied by Horizon A.
Three holes were drilled at Site 1248. LWD measurements were made in Hole 1248A down to 194 mbsf. Hole 1248B was abandoned at 17 mbsf after three cores. Coring disturbance because of massive near-seafloor gas hydrate presence and a shattered liner during retrieval of Core 204-1248B-3H resulted in poor core recovery (44%). Of the 17 cores from Hole 1248C, 5 XCB cores were drilled to 48 mbsf, followed by 11 APC cores and 1 XCB core to 149 mbsf. After poor core recovery (23%) in Cores 204-128C-1X through 5X, recovery increased to 90%. Temperature measurements were made using one DVTP (at 19 mbsf), three APCT (at 26, 86, and 105 mbsf), and two DVTPP (at 105 mbsf and at the bottom of the hole at 149 mbsf) runs.
Three lithostratigraphic units were recognized at Site 1248. The uppermost sediments that comprise Units I and II (Holocene–late Pleistocene age) are characterized by dark greenish gray diatom-bearing clay and silty clay and extend from the seafloor to 39 mbsf. These fine-grained lithologies are commonly structureless except for sulfide mottles. Lithostratigraphic Unit III (39–149 mbsf), of middle–early Pleistocene age, is dominated by homogenous silty clays with varying amounts of biogenic components. Sand- and silt-sized turbidites are intercalated as minor lithologies throughout this unit.
High concentrations of beige to white volcanic glass shards were observed in the tail of a few turbidites near 130 mbsf in Core 204-1248C-14H (Fig. F9). These glass-rich layers are the lithologic signature of seismic Horizon A, which appears in the LWD data as a 2-m-thick interval characterized by high resistivity and low density values within the depth interval from 126 to 128 mbsf. Physical property measurements on discrete samples of the cores from Site 1248 confirm the low-density character of Horizon A sediments, which is interpreted to be a result of the reduced grain density of the ash. The presence of volcanic ash in Horizon A reduces the grain density because amorphous silica particles in the ash have distinctly lower grain densities than other sedimentary components like quartz, feldspar, and clay minerals. Sediments from the interval of Horizon A do not show higher porosity values than the surrounding sediments. The distinctly larger grain size of the ash-rich sediments, however, implies a different packing structure and possibly higher permeability in these intervals, supporting the idea that Horizon A is a potential fluid migration conduit.
Organic geochemistry measurements on gases from Site 1248 reveal high methane contents throughout. In addition to the high methane levels, there is a surprising variation in ethane content with depth. C1/C2 is < 1,000 near the seafloor, increasing to 10,000 near the base of the GHSZ, then decreasing sharply below the BSR. In addition to ethane, propane (C3) is present in relatively high concentrations in the upper 120 mbsf and is even more abundant in headspace gas below that depth. The gas analyses at Site 1248 reflect the complex mixing of gases from two hydrocarbon sources (Fig. F15). Mixed microbial and thermogenic gases are present in the uppermost 40 mbsf followed downhole by an intermediate interval (40–100 mbsf) dominated by microbial gas. Mixed microbial and thermogenic gas is also present in the deepest sediments at Site 1248 (below 100 mbsf), with thermogenic gas possibly being injected at Horizon A at ~130 mbsf. The data, thus, suggest rapid advection of deeper gas to the seafloor, bypassing the lower part of the GHSZ.
Analyses of gases from dissociated hydrate samples collected from the shallow zone above 40 mbsf showed that, even though methane occupies most of the water cages of the hydrate structure, higher-order hydrocarbons are also present. Gas hydrates at Site 1248 are probably primarily Structure I hydrate that incorporates ethane molecules within their cage structure. However, in Sample 204-1248C-2H-2, 0–10 cm (7.37 mbsf), a higher concentration of propane than ethane suggests that Structure II hydrate is present. Although thermogenic Structure II gas hydrates are common in petroleum provinces such as the Gulf of Mexico and the Caspian Sea (Sassen et al., 2001; Ginsburg and Soloviev, 1998), this hydrate type was not known to be at Hydrate Ridge prior to Leg 204.
Interstitial water geochemistry results clearly show the influence of gas hydrate formation at Site 1248 (Fig. F14). Based on the chloride distribution in the pore water, the presence of hydrate is suggested from the seafloor to the BSR at 115 mbsf. The data indicate 25% gas hydrate content in the pore space of the uppermost 20 mbsf, whereas LDW resistivity data indicate up to 50% occupancy by gas hydrates. Below 20 mbsf, gas hydrate content calculated from chloride anomalies ranges from 2% to 5% pore volume saturation. This pattern of chloride distribution is well documented by direct gas hydrate sampling and by the hydrate-related fabrics observed at this site by the sedimentologists. Soupy and mousselike textures are predominantly present in silty clay and diatom-bearing silty clay after dissociation of hydrates. These were particularly common in the uppermost 20 mbsf. At these depths, more massive gas hydrate samples were recovered; in contrast, farther downcore, small nuggets and thin veins of hydrate were sampled. The samples were identified by thermal IR data imaging of cores on the catwalk using a hand held IR camera. Postacquisition processing of the IR data shows a good correlation with the pore volume saturation derived by LDW resistivity logs, the chloride pore water data, and the sedimentological observations of the presence of dissociated hydrate layers.
High advective flow rates in the uppermost 20 mbsf of the sediments drilled at Site 1248 are indicated by several findings. Sulfate concentrations were near zero even in the shallowest pore water sample, implying a high methane flux from below that feeds microbial AMO. By consuming the near-seafloor sulfate, a consortium of bacteria and Achaea is responsible for AMO close to the seafloor, during which millimolar quantities of dissolved sulfide are created. High sulfide concentrations were found within the the uppermost core. In addition, authigenic carbonates that were described in the cores are probably caused by higher dissolved carbon dioxide production, as evidenced in high alkalinity values.
Temperature measurements from downhole tools (three APCT, one DVTP, and two DVTPP runs) were used to calculate a temperature gradient of 0.038°C/m, considerably lower than expected. However, if two outlier measurements are excluded, the gradient is 0.055°C/m, identical to the gradient determined from nine measurements at Site 1245. This temperature gradient predicts the base of the GHSZ at 130 mbsf, assuming methane and standard mean seawater for the calculation. This is 15 m deeper than that indicated by the seismic and LWD data and is consistent with a general pattern of greater mismatch between measured in situ temperature and BSR depth near the summit of Hydrate Ridge, the cause of which has not yet been determined.
Gas hydrate is present throughout the sediment column from the seafloor to the BSR at Site 1248, as documented by IR imaging, LWD data, chloride anomalies in the pore water, analyses of sedimentary fabric, and direct sampling of gas hydrates in a couple of intervals. The Holocene–Pleistocene sediments drilled here indicate strong advective fluid flow near the summit of southern Hydrate Ridge. There is abundant hydrate in near-surface sediments and no sulfate in the shallowest interstitial water samples collected at Site 1248, clearly indicating active flow of methane-bearing fluids. Authigenic carbonate formation is present close to the seafloor, probably induced by AMO. The presence of authigenic carbonate as well as gas hydrate probably causes the high reflectivity that was mapped during the deep-towed side-scan sonar survey of the seafloor. Advective flow is also indicated by the shallow presence of thermogenic hydrocarbons mixed with microbial gases. Gases obtained from dissociation of a shallow gas hydrate sample revealed a higher concentration of propane than ethane (C3 > C2) in addition to methane. Such a gas composition should form Structure II hydrate, although we were not able to confirm this on board. Structure II gas hydrate is well known from petroleum basins like the Gulf of Mexico (Sassen et al., 2001), although this is the first indication of Structure II hydrate along an accretionary margin.