Site 1250 (proposed Site HR4a) was drilled in 792 m of water, ~100 m west of the southern summit of Hydrate Ridge and ~100 m east of the Pinnacle (Fig. F1). On southern Hydrate Ridge, the Pinnacle is the only carbonate mound, whereas at northern Hydrate Ridge several major chemoherms are known. The Pinnacle has near-vertical flanks rising ~40 m above the seafloor and a diameter of ~150 m. The carbon source for formation of the Pinnacle is known to be biogenic methane from the very low ¹³C values (Teichert et al., in press). ²³⁰Th/²³⁴U data indicate that the Pinnacle formed during the last 12 k.y. (Teichert et al., in press). It is located in the middle of a high-reflectivity patch (mapped by a deep-towed side-scan sonar survey) (Johnson and Goldfinger, pers. comm., 2002), which might be created by scattered carbonates close to the seafloor and/or the presence of shallow gas hydrates. Site 1250 lies close to the eastern rim of the high-reflectivity patch (Fig. F7). The precruise 3-D seismic reflection survey data show that seismic Horizon A (~150 mbsf at Sit 1250) meets the BSR (~114 mbsf at Site 1250) just below the Pinnacle (Fig. F7).

The primary objective at Site 1250 was to sample the sediments, fluids, gases, and gas hydrates under the high backscatter seafloor flanking the Pinnacle. The sediments at Site 1250 were expected to be strongly affected by the upward fluid migration that has resulted in the formation of the Pinnacle chemoherm. In this context, understanding the role of Horizon A as a conduit for fluid flow and its interaction with the BSR was of special interest.

Operations

Five holes were drilled at Site 1250. Recording of the LWD RAB tool failed during the first run in Hole 1250A because of depleted batteries; therefore, the LDW operation was repeated in Hole 1250B, which was drilled to 180 mbsf. Holes 1250C and 1250D were APC/XCB cored down to 145 mbsf. At Holes 1250C and 1250D 19 cores were recovered, with average recovery of 82% of the total penetration (Table T1). Two cores were recovered at Hole 1250E, which was dedicated to biogeochemical sampling, with 92% core recovery. Because of relatively high levels of higher-order hydrocarbons encountered near Horizon A at Site 1248, we decided not to penetrate Horizon A until a better understanding of possible hazards associated with this horizon had been obtained from drilling through it further downdip. After coring through Horizon A at Sites 1245 and 1247, we returned to Site 1250 to APC/XCB core Hole 1250F from 100 to 180 mbsf.

The PCS was deployed two times in Hole 1250C (71 and 130 mbsf), three times in Hole 1250D (35, 103, and 135 mbsf), and three times in Hole 1250F (119, 130, and 132 mbsf). HYACINTH autoclave pressure coring tools were deployed in Hole 1250C (FPC; 137.5 mbsf) and Hole 1250D (HRC; 134.2 mbsf). Special tools were used for temperature measurements in Hole 1250C, including five APCT runs at 33, 52, 71, 82.5, and 92 mbsf and two DVTP runs. Temperature measurements in Hole 1250D included four APCT runs at 35, 46.5, 65.5, and 103.5 mbsf and two DVTP runs. Wireline logging was performed in Hole 1250F using separate runs of the triple combo tool (Temperature/Acceleration/Pressure [TAP] tool/Dual Induction Tool [DIT]/Hostile Environment Litho-Density Tool [HLDT]/Accelerator Porosity Sonde [APS]/Hostile Environment Gamma Ray Sonde [HNGS]/QSST) and the FMS-sonic tool string (FMS/Dipole Sonic Imager [DSI]/Scintillation Gamma Ray Tool [SGT]) down to 180 mbsf. Vertical and offset VSPs were acquired with the JOIDES Resolution and the Ewing (located at an offset of ~700 m) alternating shots. This was followed by walk-away VSPs shot by the Ewing to downhole seismometers clamped at 91, 138, and 172 mbsf.

Principal Scientific Results

On the basis of visual sediment descriptions, physical property measurements, LWD data, and seismic correlation, the sedimentary sequence at Site 1250 was divided into three lithostratigraphic units. Lithostratigraphic Unit I, Holocene–late Pleistocene in age, extends from the seafloor to 9.5 mbsf and is mainly composed of dark greenish gray clay or silty clay, which is generally diatom bearing or diatom rich. Lithostratigraphic Unit II (9.5–14 mbsf), of late Pleistocene age, is principally composed of lithologies similar to those in Unit I but intercalated with thin silt and fine sand layers not found in Unit I. Based on their appearance, these multiple layers of coarser grain size are clearly single events that were deposited rapidly from turbidity currents. Lithostratigraphic Unit III (14–181 mbsf) consists of silty clay that is nannofossil rich or diatom rich, with an age range of late–early Pleistocene.

The boundary between lithostratigraphic Units II and III is correlated with Horizon Y, a seismic reflector that corresponds to a regional stratigraphic unconformity. This boundary is well defined by a 6-m-thick sequence of coarse-grained high-frequency thin turbidite layers, which includes individual sand layers up to 20 cm thick. LWD recorded a high-density peak around seismic Horizon Y, which was confirmed by shipboard physical property measurements of core samples. In addition, shipboard multisensor track (MST) data reveal a large positive excursion of MS caused by a higher content of magnetic minerals within the sand layers at the boundary between lithostratigraphic Units II and III.

Lithostratigraphic Unit III at Site 1250 is divided in two subunits. Subunit IIIA includes several mass-wasting deposits, of which a debris-flow layer between 86.5 and 100 mbsf is the most pronounced example. This deposit is characterized by the presence of mud clasts up to 5 cm in diameter, similar to those observed at other sites (Fig. F9) and several soft sediment deformation features. Subunit IIIB has a distinctly higher abundance of calcareous nannofossils and foraminifers than Subunit IIIA. The boundary between the stratigraphic Subunits IIIA and IIIB is marked by several light-colored ash-rich layers that are composed of volcanic glass–rich silt to silty volcanic ash, typically a few centimeters thick. This ash-rich interval is well defined in the LWD data by a low-density anomaly and corresponds to the regional seismic reflector known as Horizon A. Physical property measurements on discrete samples at Site 1250 revealed low grain densities in this depth interval, which are partly explained by the low grain density of the amorphous silicate components of the ash. Free gas in this interval probably also contributes to the low densities and high resistivities recorded in the LDW data and to the low seismic velocities recorded by the sonic logs and VSPs.

IR imaging of the cores on the catwalk using a hand held camera enabled us to identify 20 whole-round samples likely to contain gas hydrates. The hydrate samples show a wide range of morphologies, ranging from massive to nodular, and are often embedded in soupy sediments, which are interpreted to result from decomposition of disseminated hydrate and fluidization of the sediment by hydrate water.

In addition, IR imaging with the track-mounted camera revealed 40 temperature anomalies between 14 and 109 mbsf in Hole 1250C and 57 anomalies between 6 and 113 mbsf in Hole 1250D. The depth range of the cold-temperature anomalies correlates well with the depth distribution of moussey and soupy textures observed by the sedimentologists during core description. The lowermost gas hydrate piece was sampled in Hole 1250F at 100.23 mbsf, which is slightly above the base of the GHSZ at 114 mbsf as defined in the P-wave sonic log.

Chloride concentrations in the pore water at Site 1250 document the different geochemical processes linked to the presence of gas hydrates. As observed at Site 1249, an enrichment in dissolved chloride in the upper 20–30 mbsf at Site 1250 shows the effect of rapid and recent gas hydrate formation. Below 20–30 mbsf, the chloride shows a gently sloping baseline toward fresher chloride values. Using this baseline curve, the negative chloride anomalies were used to calculate the amount of gas hydrate responsible for the dilution of each sample. The peak amounts are up to 15% gas hydrate filling of the available pore space with average values ranging from 0% to 6%.

Interstitial water chemistry documents upward fluid advection and near-surface diagenetic processes. Sulfate is depleted even in the shallowest sample because of the upward methane flux and methane oxidation. Alkalinity is anomalously high in the upper tens of meters, reflecting fluid advection. Authigenic carbonate formation is documented by very low Ca concentration in the pore fluids and discrete carbonate samples close to the seafloor. Interstitial water chemistry also documents migration of fluids from a deep source. A downhole linear increase in lithium concentration with depth is believed to reflect the diagenetic remobilization of lithium at depth in the accretionary wedge where the temperature exceeds 80° C. Superimposed on this downhole lithium increase there is a peak in the pore fluid concentration around seismic reflector Horizon A, indicating focused fluid transport along this high-permeability pathway.

Gas samples from Site 1250 show high methane content throughout the sediment sequence, and there is no decrease in the in the shallow samples. This is in agreement with the lack of sulfate in the pore water and the inferred high advection rates. Void gas samples show that the ethane content is relatively high. The observed enrichment of higher hydrocarbons (C₃–C₅) close to the seafloor indicates lateral migration of wet gas hydrocarbons that must have originally been derived from a deep source. A distinct increase in ethane observed near the BSR could be a result of release of ethane from decomposed gas hydrate. An increase in propane and n-butane probably reflects migration of hydrocarbons from deeper depths.

In order to obtain in situ gas concentrations, the PCS was deployed successfully five times. Three PCS deployments above the BSR show concentrations clearly above methane saturation, which predicts gas hydrate concentrations of 0.6%–2.2% within the pore volume of the cores. A second sample, from Core 204-1250D-18P, collected ~24 m below the BSR, indicates a gas concentration below saturation at in situ conditions.

Nine APC temperature measurements were made at this site, and they yielded a temperature gradient of 0.049°C/m, which is lower than expected and predicts the base of the GHSZ at 138 mbsf, assuming methane and standard mean seawater for the calculation. This is 26 m deeper than the level of the GHSZ 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 this discrepancy is yet not known.

Summary

At Site 1250, hemipelagic fine-grained sediments interbedded with turbidites are Quaternary in age (younger than 1.6 Ma) and contain gas hydrates in varying amounts. Positive chloride anomalies in the pore water in the upper 20–30 mbsf reveal rapid and active formation of gas hydrates during recent times, consistent with LWD resistivity data and direct sampling. High methane concentration, no sulfate, high alkalinity, and carbonate diagenesis in the uppermost sediments document high advection rates of fluid migration, similar to what was observed at the other seep-related sites (Sites 1248 and 1249). Below 30 mbsf, chloride anomalies in interstitial water samples and direct measurement of in situ gas concentrations using the PCS indicates that gas hydrates occupy <1% to a few percent of the pore space. The presence of free gas just below the BSR is documented by the concentrations of methane well above in situ solubility found in a PCS core from below the BSR and from P-wave data obtained by sonic logs and VSPs.