Site 1251 (proposed Site HR2alt) was drilled in a water depth of 1216 m, ~5.5 km east of the southern summit of Hydrate Ridge (Fig. F1). The site is located in a slope basin where well-stratified sediments were apparently deposited at a rapid rate. Seismic data record a history of deposition, tilting, folding, and depositional hiatuses in the basin that is probably related to the evolution of Hydrate Ridge. A strong BSR suggests that the base of the GHSZ is at ~196 mbsf at this site.

The principal objectives at Site 1251 were to (1) determine the source of water and gases forming gas hydrates in a setting that is characterized by rapid deposition of hemipelagic sediments and mass-wasting deposits, in contrast to the uplifted sediments of the accretionary complex; (2) determine the distribution of gas hydrates in relation to the typical lithologic parameters for the basin; (3) test general models for hydrate formation in regions of rapid sediment accumulation that were developed in the Blake Ridge area from results of ODP Leg 164; and (4) provide age constraints on the geological history recorded by seismic stratigraphy.

Operations

Six holes were drilled at Site 1251. In Hole 1251A, LWD measurements were made using a variable, but generally low, rate of penetration (ROP) in the upper 30 mbsf followed by an increased ROP of ~50 m/hr from 30 mbsf to the bottom of the hole (at 380 mbsf). The LWD string included the RAB tool, measurement-while-drilling [MWD], NRT (natural magnetic resonance [NMR]-LWD), and Vision Neutron Density (VDN) tools. Hole 1251B was APC cored (Cores 204-1251B-1H through 24H, including PCS deployments at 105 and 154 mbsf and an FPC deployment at 172 mbsf) with an average core recovery of 80.6% down to 194.6 mbsf, where more lithified sediments significantly reduced the penetration of the bit. Coring continued using the XCB down to 445.1 mbsf (Cores 204-1251B-25X through 53X) with an average recovery of 85.5%. In addition to the XCB coring, the three pressure coring tools (PCS, FPC, and HRC) were used at 291, 330, and 397 mbsf, respectively. Special tools in Hole 1251B included four APCT (37.6, 66.1, 94.6, and 125.1 mbsf) and two DVTPP (156.6 and 293.6 mbsf) runs. Hole 1251C was terminated after two cores. In Hole 1251D, which comprises 30 cores, 3 XCB cores were drilled to 26.9 mbsf followed by 15 APC cores to 173.4 mbsf and 6 XCB cores to 226.5 mbsf. Furthermore, a series of special tools were deployed in Hole 1251D: one APCT (173.4 mbsf), two DVTPP (175.4, and 198.2 mbsf), four PCS (46, 77, 173, and 228 mbsf), one FPC (227 mbsf), and one HRC (230 mbsf). Holes 1251E and 1251F were each cored by APC to 9.5 mbsf for high-density sampling. Hole 1251G was washed to 2.5 mbsf before one APC core was taken for special sampling of turbidite layers. The hole was then washed down to 20 mbsf before an additional PCS (at 21 mbsf) was deployed.

Principal Scientific Results

Based on the major and minor lithologies and additional criteria like sediment fabric, physical properties, microscopic, chemical, and XRD analysis, the hemipelagic strata and turbidite sequences recovered at Site 1251 were divided in three lithostratigraphic units. Lithostratigraphic Unit I, subdivided into three subunits, extends from the seafloor to 130 mbsf and is characterized dominantly by dark greenish gray clay to silty clay ranging from Holocene to Pleistocene age (0–~0.3 Ma). The sediments of Subunit IA (0–23 mbsf) are characterized by clay and silty clay, some of which is diatom bearing, interlayered with several coarse-grained turbidites. Subunit IB (23–34 mbsf) is characterized by unsorted pebble-sized mudclasts in a clay matrix and a series of soft-sediment deformation fabrics representing a debris flow unit. This unit can be traced regionally based on its chaotic character in seismic reflection records (Fig. F8) and reaches a maximum thickness of ~70 m in the center of the slope basin. Stratified diatom-bearing silty clays comprise Subunit IC (34–130 mbsf), which shows clear seismic stratification. The base of Subunit IC is defined to correspond to a prominent angular unconformity imaged in the seismic data, although there is no apparent lithologic discontinuity at that boundary.

Hemipelagic clays of middle Pleistocene age, partly enriched in siliceous and calcareous biogenic components, form lithostratigraphic Unit II (130–300 mbsf). Unit III (300–443 mbsf) consists of partly lithified clays that show a downcore transition to claystones. A distinct enrichment of green glauconite grains in a 120-cm-thick interval on top of lithostratigraphic Unit III is probably associated with a major unconformity or period of very low sedimentation rate that lasted from 1.6 to 1.0 Ma, as defined by biostratigraphic data (Fig. F11). Between 320 and 370 mbsf, the sediments contain a higher amount of biogenic opal, which is well documented by smear slide estimates and XRD analyses. Authigenic carbonates of various morphologies and mineralogical compositions as well as glauconite grains are scattered throughout this unit. Biostratigraphic investigations using diatoms and calcareous nannofossils assign these sediments to an early Pleistocene and late Pliocene age. The Pleistocene/Pliocene boundary is present at ~375 mbsf.

Major downcore changes in physical property data are generally in agreement with seismic stratigraphy and lithostratigraphic boundaries. The uppermost sediments of lithostratigraphic Units I and II are characterized by increasing bulk density values that follow a standard compaction curve. A generally increasing trend in bulk densities measured with the MST and in density measurements on discrete samples (MAD) is interrupted by a 50-m-thick sediment sequence between 320 and 370 mbsf, in which the bulk densities drop significantly and porosity values increase. This change in physical properties is caused by higher amounts of biogenic opal-A, an amorphous silica phase having low grain density and high skeleton porosity. MS values at Site 1251 are characterized by generally uniformly low values. Various high-amplitude MS peaks are correlated with either turbidites, enrichments of magnetic minerals resulting from low sedimentation rates, or diagenetic iron sulfide minerals.

Thermal imaging of cores using the IR cameras was used to detect intervals of disseminated gas hydrate between 40 and 200 mbsf. Discrete samples of hydrate were not seen in Hole 1251B, although several cold anomalies were observed. As a result of low recovery of this interval, the BSR was not sampled in Hole 1251B. The IR temperature anomalies observed in Hole 1251B were relatively small (T = ~1°–1.5°C) compared to an IR temperature anomaly (T = ~6°C) encountered in Hole 1251D between 190 and 197 mbsf, which corresponds to the interval just above the BSR.

Interstitial water geochemistry at Site 1251 focused on hydrate-related changes in chloride concentration pattern, changes in fluid composition in relation to dewatering of the sediments, and biogeochemical processes within the sediments. As observed at Sites 1244 and 1252 as well as in other accretionary wedges, the profile of dissolved chloride at Site 1251 decreases downcore from present seawater values close to the seafloor. At Site 1251, the chloride decrease in the interstitial water corresponds to an increase in dissolved lithium, revealing a fluid source from deeper sediments of the accretionary complex. The dissolved chloride distribution at Site 1251 shows only one distinct negative anomaly, seen in several samples taken just above the BSR from ~190 to 200 mbsf. This is consistent with the IR temperature and visual observations, indicating the presence of disseminated hydrate just above the BSR. Based on the lowest chloride value measured relative to an estimated background concentration, gas hydrate occupies up to 20% of the pore space in this zone. This finding is in contrast to other sites drilled during the leg, where repeated excursions to low chloride concentration values record the presence of gas hydrate over much larger depth intervals above the BSR (Fig. F21). We note that this low chloride anomaly was missed completely in Hole 1215B because of poor recovery of this interval. Special care was taken to sample this interval in Hole 1251C.

As at other sites on Hydrate Ridge, a number of different processes control whether methane reaches levels above saturation within the GHSZ. An important process controlling the methane distribution in the sediments at Site 1251 is methane consumption by AMO using sulfate as an oxidant. Methane flux at the SMI can be estimated from the sulfate and methane concentration gradients. At Site 1251, over half the sulfate being reduced is a result of AMO. Sulfate depletion at the SMI at 4.5 mbsf also leads to high dissolved barium concentrations below the SMI.

Methane concentrations obtained from headspace analyses increased rapidly below the level of sulfate depletion. Nine PCS deployments revealed methane concentrations from 46.4 to 158.4 mM at depths ranging from 20 to 290 mbsf. The methane concentration at 20 mbsf is compatible with the shallower headspace methane estimates and provides the gradient from which the methane flux is calculated. Based on measured methane concentrations slightly above solubility, two PCS deployments within the GHSZ (at 45 and 104 mbsf) show methane concentrations with values above saturation, implying the presence of methane hydrate. Observations below the BSR are ambiguous. A PCS sample from 32 m below the BSR did not show methane concentration above solubility, whereas one from 100 m below the BSR did.

In addition to methane (C₁), higher molecular weight hydrocarbons such as ethane (C₂), ethylene (C₂⁼), and propane (C₃) traces were also detected throughout the sequence cored at Site 1251. As at Sites 1244 and 1246, the composition of gas samples recovered from both expansion voids in the core liner and headspace measurements show a systematic in decrease in C₁/C₂ ratios below the BSR (Fig. F15). This order of magnitude decrease in the C₁/C₂ ratio is caused by an abrupt increase of ethane rather than by a change in methane concentration. Two possible mechanisms have been considered to explain this observation. In the first mechanism, ethane preferentially stored in hydrates and then released when the ethane-enriched gas hydrates at the base of the GHSZ dissociate in response to subsidence of the slope basin and the resulting upward migration of the GHSZ through the sediment column. In the second mechanism, the GHSZ is a barrier to C₂ migration. Additional analysis of the gas composition in hydrate samples from Leg 204 should permit us to distinguish between these two mechanisms.

Temperature measurements obtained with five APCT and three DVTPP deployments were used to calculate a linear temperature gradient of 0.0575°C/m at Site 1251, which is very similar to temperature profiles from other sites on Hydrate Ridge. Extrapolating laboratory measurements of gas hydrate stability for pure methane in water of 3.5% salinity, this temperature gradient predicts the base of the GHSZ to be at ~196 mbsf, which is in excellent agreement with the calculated BSR depth of ~196 mbsf determined from 3-D seismic reflection and OBS-derived seismic velocity data.

The recorded LWD data in the basin sediments in Hole 1251A are of high quality even though data were collected at a faster penetration rate than at other sites. There is minimal loss of vertical resolution. Resistivity and density log variations indicate thin-bedded changes in lithologies throughout the hole below 130 mbsf, which most likely reflect the downhole presence of interbedded turbidites that were observed during core description. There is a little direct evidence for the presence of gas hydrate in the LWD data, except for the depth interval from 90 to 110 mbsf and again immediately above the BSR. The Archie's Law relation for estimating gas hydrate saturation from the resistivity log data implies gas hydrate saturation up to 18% in the sediments just above the BSR, in excellent agreement with the estimate based on the maximum chloride anomaly. The resistivity data also indicate the presence of free gas extending for ~100 m below the BSR. Borehole breakouts are well developed below 300 mbsf and indicate an east-west axis of compressive stress.

Summary

Drilling at Site 1251 recovered a sequence of well-stratified hemipelagic sediments of the slope basin adjacent to Hydrate Ridge. Major lithostratigraphic units were characterized and are, in most cases, separated by clearly defined unconformities in the seismic record. Thermal, sedimentological, geophysical, and geochemical proxies for the presence of hydrate, as well as direct sampling, were used to document the presence of gas hydrates in host sediments younger than 500,000 yr old. At this site, significant hydrate accumulations seem to be limited to two intervals, 90–110 mbsf and just above the BSR at 190–200 mbsf. This contrasts with the hydrate distribution at the other sites cored during Leg 204, where hydrate is found throughout most of the hydrate stability zone.