RESULTS

Two sites were drilled at the summit of southern Hydrate Ridge. Site 1249 (778 m water depth), located on the summit, is characterized by massive gas hydrate deposits at the seafloor and methane venting into the water column (e.g., Suess et al., 2001). Site 1250 (792 m water depth) is located 100 m west of the summit and 100 m east of a highly reflective active carbonate chemoherm mound (Suess et al., 2001; Johnson et al., 2003) known as the Pinnacle (Figs. F1, F2B).

The uppermost 59.3 meters below seafloor (mbsf) of Hole 1249C and the uppermost 43.75 mbsf of Hole 1250C, corresponding to lithostratigraphic Units I and II (undifferentiated at Site 1249), are dominated by fine-grained silty clays to clays (mean size = 8.5) (Fig. F3). Relatively high values of calcium carbonate (11 wt% in Sample 204-1249C-4H-2, 6–8 cm [15.4 mbsf] and 9.5 wt% in Sample 204-1250C-4H-5, 50–52 cm [29.61 mbsf]) characterize these units and are probably associated with the presence of authigenic carbonates. Mousselike and soupy textures, related to gas hydrate dissociation, are abundant throughout Units I and II, especially at Site 1249, where the highest concentration of gas hydrates was encountered (Shipboard Scientific Party, 2003a). Large negative values of skewness were found in Samples 204-1249C-8H-2, 50–52 cm and 8H-4, 60–62 cm (46.51 and 49.55 mbsf, respectively), where as much as 2.24% of gravel sized sediments (iron sulfide nodules identified by scanning electron microscope) have been identified in a relatively fine grained matrix (>62% clay) (Fig. F3A).

Lithostratigraphic Unit III is characterized by the presence of gravity-driven deposits (i.e., turbidites and debris flows) interbedded with clays and silty clays (Fig. F3). A 13.5-m-thick clay clast dispersed in a muddy matrix debris flow disrupts toward the middle of this unit from 86.5 to 100 mbsf in Hole 1250C, with a texture of 1.09% sand, 37.68% silt, and 61.22% clay. The mean size of the matrix is 8.59 (Sample 204-1250C-11H-5, 49–51 cm; 88.51 mbsf). Turbidite layers are mainly dominated by the silt fraction (mean size = 7.4), such as Sample 204-1250C-8H5, 50–52 cm (68.01 mbsf) with a grain-size distribution of 8% sand, 53.4% silt, and 38.6% clay. A turbidite base with 15.97% sand, 39.11% silt, and 44.92% clay (mean size = 7.48) was sampled at 204-1250C-17H-2, 50–52 cm (134 mbsf) (Fig. F3B). Turbidite samples are associated with moderate positive peaks (<28 x 10^–8 m³/kg) in the magnetic susceptibility record (Fig. F3A, F3B). The highest value of magnetic susceptibility in Hole 1249C (39 x 10^–8 m³/kg) corresponds to a hemipelagic sample (204-1249C-13H-2, 50–52 cm; 86.01 mbsf) characterized by fine-grained texture (0.5% sand, 36.88% silt, and 62.6% clay) and collected from an interval containing abundant sulfide stains and gas-rich sediments (Fig. F3A). These data support the results presented by Larrasoaña et al. (this volume) on the relationship between rock magnetic properties of sulfides and presence of gas and hydrates. Similar data are obtained from Hole 1250C, where Sample 204-1250C-15H-5, 50–52 cm, located below the BSR (126.57 mbsf), shows the largest value in magnetic susceptibility (52 x 10^–8 m³/kg) with a silty clay texture (3.03% sand, 42.29% silt, and 54.67% clay) (Fig. F3B). No samples of Horizon A were available from Hole 1250C.

Bulk and clay mineral composition in Hole 1250C (Fig. F3C) revealed that the main mineral components at the summit of southern Hydrate Ridge are clays (30%–55%), quartz (25%–35%), and feldspars (15%–20%), with a minor amount of calcite (usually <5%) except for some carbonate-rich intervals at 16–20 mbsf corresponding to the uppermost sampled level and in the sediments above the BSR (~97 mbsf). The most abundant mineral observed in the clay association is detrital mica (35%–50%), whereas smectites, kaolinite, and chlorite are less abundant (Fig. F3C). Smectite content ranges from 10% to 30%, slightly lower in Unit II with respect to Unit III. The smectite content increases in Unit III close to Horizon A (140–145 mbsf) and corresponds with a decrease in detrital mica content.

Two sites were drilled at the east flank of southern Hydrate Ridge. Site 1244 (890 m water depth) is located ~3 km northeast of the summit, and Site 1246 (850 m water depth), near the crest, is located ~3 km north of the summit (Figs. F1, F2A). Only lithostratigraphic Units I and II were recovered from Hole 1244E, cored to 136 mbsf (Fig. F4), from where our samples were obtained.

Lithostratigraphic Unit I (0–77.6 mbsf in Hole 1244E and 0–21.7 mbsf in Hole 1246B) is dominated by dark greenish gray clay with occasional silty clay layers with a mean size of 8.45. The uppermost 20 mbsf of Unit I is characterized by a relatively high content of calcium carbonate showing peaks of 11 wt% in Hole 1244E at 2.57 mbsf (Sample 204-1244E-1H-2, 106–108 cm) and as much as 25 wt% in Hole 1246B at 3.51 mbsf (Sample 204-1246B-1H-3, 50–52 cm) (Fig. F4) that corresponds to near-surface authigenic carbonates. A large positive peak in magnetic susceptibility (up to 105 x 10^–8 m³/kg) is observed in Sample 204-1244E-4H-1, 50–52 cm (20.71 mbsf) and also in samples from Sections 204-1244E-4H-2 and 4H-3. High magnetic susceptibility values within this 3-m-thick interval do not seem to be associated with any evident textural change but are related to a compositional change (Fig. F4A). A fine-grained turbidite layer in Sample 204-1244E-7H-CC, 36–38 cm (50.83 mbsf) (0.54% sand, 49.61% silt, and 49.85% clay; mean size = 8.24) corresponds to a small positive peak of magnetic susceptibility (30 x 10^–8 m³/kg) (Fig. F4A).

Lithostratigraphic Unit II presents a grain-size distribution similar to Unit I except when crossing specific horizons. Hole 1246B Horizon B (from 54.1 to 65.22 mbsf) is characterized by the two largest peaks of the entire magnetic susceptibility data set (Fig. F4B). The younger peak, located at 55.6 mbsf, reaches values of 132 x 10^–8 m³/kg and corresponds to a clayey silt turbidite composed of 0.1% sand, 52% silt, and 47% clay with a mean size of 8 (Sample 204-1246B-7H-3, 51–53 cm). The older peak, located at 65.2 mbsf, has a higher magnetic susceptibility of 172 x 10^–8 m³/kg and corresponds to a silty clay interval composed of 0.47% sand, 42.21% silt, and 57.33% clay (Sample 204-1246B-8H-3, 51–53 cm) (Fig. F4B). This latter event is located just above an infrared thermal anomaly of –4ºC (IR185) where gas hydrates were sampled (Shipboard Scientific Party, 2003a). Horizon B´, between 88 and 95 mbsf, is composed of two volcanic glass-bearing layers (Shipboard Scientific Party, 2003b). The upper layer (Sample 204-1246B-11H-1, 50–52 cm; 90.71 mbsf) is characterized by a textural distribution of 7.98% sand, 41.85% silt, and 50.17% clay with a mean size of 7.95 and standard deviation of 2.4 (Fig. F4B).

XRD data from Hole 1244E (Fig. F4C) show a mineral composition similar to the sediments recovered from the summit sites. The main mineral components are clays, quartz, feldspars, and calcite. The clay content ranges from 35% to 60%, quartz from 20% to 40%, and feldspars from 10% to 25%. Calcite content is below 5% through most of the sampled interval except for the samples located near the surface and above the BSR, where a significant increase likely related to fluid circulation and precipitation of authigenic carbonates is recognized. The clay mineral assemblage is also dominated by detrital mica (usually ranging from 30% to 60%), smectite (5%–30%), kaolinite (15%–30%), and chlorite (10%–30%) (Fig. F4C). Chlorite is relatively more abundant in Unit I than in Unit II, whereas kaolinite presents an opposite trend. Below the BSR and in the upper part of the site, a slight increase in smectite content corresponds to a decrease in the detrital mica values.

Three sites were drilled along the west flank of southern Hydrate Ridge following a north–south transect (Figs. F1, F2C). Site 1245 (870 m water depth) is located ~3 km northwest of the summit. Site 1247 (845 m water depth) is located ~800 m northwest of the summit. Site 1248 (830 m water depth) is located 300 m northwest of the summit (Fig. F1) in the middle of an area characterized by high backscatter and interpreted as related to the presence of authigenic carbonate resulting from fluid seafloor venting (Johnson et al., 2003) (Fig. F2C).

Lithostratigraphic Units I and II are undifferentiated at Site 1248 and are mainly composed of fine-grained silty clays (mean size = 8.28). Soupy and mousselike facies induced by gas hydrate dissociation are common above the BSR at Site 1248, located very close to the summit of Hydrate Ridge (Shipboard Scientific Party, 2003d) (Fig. F5C). At 33.39 mbsf, a large peak in the magnetic susceptibility record (>140 x 10^–8 m³/kg) corresponds to a 10-cm-long sulfide vein oblique to the stratification in a silty clay sediment composed of 2.38% sand, 36.93% silt, and 60.68% clay (Sample 204-1247B-5H-6, 128–130 cm) (Fig. F5B). Thin sand-silt turbidite layers are locally observed. One of these was sampled at 45.67 mbsf (Sample 204-1247B-7H-2, 61–63 cm) and contains 30.37% sand, 32.41% silt, and 37.22% clay with a mean size of 6.67. Seismic Horizon Y marks the boundary between Unit II and Subunit IIIA but does not seem to correspond to any major textural change, although a peak in the magnetic susceptibility data (47 x 10^–8 m³/kg) is measured for Sample 204-1245B-9H-1, 50–52 cm (76.5 mbsf) (Fig. F5A).

Near the top of Subunit IIIA at 69.02 mbsf, the largest peak in magnetic susceptibility (160 x 10^–8 m³/kg) is measured in a clay sample (1.44% sand, 34.73% silt, and 63.83% clay; mean size = 8.7) characterized by abundant sulfide stains (Sample 204-1247B-9H-5, 50–52 cm) (Fig. F5B). The highest value of calcium carbonate (16.7 wt%) is found at 96.67 mbsf in a clay-rich sample (Sample 204-1248C-11H-2, 50–52 cm; mean size = 8.61) (Fig. F5C). Turbidites are present as sandy silt layers with a textural distribution of 26.74% sand, 30% silt, and 43.26% clay with a mean size of 7.3 and standard deviation of 2.7 (e.g., Sample 204-1248C-12H-5, 50–52 cm; 112.12 mbsf) that correlate with a small peak in magnetic susceptibility (16.6 x 10^–8 m³/kg) (Fig. F5C). Only a few centimeters above this sample depth, a temperature anomaly of –4.7°C was recorded by the infrared camera (Anomaly IR162) (Shipboard Scientific Party, 2003d).

Seismic Horizon A was drilled at the three sites of the western flank of southern Hydrate Ridge. In Holes 1245B and 1248C, Horizon A corresponds to a series of volcanic glass-rich and ash sequences (Figs. F5A, F5C), whereas in Hole 1247B it corresponds to a clay clast-rich debris flow characterized by 0.76% sand, 33.35% silt, and 62.89% clay with a mean size of 8.7 (Sample 204-1247B-21X-5, 50–52 cm; 160.41 mbsf), and is correlated with subdued reflectivity in the MCS data (Shipboard Scientific Party, 2003c). In Hole 1245B, Horizon A spans all of Core 204-1275B-21X with a textural distribution of 24.16% sand, 56.54% silt, and 19.3% clay with a mean size of 6.2 and skewness of 0.5 (Sample 204-1245B-21X-4, 50–52 cm; 181 mbsf) (Fig. F5A). In Hole 1248C, Horizon A is characterized by a grain-size distribution of 11.35% sand, 63.67% silt, and 24.98% clay with a mean size of 6.75 and skewness of 0.27 (Sample 204-1248C-14H-6, 47–49 cm; 130.72 mbsf) (Fig. F5C). Both coarse-grained ash-rich layers are correlated with exceptionally low values in the magnetic susceptibility record (3.72 x 10^–8 and 1.8 x 10^–8 m³/kg for Sites 1245B and 1248C, respectively). From 5 to 20 m below Horizon A, a gravel-grained turbidite depicting a small peak in magnetic susceptibility is identified on the three west flank sites. This layer is characterized by 6.69% gravel, 4.69% sand, 34.75% silt, and 53.87% clay with a mean size of 7.24, standard deviation of 4.4, and skewness of –2.15 (Sample 204-1247B-23X-5, 102–104 cm; 180.13 mbsf) (Fig. F5B).

Regarding mineral composition, sediments from the west flank have similar components as those reported for Site 1244 on the east flank (Fig. F5D). Slight fluctuations are observed in bulk mineralogy with the exception of a very modest increase in calcite content with depth. The smectite content, ranging between 10% and 20% in Unit I, increases in Units II and III to 50%. Below the BSR, the smectite content increases downcore, reaching as much as 60% in the middle part of Subunit IIIB (Fig. F5D). Detrital mica content presents an opposite trend.