Site 997 is located on the topographic crest of the Blake Ridge in an area where an extremely well developed and distinct BSR exists (Fig. 5). It was the last of three sites drilled along the Blake Ridge transect.
The three lithostratigraphic units recognized at Site 997 are very similar to those documented at Sites 994 and 995 (Fig. 6). Unit I (0-6.2 mbsf) consists of interbeds of foraminifer-bearing nannofossil-rich clay with color repetition of light gray and greenish gray. Beds with disseminated and concentrated glauconite occur, especially in the lowermost portion of the unit. Total carbonate contents range from 11 to 50 wt% CaCO3. Unit II (6.2-107.3 mbsf) consists of interbeds of greenish gray, intensely burrowed nannofossil-rich clay, and darker-colored bioturbated clay with lower nannofossil abundance. The top of the unit is marked by a sharp decrease in nannofossil abundance to <20% and the appearance of diatoms. Diatoms remain abundant (35%) in the interval from 37.9 to 50.4 mbsf. Total carbonate contents decrease to <30%. Unit III (107.3-750.0 mbsf) consists of homogeneous, diatom-bearing nannofossil-bearing clay and claystone, with two diatom-rich intervals at 146.9-183.2 and 452.6-587.2 mbsf. The top is marked by the last occurrence of a mottled nannofossil-bearing clay bed. Total carbonate contents decrease to <20% with nannofossil abundances <10%. Two intervals of diatom-rich sediments in Unit III from all three Blake Ridge sites (160-232 mbsf and 430-660 mbsf at Site 994, 131.9-252.3 mbsf and 464.2-656.0 mbsf at Site 995, and 146.9-183.2 mbsf and 452.6-587 mbsf at Site 997) occur at times of higher sedimentation rates, as deduced from nannofossil biostratigraphy (approximately 5.8 to 5.0 Ma and 3.0 to 2.6 Ma).
The 750-m sequence recovered at this site is Holocene to late Miocene in age (Fig. 7). Three short hiatuses were detected in the upper lower Pleistocene and upper lower Pliocene, and the fourth short hiatus is likely in the upper Miocene. Sedimentation rates increased with depth throughout the sedimentary sequence. Because of a hiatus and coring gap detected between Cores 164-997A-1H and 2H, the Quaternary sedimentation rate at Site 997 (40 m/m.y.) is only two-thirds of the value observed at Sites 994 and 995. The Pleistocene rates (129-194 m/m.y.), calculated by excluding the hiatuses, are the highest among the three sites. The minimum rate for the upper Miocene is more than 342 m/m.y., very close to the value obtained for the uppermost Miocene sequence at Hole 994C. The oldest age estimated at Hole 997B is approximately 6.4 Ma.
A well-defined magnetostratigraphy was determined for the upper part of Site 997. The lower boundary of the C1r.1n subchron (Jaramillo normal chron) is at 52 mbsf, the upper and lower boundaries of the C2n (Olduvai normal chron) are at 72 and 84 mbsf, respectively, and the C2An/C2r boundary (Gauss/Matuyama boundary) is at 122 mbsf. The C1n/C1r boundary (Brunhes/Matuyama boundary) and the upper boundary of the C1r.1n (top of Jaramillo normal chron) are tentatively defined at 36 and 41 mbsf, respectively.
Sediment physical properties at Site 997 are generally similar to those at Sites 994 and 995. Most notably, all three sites have a well-defined lowermost unit (upper boundary at 610-625 mbsf) in which wet-bulk density increases with depth more rapidly than in the overlying and largely homogeneous unit that makes up most of the sedimentary section. All three sites also are characterized by an interval in which water contents increase with depth in at least one 50-m-thick section between 50 and 200 mbsf. At Site 997, the major interval of increasing water content and porosity with depth (approximately 88-177 mbsf) corresponds to a diatom-rich layer.
Rock magnetism defines a similar downhole sequence of reduction steps to that seen at Sites 994 and 995. Magnetite authigenesis from 0 to 2 mbsf is followed by reduction of magnetite to pyrite via magnetic sulfides. Two reduction styles are apparent. The first is a trend of magnetite-greigite pyrite conversion in which greigite apparently develops within the first 20 mbsf in response to bacterial oxidation of organic material and is progressively reduced to pyrite downhole. A second generation of greigite extends downward to ~260-300 mbsf, as observed at Sites 994 and 995. Rock-magnetic data suggest that the second generation greigite has been largely reduced to pyrite below 300 mbsf.
Seventeen in situ temperature measurements were obtained between 0 and 414.1 mbsf in Hole 997A using the Adara, WSTP, and DVTP (Fig. 9). The data indicate that Site 997 is characterized by a linear geothermal gradient of 36.9 ± 0.4 mK/m, which is within one standard deviation of the estimated gradient at Site 994 (36.4 ± 1.3 mK/m), and 9% higher than the gradient at Site 995 (33.5 ± 0.9 mK/m). The estimated temperatures at the BSR are 18.7°C (440 mbsf) at Site 995 and 20.0°C (450 mbsf) at Site 997. At Site 994, where there is no BSR, the temperature at a comparable depth (440 mbsf) is estimated at 20.1°C. On the Blake Ridge, sediments overlying a BSR (Sites 995 and 997) are not uniformly cooler than those at a comparable depth in a location with no BSR (Site 994).
The concentration of methane in headspace gas samples increases from 0.022 ml/kg of sediment near the sediment-water interface to a maximum of 180 ml/kg at a depth of 50.4 mbsf. The concentration steadily declines to about 6.6 ml/kg near 210 mbsf, and thereafter remains at concentrations ranging from 0.4 to 61 ml/kg. Notable spikes in methane concentrations occurred in the same intervals in which gas hydrate was found or inferred by temperature measurements. Methane-to-ethane ratios increase with depth and reach a minimum of 109 at 725.8 mbsf. Higher molecular-weight hydrocarbons are present throughout the hole in <100 ppm concentrations and increase with depth. Heptane occurs from 153.3 mbsf to 750.0 mbsf. The total organic carbon (TOC) contents of the sediments are near 1% above 200 mbsf and ~1.5% in the interval from 200 to 700 mbsf. The amount and composition of volatile and nonvolatile organic matter is very similar to that found at Sites 994 and 995.
The interstitial-water chemistry at Site 997 is very similar to that documented at Sites 994 and 995 (Fig. 8). Downhole profiles of all dissolved species show the same general trends with the same approximate depths for maxima and minima as those seen at the other Blake Ridge sites. Chloride profiles were used as a proxy indicator of the amount of in situ gas hydrate. The chloride excursion zone occurs at approximately the same depths (~200 to 440 mbsf) over the 10-km transect. Furthermore, the two depth intervals showing the largest chloride excursions, at ~250 and 400 mbsf, are also depth correlative. Thus, the similarity of the Cl- profiles at the ridge sites strongly suggests that gas hydrate is correlative over the expanse of the Blake Ridge.
The PCS was successfully deployed 11 times at Site 997, including one deployment to determine the volume and composition of gas in sediment immediately below the prominent BSR at 462 mbsf. The volume of methane evolved from this particular PCS core suggests that interstitial waters at this depth are 10 times oversaturated with methane. Chemical analyses also show that pore waters at this depth do not have the low chloride concentrations associated with the presence of gas hydrate. Together these observations provide the first direct evidence for significant free gas immediately below a BSR.
Hole 997B was logged from 113.0 to 715.0 mbsf with four different tool strings (Quad combination, geochemical combination, Lamont-Doherty dipole shear tool, and the FMS). Preliminary analysis of the acoustic velocity and resistivity logs shows a pattern similar to that at Sites 994 and 995 (Fig. 10 and 11). Low acoustic velocities (about 1600 m/s) in the top of the hole begin to increase at 186 mbsf to a maximum of 2000 m/s at 450 mbsf before dramatically decreasing again to below 1200 m/s. The interval from 186 to 450 mbsf also exhibits higher resistivities. In addition, the resistivity log reveals three conspicuous high electrical resistivity intervals near 210, 365, and 440 mbsf. The anomalously high resistivities and velocities measured in the zone from 186 to 450 mbsf are likely due to the presence of gas hydrate. Numerous low velocity zones are observed within the depth interval below 440 mbsf in Hole 997B. These apparent low-velocity zones likely contain free gas.
The VSP program at Site 997 consisted of two lowerings of the three-component WHOI borehole seismometer, each of which provided complete coverage of the hole from ~160-700 mbsf. Preliminary velocity inversions from the two lowerings yield the same background velocity structure, thus verifying the repeatability of the experiment. Stations on the second lowering were interleaved with stations from the first lowering to provide 4-m trace spacing throughout most of the hole. Results show that velocities decrease sharply from 1800 m/s to 1400 m/s at the BSR (464 mbsf) and remain low to the base of the hole. Average velocities beneath the BSR at Site 997 are substantially lower than at Sites 994 and 995 (Fig. 12), providing evidence for increased concentrations of gas beneath the crest of the Blake Ridge.