Site 1247 (proposed Site HR4c) is located in ~845 m of water on the western flank of Hydrate Ridge, ~800 m northwest of the southern summit and approximately halfway between Site 1245 and the summit (Fig. F1). The 3-D seismic data indicate that the seismic stratigraphic setting is similar to that of Site 1245 (Fig. F5). The BSR is at a depth of ~121–124 mbsf at this site. Horizon A is brighter and shallower (~160 mbsf) at Site 1247 than at Site 1245; Horizon Y is also shallower (~60 mbsf) at this site.
A faint negative-polarity reflection ~40 m below the BSR is observed in this region. The possibility that this event is a source artifact has not been definitively ruled out. However, the abrupt decrease in the amplitude of Horizon A as it crosses this reflection and approaches the BSR from below suggests that it might be a "second BSR" resulting from the presence of more stable hydrate structures that contain higher-order hydrocarbons, as has been suggested for similar features observed elsewhere.
The primary objective at Site 1247 was to sample sediments and fluids from Horizon A, approximately halfway between Site 1245 and the summit (Site 1249), in order to determine updip variations in the physical and chemical characteristics of this horizon and, thus, understand the role it plays in fluid migration and formation of hydrate on the seafloor at the summit. A second related objective was to investigate the origin of the second BSR. Although they are only 75 m apart, the two holes drilled at this site sample parts of Horizon A with distinctly different seismic characteristics.
Hole 1247A was drilled without coring to a depth of 270 mbsf to obtain LWD data for this site. We returned to core Hole 1247B, which was offset from Hole 1247A ~75 m to the east. Hole 1245B was cored to 220 mbsf using the APC and XCB. The APCT was run six times (including a mudline run to obtain seafloor temperature), the DVTP was run twice, and the PCS was run three times in this hole. The HYACINTH, HRC, and FPC devices were not run at this site. Hole 1247B was then logged using the triple combination (triple combo) and Formation MicroScanner (FMS)-sonic tools. After wireline logging, a vertical and an offset VSP covering the interval of 104–214 mbsf was acquired by alternately shooting from the JOIDES Resolution and the Ewing, which held station ~700 m away. Plans to conduct walk-away VSPs were abandoned when the Schlumberger VSI tool would no longer clamp in the hole.
Samples suspected of containing hydrate based on IR temperature anomalies were recovered from 93 and 113 mbsf and were stored temporarily in liquid nitrogen. However, on later inspection, it was discovered that although the samples showed textures indicative of hydrate dissociation, no hydrate was actually preserved.
Biostratigraphic observations from the 220 m of core recovered in Hole 1247B indicate that the entire sequence is younger than 1.65 Ma (Fig. F11). Sediments deeper than ~165 mbsf were deposited at a rapid but poorly constrained rate. These strata can be correlated with strata yielding a linear sedimentation rate of 62 cm/k.y. at Site 1245. The interval from ~150 to 165 mbsf, which contains Horizon A, yields a relatively slow sedimentation rate of 4 cm/k.y. based on nannofossils. The overlying sediments were deposited at a rate of 9–22 cm/k.y., similar to what was observed at Site 1245.
Lithostratigraphic analysis indicates that the dominant lithologies are clay with authigenic carbonates and foraminifer-rich interlayers in the upper 0–27 mbsf (lithostratigraphic Unit I). This unit is underlain by diatom-bearing clay and silty clay with frequent sand-rich turbidites containing a few glass-rich layers from 27 to 212.7 mbsf (Unit II). Lithostratigraphic Unit III (60–220 mbsf) is distinguished from Unit II by an increase in turbidites and biogenic components. Included in lithostratigraphic Unit II is seismic Horizon A. Unlike at the other sites where Horizon A was sampled and found to correspond with volcanic ash layers, at Site 1247B a soft-sediment debris flow bounded by turbidites was found. Because the signature of Horizon A in the LWD data from Hole 1247A (located only 75 m from Hole 1247B) is very similar to that observed at Sites 1245, 1248, and 1250, we conclude that the change in amplitude of Horizon A between the two holes at Site 1247 results from a dramatic local change in lithology rather than from processes related to gas hydrates.
As at other sites, the apparent top of the zone where hydrate is present, as indicated by a variety of different proxies, is generally consistent. The onset of high and variable electrical resistivity and of thermal anomalies observed with the IR camera on the catwalk are both at ~45 mbsf (Fig. F13). High-resistivity layers are subhorizontal, indicating accumulation of gas hydrate parallel to bedding, and steeply dipping, indicating that hydrate fills fractures. The onset of low chloride concentration anomalies is at ~55 mbsf (Fig. F14). The onset of in situ methane oversaturation as projected from headspace and PCS measurements is at ~38 mbsf (Fig. F16).
The above proxies are also consistent with similar depths for the base of the GHSZ. The deepest IR thermal anomaly is at 118 mbsf (Fig. F13). The deepest chloride concentration anomaly is at 114 mbsf (Fig. F14). A PCS core, indicating a volume of methane greater than in situ concentration, was taken at 123 mbsf. Seismic velocities from the sonic log and the VSP complement these interpretations by clearly resolving a velocity decrease indicative of the presence of free gas beneath 129–134 mbsf.
The significant concentrations of higher-order hydrocarbons found beneath the BSR at Site 1245 were also observed at Site 1247 (Fig. F15). Here, low values of the methane/ethane ratio (C1/C2 < 100) persist to a slightly deeper depth than at Site 1245 (~220 mbsf compared with ~180 mbsf at Site 1245). Hole 1247B did not extend deep enough to resolve whether the C1/C2 ratio returns to normal at greater depth, as it did at Site 1245.
One of the notable results from this site was identification of a new hydrate proxy that has the potential to provide valuable constraints on the dynamics of hydrate formation. It was found that many samples collected from the depth range in which other proxies indicate the presence of hydrate showed ethane enrichment and propane depletion. This was attributed to fractionation of ethane and exclusion of propane (C3) during the formation of Structure I hydrate. The observed pattern of anomalies, relative to baseline C1/C2 and C1/C3 concentrations defined by the majority of samples, can be explained by invoking the presence of gas hydrates within those samples that dissociated on recovery.
Another interesting result was that Horizon A shows a low-amplitude chloride concentration low and methane high. Given the expectation that Horizon A should be more permeable where it is characterized by coarse-grained ash-rich layers than where it consists of a clay-rich debris flow, these observations are surprising and have not yet been explained. At this horizon, there is also Li enrichment similar in magnitude to that observed at other sites, supporting the interpretation of a stratigraphic horizon that transports fluids from greater depth.
The depth of the SMI at this site is well constrained by high-resolution samples and is determined to be at 11 mbsf. Assuming that this depth is entirely controlled by AMO, a methane flux of 2.5 x 10–3 mM/cm2/yr is inferred, which is ~1.4 times greater than at the Blake Ridge and ~30% less than at Site 1251. However, the assumptions on which this estimate is based may not be valid for the entire interval above the SMI, leading to considerable uncertainty in this estimate.
The in situ temperature measurements in Hole 1247B yielded a very precisely defined slope of 0.0524°C/m (correlation coefficient = 0.999) and did not reveal any sign of a positive temperature anomaly at Horizon A. This observation will be used to place an upper bound on the rate of fluid transport from depth along this horizon that can be compared to rates obtained from the chemical anomalies.
Site 1245 provided further confirmation that multiple proxies for the presence of in situ hydrate are consistent with direct measurements of gas concentration in predicting the distribution and concentration of gas hydrates in the subsurface. These include electrical resistivity and porosity measured downhole via LWD, core temperatures measured on the catwalk, and chloride anomalies measured in interstitial waters extracted from sediment samples. A new proxy, ethane enrichment and propane depletion, was discovered and holds promise for constraining hydrate dynamics. Results from this site also indicate that lateral changes in the amplitude of Horizon A in the interval between the BSR and a faint second BSR probably result from lithologic changes rather than from the presence of more stable hydrates of higher-order hydrocarbons, as had been speculated, which leaves open the question of the origin of this second BSR. Finally, the small chloride concentration depletion and methane enhancement associated with Horizon A, which is not observed at Sites 1245, 1248, and 1250, leads to the apparently contradictory conclusion that Horizon A is a more active conduit for deeper fluids where it is composed of a clay-rich debris flow than where it is composed of coarse-grained ash-rich sandy silt.