Site 1249 (proposed Site HR4b) was drilled in 778 m of water on the summit of southern Hydrate Ridge (Fig. F1). This area is characterized by massive gas hydrate deposits at the seafloor (Suess et al., 2002). Vigorous streams of bubbles are known to emanate from the seafloor, as documented by submersible observations and high-frequency echo-sounding surveys, which have repeatedly imaged bubble plumes in the water column (Trehu and Bangs, 2000; Heeschen et al., pers. comm., 2002). These observations are interpreted to indicate that some of the methane rising through the sediment column is trapped as hydrate near the seafloor and that some escapes into the water column (Suess et al., 2001).
The seafloor in this area is anomalously reflective (Johnson and Goldfinger, pers. comm., 2002), and the seafloor reflectivity is spatially correlated with subsurface seismic reflectivity that extends to ~30 mbsf (Fig. F7). These geophysical observations have been interpreted to indicate the spatial extent of lenses of massive hydrate intercalated with sediment (Trehu et al., 2002). Seismic data also indicate that the BSR at this site is at ~115 mbsf. However, coring was only permitted to 90 mbsf because of the possibility of trapped gas beneath the BSR at this structural high.
The objectives at Site 1249 were to (1) determine the distribution and concentration of gas hydrate with depth at the southern summit of Hydrate Ridge and to investigate processes that allow methane gas bubbles to coexist with gas hydrate and pore water within the hydrate stability field and (2) test whether the pattern of chaotic reflectivity accurately predicts the spatial extent of massive hydrate lenses.
Twelve holes were drilled at Site 1249 (Table T1). LWD measurements were made during drilling in Hole 1249A. Hole 1249B was drilled using the new RAB-8 LWD and coring system, which permits simultaneous acquisition of core and logging data. For this test, we washed down to 30 mbsf before beginning RAB coring operations; eight cores (4.5 m long) were taken with liners followed by 9-m-long cores without liners until we reached the permitted penetration depth of 90 mbsf. Following this test, the 90-m sediment sequence was APC sampled in Holes 1249C through 1249F, with core recoveries of <30% in the uppermost 20 mbsf and increased core recovery (up to 70%) deeper in the holes. Six holes (Holes 1249G–1249L) were APC/XCB cored for a special shore-based "geriatrics" study, in which several means of preserving gas hydrates for future study will be compared. During this effort, 244 m of gas hydrate–bearing sediments were cored with 35% core recovery. The samples were either stored in liquid nitrogen or steel pressure vessels, which were repressurized using methane gas and water.
All pressure coring systems available (PCS, HRC, and FPC) were used at Site 1249 (Tables T2, T3). The ODP PCS was deployed seven times in Hole 1249C (33.5, 63.5, and 88.5 mbsf), in Hole 1249E (9.08 mbsf), and in Hole 1249F (13.5, 58.9, and 71.4 mbsf). The FPC was used in Hole 1249D (8 mbsf), in Hole 1249H (70.4 mbsf), and in Hole 1249G (13.5 mbsf). The HRC was deployed in Holes 1249F and 1249G (8 and 13.5 mbsf, respectively). Temperature measurements were made using the DVTP (30.4 and 90 mbsf) and the APCT (16.5, 24.0, 32.5, 37.5, 39.9, 58.9, 70.4, and 90.0 mbsf).
Based on visual observations, smear slide analyses, physical property measurements, seismic stratigraphy, and logging data, the sediments at Site 1249 were divided into three lithostratigraphic units. Each of the three units correlates well with lithostratigraphy of other sites on the western flank of Hydrate Ridge (Fig. F10). Because of poor core recovery resulting at least, in part, from the presence of massive hydrate, lithostratigraphic Units I and II at Site 1249 were combined into a single unit referred to as Unit I-II. This unit, of Holocene–early Pleistocene age, is composed of clay and silty clay; the biogenic component changes from nannofossil bearing to diatom bearing and diatom rich. Lithostratographic Unit III, of early Pleistocene age, has similar lithologies to Unit I-II. The boundary between Unit III and Unit I-II is defined by the presence of visible turbidites in the cores, an increase in grain size, a slight increase in calcareous components, and a slight decrease in biogenic opal. This boundary varies in depth among the holes (from 51 to 59 mbsf) and is coincident with seismic Horizon Y (Figs. F5, F7), which is interpreted to be a regional angular unconformity.
During Leg 204, the highest concentration of gas hydrates was encountered at Site 1249, leading to considerable whole-round sampling. Massive gas hydrate pieces were recovered in the uppermost two cores. Layers of apparently pure gas hydrate up to several centimeters thick were interbedded with soft sediment. Temperature anomaly profiles from the catwalk-track IR camera support this generalized model for gas hydrate distribution. Downhole gas hydrate presence can also be inferred from LWD resistivity data, and the Archie's Law relationship between resistivity and porosity implies gas hydrate saturations in the pore space at Site 1249 that range from 10% to 92% of the pore space.
As a result of dissociation of gas hydrate during core recovery, cores were highly disturbed and most of the original gas hydrate fabric was probably not preserved. Soupy and mousselike textures, probably related to gas hydrate presence, were commonly observed. Soupy textures are thought to result from the dissociation of massive gas hydrate, a process that releases a considerable amount of water. Mousselike textures result from the dissociation of disseminated gas hydrates in fine-grained sediments.
Pore fluids recovered from the upper 20 mbsf show pronounced enrichment in dissolved chloride concentration. The highest chloride concentration measured is 1368 mM in a sample collected from the working half of the core. This sample was selected because it showed a dry-looking coherent fabric and is, therefore, thought to represent the in situ pore fluid with minimal overprint from addition of water from hydrate dissociation during recovery. The observed enrichment in dissolved chloride is only possible in a system in which the rate of gas hydrate formation exceeds the rate at which excess salts can be removed by diffusion and/or advection. The presence of brines in the upper 20 mbsf is also reflected by the concentration of other dissolved species such as Na+, K+, Ba2+, Sr2+, and Mg2+ because these ions are excluded from the hydrate structure and enriched in the residual pore water. Superimposed on the dissolved ion enrichment resulting from brine formation, the interstitial chemistry water also reflects the effect of rapid advection of deeply sourced fluids and diagenetic processes occurring in near-surface sediments. Below 20 mbsf, small negative chloride anomalies have been attributed to hydrate dissociation during core recovery.
In comparison to other sites, headspace samples at Site 1249 showed extremely high methane contents consistent with the presence of gas hydrate in the headspace samples. Ethane and propane are also present in high concentrations, indicating migration of thermogenic hydrocarbons. Gases from decomposed hydrate samples show that some of the gas hydrates contain propane and butane, suggesting the presence of Structure II hydrate.
In order to determine in situ methane concentrations, PCS cores were successfully obtained at 14, 34, and 72 mbsf (Fig. F16). The degassing experiments document methane concentrations that range from 200 to 6000 mM, methane concentrations above saturation at in situ temperature and pressure conditions.
One of the highlights of Site 1249 was the successful recovery of gas hydrate at in situ pressure using the new HYACINTH pressure sampling tools. HRC and FPC cores from 14 mbsf both contained high concentrations of gas hydrate. Gamma density logs show a spectacular interlayering of sediments, with some layers having density slightly lower than 1 g/cm3. Since pure methane hydrate has a density of ~0.92 g/cm3, we interpret these low-density layers to be relatively pure hydrate layers. In addition, a low-density spike (0.75 g/cm3) in a 8-cm-thick gas hydrate layer reveals the presence of free gas within a massive gas hydrate layer. A second indicator of free gas came from the HRC core. A small explosion pushed gas hydrate and sediment interlayers out of the liner from two intervals while the core was being transferred from the pressure vessel to liquid nitrogen. This sudden gas release can only be explained by the expansion of small volumes of free gas that existed in situ within the gas hydrate layer.
Downhole temperatures derived from the APCT define a temperature gradient of 0.047°–0.051°C/m and predict a depth to the base of the GHSZ that is 20 m deeper than the BSR. The data at this site show more scatter than is observed at other sites, where a comparable number of measurements were made. Some possible explanations for this scatter are that thermal conductivities measured on core samples are not representative of in situ thermal conductivity when large concentrations of hydrate are present or that dissociation of hydrate resulting from frictional heating when the temperature probe is inserted affects the measurements. This will be investigated further as part of shore-based recalibration and reanalysis of the downhole temperature data.
Holocene–early Pleistocene sediments of lithostratigraphic Units I-II and II at Site 1249 are well correlated with other sites along the western flank of southern Hydrate Ridge. Site 1249 was cored to a depth of 90 mbsf; thus, this entire sequence lies within the GHSZ, and large quantities of gas hydrates were sampled. Core recovery at this site was limited because of the presence of massive gas hydrate close to the seafloor. Rapid formation of massive hydrates in the uppermost 20 mbsf at this site induces a brine formation, with chloride values in the interstitial water of up to 1368 mM. This is the greatest chloride enrichment as a result of gas hydrate reported to date. Degassing of the PCS from Core 204-1249F-4P revealed 95 L of gas, which is the largest volume of gas ever measured with the PCS. At this site, we obtained the first density measurements from gas hydrates under in situ conditions using the HYACINTH pressure coring and laboratory transfer systems. One HYACINTH core showed direct evidence for free gas within gas hydrate layers at 13 mbsf.