ODP Leg 164 drilled holes at seven sites on the eastern margin of the United States and recovered a number of massive gas hydrate samples from Sites 994 and 997 on the Blake Ridge and from Site 996 on an active vent on the Blake Ridge Diapir (Fig. 1). Blake Ridge hydrates were white, lucent, solid materials of a few centimeters to more than 35 cm in length, whereas those from the diapir site occurred as fizzing and friable veins, a few millimeters thick and >10 cm long, filling nearly vertical to inclined fractures (Paull, Matsumoto, Wallace, et al., 1996). After a brief inspection (<10 min.) on the catwalk, solid gas hydrate were removed or cut from the sediment cores, transferred to Parr Pressure vessels, and stored in a freezer of -20°C. The pressure vessels were shipped to Tokyo by air cargo from Miami within 36 hr after the end of the cruise.
In this report, two samples from Site 994 and three samples from Site 997 were subjected to analysis and measurements. These samples were selected as the best preserved after more than two months storage in pressure vessels at <-20ºC.
Gas hydrate samples provided for the present study were retrieved from nannofossil-rich clay at a sub-bottom depth of 260-330 m, about 200-120 m above the BSR.
Five large pieces of white, irregularly shaped nodules with soapy luster were observed to be embedded in nannofossil-rich clay in interval 164-994C-31X-7, 10-50 cm (259.90-260.30 mbsf). Samples were stored in two pressure vessels--1A and 2A. A broken appearance of hydrate samples and disturbance of the host sediments suggest that the recovered samples were part of the original subsurface gas hydrate layer that was broken into pieces during coring and recovery. The sediment cores in this section were slightly soupy, suggesting dissociation of gas hydrate during coring/recovery. Occurrence of massive gas hydrate from this horizon seems to correspond to the zone of anomalously high electric resistivity in interval 220-260 mbsf (Paull, Matsumoto, Wallace, et al., 1996).
A number of large pieces of massive gas hydrate were recovered from Section 164-997A-42X-3 at 330.03-331.17 mbsf. This section was anomalously cold, white-colored, and actively bubbling. After splitting the plastic core liner, an approximately 50-cm-long hydrate-bearing piece of sediment was revealed (Fig. 2). The top 30 cm of this section contained 10-15 angular fragments of white, fizzing hydrates, within 3-10 cm across, in drilling disturbed nannofossil-rich clay. The next interval appeared to consist of solid gas hydrate, ~6 cm in diameter and 30 cm in total length, which was broken into three pieces, 5, 7, and 15 cm in length (Fig. 3). Collected samples were stored in four pressure vessels--14, 16A, 16B, and 16C. Sample 16A was taken from the first 5-cm-long piece and 16B and 16C from the second 7-cm-long piece. The surfaces of these solid gas hydrate pieces was coated by a thin (<1 mm), transparent, shiny layer of freshwater ice. The interval contained rectangular fragments of gas hydrate in slightly soupy, disturbed clay. Well-logging conducted in Hole 997B, about 25 m northeast of Hole 997A, showed a resistivity anomaly at 360-365 mbsf, 30-35 m deeper than the thick, massive gas hydrate horizon (Paull, Matsumoto, Wallace, et al., 1996). This data suggests that the gas hydrates are not horizontal but slightly inclined with respect to the bedding plane.
Analyses of methane hydrate by the X-ray CT scanner is useful for recognizing occurrences of gas hydrate in sediments and its relationship to the surrounding host sediments without any disturbance to the gas hydrate samples.
A core plug sample, 1-in diameter, was cored from the large gas hydrate sample (Sample 164-997A-42X-3, 25-35 cm) by means of a desktop core driller and was subjected to X-ray CT imagery. The X-ray CT images were measured by the Toshiba X-ray CT scanner "X-force" of Technology Research Center of JNOC. Because the analyzing system was originally designed for medical use for human subjects, the system was slightly modified for observation of sediment/rock samples. Whereas the resolution of images is 350 µm2/pixel and 1 mm in thickness due to the low output voltage of 130 kV, 50 profiles were obtained within 3 min., which benefits the observation of gas hydrate.
The hydrate samples were first placed in an aluminum container, pressurized by helium and cooled down by dry ice, then placed in the X-ray CT equipment. Although we can observe hydrate in an aluminum vessel, CT images of gas hydrate samples were directly measured without an aluminum container to obtain stronger images. For comparison, freshwater ice and dry ice were also measured simultaneously.
Figure 4 shows the X-ray CT imageries of methane hydrate-bearing sediments. Significant difference in CT values among methane hydrate, sediment, water ice, dry ice, and an acrylic tube are represented by color changes. Methane hydrates appear as blue and yellowish blue to light blue, although the CT values are not highly homogeneous and range from -100 to -250, whereas water ice is shown as yellow (CT value = -65). Although the boundaries between sediments (red) and methane hydrates (light-blue) show yellow, they are not considered to designate water ice but the boundary effects. The histogram in Figure 5 depicts the distribution of CT values for methane hydrate, dry ice, and freshwater ice. Sediments are represented by red and yellow, whose CT values are around 1000. Dry ice is shown as pink (CT value = 400), and an acrylic tube is as orange (CT value = 140). Figure 4B is an enlarged picture of a portion of Figure 4A showing a nodular methane hydrate in sediments.
Shape, occurrence, and textural relation to surrounding host sediments of natural gas hydrate are easily recognized by the X-ray CT scanner. Figure 4B and Figure 4C appear to indicate that hydrate forms nodules and veins in the host sediments. Measured CT values don't have units, and chiefly depend on the X-ray absorption coefficient of a substance. Because X-ray CT values eventually depend on the density of a substance, and the densities of liquid water (d = 1.000), freshwater ice (d = 0.917), and methane hydrate (d = 0.91) are nearly the same, it has been believed that methane hydrate would not be distinguished from freshwater and freshwater ice by X-ray CT imagery. However, the CT values of these substances are observed to be 0 (zero), -65, -100 to -250, respectively. As a result, natural methane hydrate can be clearly distinguished from freshwater ice and freshwater on CT images.