During Leg 204, numerous gas hydrate samples, as detected by IR imaging on the catwalk, were recovered as whole rounds and preserved in liquid nitrogen for detailed shore-based studies. Unfortunately, the preservation status of such hydrates was compromised somewhat because of gas hydrate decomposition during recovery, which apparently continued while the samples were still cooling after their submergence in liquid nitrogen.
X-ray diffraction (XRD) analyses on gas hydrate samples preserved in liquid nitrogen document that only Structure I hydrate is present (Kim et al., 2005; Bohrmann et al., in press), confirming conclusions from gas analyses of distinct hydrate samples (Milkov et al., 2005). XRD measurements using synchroton radiation and Rietveld analyses allowed a quantitative estimate of the preservation status of these hydrate samples (Bohrmann et al., in press). Out of 13 distinct samples from various depths at Sites 1244, 1245, and 1247–1250, 8 samples contained 1%–7% hydrate and >90% ice. The ice probably represents water from dissociated hydrate (Bohrmann et al., in press). Five samples showed higher gas hydrate amounts of 20%–70% and lower concentrations of ice (30%–75%), with other mineral phases (e.g., quartz) comprising up to 8% of the sample. Bohrmann et al. (in press) have recognized a correlation between the volume of gas hydrate originally present in the host sediment and the degree of preservation of the gas hydrate samples.
Abegg et al. (this volume, submitted [N1]) investigated a total of 65 frozen whole-round samples from six sites using X-ray computerized tomographic (CT) imaging. Although most of the whole-round samples stored in liquid nitrogen showed widespread decomposition of gas hydrate, the original presence of gas hydrate in the sediment cores was indicated by low-density anomalies. In many cases the gas hydrate itself was no longer present and had been replaced by a distinctive fabric that reflects the frozen state of a soupy sediment full of bubbles. Detailed fabric analyses of the samples showed that the hydrates had been present in layers with a variety of dips. Shallow hydrate layers parallel or subparallel to bedding are interpreted to result from gas bubble injections parallel to the layering of sedimentary deposits. Deeper than 40 mbsf, hydrate layers are characterized by steeper dip angles of 30°–90° and are interpreted as precipitates in open fractures or joints, consistent with a model in which free gas migrates through the sediments by opening cracks, which close as gas hydrate is precipitated, leading to formation of new cracks (Weinberger and Brown, 2006).
Isotopic data also suggest structural differences between gas hydrate formed in a transport-dominated regime and gas hydrate formed in the reaction-dominated regime. Tomaru et al. (in press) show that the isotopic fractionation of the water in the lattice of the massive gas hydrate (
O = 1.0010 and H = 1.008) is significantly lower than that observed in the disseminated, nodular, or vein-filling deposits (O = 1.0025 and H = 1.022). The authors suggest that the anomalous fractionation may be caused by lower gas occupancy in the massive deposits.
