The Blake Ridge is a large sediment drift deposit in the Atlantic Ocean off the east coast of the United States (Paull, Matsumoto, Wallace, et al., 1996, p. 5). The primary objective of Ocean Drilling Program (ODP) Leg 164 was to understand the amount and distribution of natural gas hydrate in sediment pore space at three sites (Sites 994, 995, and 997) on the crest of this ridge ~200 km from the coast (Paull, Matsumoto, Wallace, et al., 1996). Critical to this endeavor was the testing of various techniques to detect the presence (or absence) of gas hydrate.
Hunt (1979, p. 160-161) presented an intriguing theoretical approach for identifying gas hydrate in sediment cores under pressure. The technique involves measurement of incremental gas volumes slowly released from a pressurized core, and subsequent comparison of observed volume and pressure changes to expected curves (see below). Kvenvolden et al. (1983) attempted this experiment at Deep Sea Drilling Project (DSDP) Site 533 but could not recover incremental gas volumes for a rigorous evaluation of the technique. Instead, they constructed time-pressure plots and made tentative interpretations concerning the presence or absence of gas hydrate on the basis of these alternative plots.
Forty-two cores were successfully recovered at high pressure with the pressure core sampler (PCS) during Leg 164 (Dickens et al., Chap. 43, this volume). Twenty-nine of these PCS cores were connected to a gas manifold system especially designed to collect incremental gas volumes over time (Paull, Matsumoto, Wallace, et al., 1996, pp. 24-26). Here we present volume-pressure plots for 20 of these cores. Our interpretation of these plots is more complex than those discussed previously (Hunt, 1979; Kvenvolden et al., 1983). In particular, we demonstrate that effects related to the coring process and differences between in situ and experimental conditions make it difficult to unambiguously detect the presence of CH4 hydrate in pressurized sediment cores.