Samples collected for our noble gas investigation were gas aliquots from Leg 164 shipboard gas hydrate dissociation experiments at Sites 994, 996, and 997. These experiments are detailed by Paull, Matsumoto, Wallace, et al. (1996, p.18, 19, 112, 113, 249-255, 257, and 286-288).
"Massive" specimens of gas hydrate were recovered in sediment cores by the advanced hydraulic piston core (APC) or extended core barrel (XCB) at Sites 994, 996 and 997 (Paull, Matsumoto, Wallace, et al., 1996). Several of these specimens were hand picked from cores and immediately placed in a storage container with liquid nitrogen for preservation. Stored samples then were transferred into a gas hydrate dissociation chamber. This chamber (Fig. 2) consists of a small (23 cm3) pressure vessel connected to a gauge block assembly and a gas sampling port. The chamber (with the hydrate specimen inside) was evacuated for 1 s at room temperature. Gas hydrate specimens were then allowed to dissociate at room temperature. After thermal equilibration and complete dissociation, pressure inside of the chamber was recorded and a gas split was collected for shipboard gas chromatography (GC). Remaining gas was released into an evacuated stainless steel gas cylinder for shore-based analyses, including our investigation. The volume of water released during hydrate dissociation and the Cl- content of this water was determined after opening the chamber. The volume of gas released during hydrate dissociation was calculated from recorded pressures, temperatures, and the ideal gas law.
Gas released from seven gas hydrate specimens were examined in our study (Table 2). For six hydrate specimens, nearly all gas remaining after removal of the split for shipboard GC analyses could be placed inside of a stainless steel container. However, the hydrate specimen recovered from Core 164-997A-42X released a sufficient quantity of gas (1250 cm3) to transfer significant amounts of gas into two stainless steel containers. Gas in these two containers, labeled A and B, are successive splits of an original gas quantity that were taken at high and low pressure, respectively. All eight gas samples in stainless steel containers were analyzed for noble gases (Table 2).
Original specimens of gas hydrate are from different depths in the sediment column and from distinct fluid flow regimes (Table 2). In particular, specimens from Sites 994 and 997 were collected at deep depth (>220 mbsf) in an area with relatively low fluid flow whereas specimens from Site 996 were collected at shallow depth (<50 mbsf) in a region of active fluid venting on the seafloor (Paull, Matsumoto, Wallace, et al., 1996).
There is no precedent for naming gas samples collected from dissociated gas hydrate specimens recovered from ODP holes. For the remainder of this paper, we give our samples the standard ODP nomenclature for the hole, core, and section from which the hydrate was recovered, followed by a designator for the sample number (i.e., the first two columns in Table 2). Thus, gas released from the second hydrate specimen recovered from Hole 996C, Core 1H, and Section 1 is simply called "Sample 996C-1H-1(2)" (Table 2, Table 3), even though it is not a sample of sediment or gas that would commonly receive this ODP designation.