METHODS AND PROCEDURES

The western Woodlark Basin is an active extension spreading center to the east of the Papuan Peninsula of Papua New Guinea (Fig. F1). Sites 1109 and 1115 are located just ahead of the spreading tip north of Moresby Seamount. Site 1109 is located on the Woodlark Rise and was cored to a depth of 802.5 meters below seafloor (mbsf). The sedimentary succession at Site 1109 shows a progressive subsidence (from subaerial to lagoonal followed by shallow marine and deep water). At the base of the section, a massive dolerite is present (~773-802 mbsf). Site 1115 is located ~35 km north of Site 1109 and was cored to 802 mbsf. The thick dolerite layer is not present at the base of this site, and the sediments are far older (~15 Ma at base). Organic carbon contents were low throughout both sites, averaging ~0.42 and 0.34 wt% at Sites 1109 and 1115, respectively. The thermal gradients were also similar at both sites, 31°C/km at Site 1109 and 28°C/km at Site 1115 (Taylor, Huchon, Klaus, et al., 1999).

Samples for sediment gas analysis were taken onboard ship as 5-cm whole-round cores (WRC) sampled using a specially designed core cutting rig (Cragg et al., 1992b). The cut ends were capped under a flow of oxygen-free nitrogen (OFN), sealed in OFN-flushed, gas-tight "wine" bags (Cragg et al., 1992a), and immediately frozen. The samples were transported back to the laboratory by air in insulated trunks containing dry ice. The samples remained frozen throughout transportation.

Sedimentary Gas Desorption

Sediment gases were analyzed using a specially designed purge-trap gas apparatus (Fig. F2). A frozen WRC sediment sample was subsampled using a metal corer (~2.5 cm diameter × 5 cm). This subcore was then extruded into a glass vessel that had been preflushed with helium. The vessel was attached to the apparatus, sealed, and the sample allowed to defrost for 1 hr at room temperature. After this time, the glass vessel headspace was flushed with helium for 15 min (flow rate = 60 mL/min). The eluent was passed through a mixed bed adsorbent trap containing, sequentially, 75 mg Porapak Q, 75 mg Carboxen 1001, and 150 mg molecular sieve 5 Å, externally cooled with liquid nitrogen. The adsorbent trap was then removed and immediately analyzed (see "GC Analysis" ). The apparatus was then sealed and the sample vessel heated (60°C) for 1 hr. After this time, a new adsorbent trap was installed in the apparatus and the sample vessel flushed with helium as before. The results from both these procedures were then summed. In general, hydrocarbon distributions were similar between the two analyses, although increased concentrations of the higher molecular weight components were usually found at 60°C (typically between 50% and 100% more). Methane could not be quantified reliably using this technique because it was not quantitatively adsorbed onto the adsorbent trap.