METHODS

Sediments collected from ODP cores were sampled and processed using standard ODP methods, except that pore waters were sampled more frequently (approximately every 1.5 m) in the upper 50 m of the sedimentary section. Pore waters were extracted from sediments using a hydraulic sediment press (Manheim, 1966) and collected in air-tight syringes (Manheim and Sayles, 1974). At Site 1059, 60-cm³ syringes attached to Reeburgh-style squeezers (Reeburgh, 1967) were used to collect pore waters and interstitial gases from selected samples. This method has the advantage of collecting interstitial gases for isotopic measurements. Sulfate concentration was determined by ion chromatography, and alkalinity was measured by titration (Gieskes et al., 1991). Methane concentrations were measured on board ship using headspace methods and employing a gas chromatograph fitted with a flame ionization detector (FID) (Kvenvolden and McDonald, 1985). Methane concentration measurements gathered using headspace methods were normalized to pore-space volume (e.g., Paull, Matsumoto, Wallace, et al., 1996). Pore-water aliquots for onshore CO₂ measurements were stored immediately after squeezing in flame-sealed, air-tight ampoules.

Onshore, CO₂ was separated and purified using cryogenic techniques (e.g., Craig, 1953). Carbon-isotopic-composition measurements of CO₂ are reported relative to the Peedee belemnite (PDB) standard, and were made using a Delta E mass spectrometer at North Carolina State University. The cumulative (vacuum line and mass spectrometer) accuracy and precision of isotopic measurements are ±0.2 and ±0.06 (N. Blair, pers. comm., 1998).

Methane and CO₂ gases were typically collected directly from core liners using a piercing tool that directed pressured gas into a 60-cm³ syringe (e.g., Paull, Matsumoto, Wallace, et al., 1996). Gas subsamples were taken from syringes, and gas composition and concentration was measured using a gas chromatograph fitted with an FID. Any remaining sample was transferred under water to 60-cm³ serum bottles. In cases where there was insufficient sample to completely fill serum bottles, nitrogen gas was used to fill the bottles leaving only small amounts of water. Thus, contamination with ambient air was avoided. The bottles were sealed with rubber stoppers and metal crimp caps, inverted, and frozen for later measurement in an onshore laboratory.

Gas samples were stripped of their water onshore using cryogenic techniques (e.g., Craig, 1953). CO₂ and methane gas fractions were then separated and ¹³C of CO₂ gas and methane was measured using a Finnigan MAT 252 gas chromatograph-isotope ratio-mass spectrometer at the University of North Carolina at Chapel Hill following the methods of Merritt and Hayes (1995) and Popp et al. (1995).

Sediment coring during ODP advanced piston coring often results in repeated coring of the same portion of the sedimentary section or lack of sediment recovery over a portion of the sedimentary section within the same hole. Coring of multiple holes at each site allowed a composite stratigraphic section to be constructed that is reported as meters corrected depth (mcd) below the sediment-water interface (see Keigwin, Rio, Acton, et al., 1998).