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Pore water geochemistry

Shipboard interstitial water samples were obtained from 10–40 cm long whole-round samples cut on the catwalk. The samples were then capped and taken to the laboratory for immediate processing. Sampling resolution was 1–2 per core in the middle of the sediment column and ~1 per section (1.5 m) in the bottom 5 m of the sediment column. When there were too many interstitial water samples to process immediately, the capped whole-round samples were stored temporarily in the refrigerator at 4°C. Sediment processing for interstitial water sampling in the laboratory was carried out in a nitrogen-flushed and pressurized glove bag. After extrusion from the core liner, the outer layer of each whole-round sample was carefully scraped with a spatula to remove potential contamination from drill water (surface seawater). The remaining sediment was placed in a titanium squeezer modified after the standard stainless steel squeezer of Manheim and Sayles (1974). The piston was positioned on top of the squeezer, and the entire unit was removed from the glove bag and placed on the hydraulic press. Pressures as high as 76 MPa were applied in the squeezer, calculated from the measured hydraulic press pressure and the ratio of the piston areas of the hydraulic press and the squeezer.

Interstitial water was passed through a prewashed Whatman Number 1 filter above a titanium screen, filtered through a 0.45 µm Gelman polysulfone disposable filter (except for microbiology aliquots, which were drawn prior to Gelman filtration), and subsequently extruded into a 50 mL gas-tight glass syringe, a new sterile 50 mL plastic syringe, or an acid-cleaned and combusted glass vial. Both types of syringes were attached to the bottom of the squeezer assembly via a three-way plastic valve.

Distinct procedures were used to collect fluids for 14C analysis and organic/microbial analyses. A 50 mL glass syringe was used to collect the first 50 mL from the squeezer for 14C analysis. The remainder of the interstitial water from the squeezer was dispensed into a plastic syringe for shipboard and shore-based analyses. Samples for organic chemistry were extruded from the squeezer into plastic syringes and Gelman filter assemblies that were first rinsed with 18 MΩ deionized water; attempts were made to dry the syringe and filter before use. Incomplete drying was observed in the chlorinity data, and this rinsing procedure was stopped after Hole U1363C. After interstitial water was collected, the syringe was removed to dispense aliquots for shipboard and shore-based analyses.

Samples were stored in acid-cleaned plastic vials and bottles pending shipboard and shore-based analyses. Aliquots for shore-based trace metal and elemental analyses were placed in warm (60°C) HDPE plastic vials and acidified with 4 mL of subboiled 6N HCl per liter of sample. Aliquots for organic carbon, amino acids, and low molecular weight organic acids were stored in acid-cleaned combusted glass vials and frozen at –20°C. Aliquots for microbiology were either (1) transferred into cryovials, fixed with formaldehyde (3.7% final), and placed at 4°C before freezing at –80°C or (2) filtered onto a 0.1 µm Supor filter that was subsequently placed at the bottom of a 15 mL sterile plastic vial and covered in lysis buffer before freezing at –80°C (see “Microbiology”). Samples for 14C analysis were placed in evacuated glass serum bottles (60 or 120 mL). These bottles contained 20 µL of saturated mercuric chloride that was evaporated before the bottle was evacuated.

Interstitial water samples were analyzed according to standard shipboard procedures (Gieskes et al., 1991). The pH was determined by ion-selective electrode. Alkalinity was determined by Gran titration with a Metrohm autotitrator. Chloride analysis was carried out by potentiometric titration using silver nitrate and a Mettler Toledo DL25 titrator equipped with a silver ring electrode (Mettler/Toledo ME 89599) with 2M KNO3 electrode filling solution. All quantifications were based on comparison with International Association for the Physical Sciences of the Ocean (IAPSO) standard seawater. Nutrients (phosphate and ammonium) were measured aboard ship using standard colorimetric methods. Sulfate and the major cations in seawater (Na, Ca, Mg, and K) were measured aboard ship using ion chromatography. Shore-based analyses include Na calculated by difference, Br measured using ion chromatography, major and minor ions measured using inductively coupled plasma–optical emission spectroscopy (ICP-OES) (Na, Mg, Ca, K, Sr, Li, B, Mn, Fe, Li, Si, and Ba), and some trace elements measured using inductively coupled plasma–mass spectroscopy (ICP-MS) with a sample dilution of 1:75 (U, V, Co, Rb, Mo, Cs, and Ba).

Because the nitrate data from shipboard samples showed evidence of contamination, additional pore water was extracted from the center of sediment whole-round samples that had been frozen at –80°C since collection. Whole-round samples were thawed overnight in a 4°C cold room, and ~50 cm3 of sediment was removed from the interior, transferred to sterile 50 mL plastic tubes, and refrozen at –80°C. Precleaned acid-washed rhizon samplers were inserted into the remaining sediment interior (at least 5 cm sediment height) to collect pore water into 10 mL syringes via a three-way stopcock. The first 1 mL of collected pore water was flushed from the syringe to clean it. Once enough pore water was collected, the sample was transferred to a sterile 10 mL plastic tube and frozen at –20°C. These samples were analyzed using standard colorimetric methods (flow injection analysis) for nutrient analysis (ammonium, nitrate, and phosphate).