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doi:10.2204/iodp.proc.327.202.2015

Methods and materials

Sample collection and processing

Samples used for this study were from sediment cores taken from Integrated Ocean Drilling Program (IODP) Holes U1363G, U1363F, U1363B, and U1363C/D located on the slope of Grizzly Bare outcrop on the eastern flank of Juan de Fuca Ridge (Table T1; Fig. F1). The sediment/basement interface at these holes are 17, 35, 57, and 231 meters below seafloor (mbsf), respectively (see the “Site U1363” chapter [Expedition 327 Scientists, 2011b]). Details of whole sediment and sediment pore water sampling are described previously (see the “Methods” chapter [Expedition 327 Scientists, 2011a]; Wheat et al., 2013). Briefly, sediment cores were collected using advanced piston coring (APC or “H” shown in Table T2) and extended core barrel (XCB or “X” shown in Table T2) systems on the R/V JOIDES Resolution. Once shipboard, roughly 10–40 cm of the whole-round sediment section was placed in a nitrogen-filled glove bag, and the outside 0.5 cm of the sediment core was carefully scraped using a polytetrafluoroethylene-coated spatula in order to remove potential contaminants, leaving a ~5.8 cm diameter core. The remaining sediment (~238–1030 cm³) was pressed using a titanium squeezer and hydraulic press piston to extract sediment pore water. The resulting pore water was then passed through a prewashed Whatman Number 1 qualitative filter paper above a titanium screen and collected in a combusted (550°C; 5 h) glass syringe, before being filtered through a prerinsed (50 mL deionized water) 0.45 µm pore-sized polysulfone membrane filter (Supor, Pall Corporation) and collected in an acid-cleaned and combusted glass scintillation vial. The pore water samples were kept frozen at –20°C until subsequently thawed on shore for analysis. Two blank samples that were collected by passing deionized water through the squeezer and filter series have DOC concentrations of 0.016 and 0.028 mM using the method described below. These blank values are an order of magnitude lower than the DOC in environmental pore water measured in the current study.

Sections of sediment after pore water extraction were carefully wrapped in combusted aluminum foil and frozen at –80°C shipboard. In a shore-based laboratory, ~5 g of frozen whole sediment from each sample was subsampled with a clean stainless steel spatula. The whole sediment subsamples were subsequently placed in combusted glass scintillation vials, loosely covered with combusted aluminum foil, and dried in an oven at 60°C. The dried sediment was powdered with a clean ceramic mortar for subsequent analysis as described below.

Pore water–dissolved organic carbon

Sedimentary pore water DOC concentrations were measured by high-temperature combustion using a Shimadzu TOC-VCSH analyzer. The combustion temperature was set at 720°C to ensure complete oxidation of organic matter. Samples were acidified to pH <2 by the addition of 45 µL of 2 M HCl to 3 mL samples. No acid contamination was observed based on monitoring the DOC value of low-carbon deionized water. Samples were purged with nitrogen gas within the autosampler syringe for 2 min in order to remove inorganic carbon. An injection volume of 150 µL was used, with five or six injections per sample. The reproducibility between replicate injections was <1 µM. Analytical reference materials (ARM) supplied by Dr. Dennis Hansell (RSMAS, University of Miami) were measured before, between, and after analysis of environmental samples (Sharp et al., 2002; Dickson et al., 2007). At least one ARM was measured every five samples. The average measured concentration of the ARM was 42 ± 2 µM (n = 44); the reported value was 41–43 µM. Our detection limit for DOC concentrations was ~2 µM.

Sediment

Whole sediment samples were analyzed for concentration and isotopic composition of total carbon, organic carbon, and total nitrogen using an elemental combustion system (Costech ECS 4010) connected inline to an isotope-ratio mass spectrometer (Thermo Finnigan Delta XP). The amount of powdered sediment used for the analyses was optimized to provide sufficient carbon and nitrogen for isotopic composition analysis and varied between 26 and 425 mg. A subset of samples was acidified by fuming with concentrated HCl (Hedges and Stern, 1984) in order to remove inorganic carbon and quantify the particulate organic carbon (POC) content. Acid fuming did not remove inorganic nitrogen, resulting in insignificant differences between whole and acid-fumed total particulate nitrogen (PN) concentrations. Calibration for C and N concentration was performed by running reference materials with the environmental samples. The isotopic compositions of carbon and nitrogen are reported as δ values (e.g., δ¹³C and δ¹⁵N) expressed in permil (‰):

where R is the ¹³C/¹²C or ¹⁵N/¹⁴N ratio. The standards for carbon and nitrogen isotopic composition are Vienna Peedee belemnite (NBS 19) and air, respectively (Coplen et al., 1992; Coplen, 1994).

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