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doi:10.2204/iodp.proc.314315316.122.2009

Inorganic geochemistry

Interstitial water collection

Shipboard interstitial water samples were generally obtained from 20 to 45 cm long whole-round cores from the bottom of the third section of each core (labeled “Section 4”). Three whole-round cores were collected from the uppermost 10 m. These cores consisted of a clay matrix, and core recovery was adequate to produce at least five sections. Whole-round samples were sectioned and capped upon core recovery in the core cutting area and taken to the laboratory for immediate X-ray CT scanning. Provided no significant sedimentary or tectonic structures were observed in X-ray CT images, these samples were placed in a nitrogen-filled glove bag and flushed with nitrogen gas three times before squeezing. When there were too many samples to process immediately, samples were sealed in a small nitrogen-filled plastic bag and stored in a 4°C refrigerator until further processing.

The core liner was extracted from a whole-round core while it remained in a nitrogen-filled glove bag. The surface of each whole-round core was carefully scraped to avoid sediments that had potentially been contaminated from seawater, drilling fluid, oxidation, and smearing in the core liner. The clean inner parts of the core were then placed into a Manheim-type titanium squeezer (Manheim, 1966) and squeezed with a laboratory hydraulic press at gauge pressures up to 24,000 lb (with a piston diameter of 5 cm, this translates to a pressure of ~55 MPa). Interstitial water was passed through two rinsed filter papers fitted on two to four 300 mesh stainless steel screens at the bottom of the squeezer. Fluids from the squeezing process passed through a 0.45 µm disposable filter into an acid-washed (10% HCl) 50 mL plastic syringe. Interstitial water was subsampled for shipboard and shore-based analyses. All high-density polyethylene (HDPE) sample vials intended for minor and trace element analysis were cleaned by immersion in 55°C 10% trace metal grade 12N HCl for a minimum of 24 h and were subsequently rinsed with Millipore 18.2 MΩ·cm Type 1 ultrapure (Milli-Q) water and dried in a class 100 laminar flow clean hood.

All samples designated for shipboard minor and trace element analysis were acidified with subboiled 6N HCl at a ratio of 4 mL of subboiled 6N HCl per liter of sample at least 24 h prior to inductively coupled plasma–atomic emission spectroscopy (ICP-AES) or inductively coupled plasma–mass spectrometry (ICP-MS) analysis to dissolve any metallic oxide precipitates that may have formed after squeezing. Subsamples were collected for shore-based isotopic (O, H, Sr, Cl, Li, B, I, and S), rare earth elements (REE), Cd, Y, transition metals, and U series analyses. These subsamples were stored in detergent or acid-washed HDPE bottles (REE, Cd, and Y samples were acidified with subboiled 6N HCl at a ratio of 4 mL of subboiled 6N HCl per liter of sample within a few days of sampling). Additional subsamples were distributed for analyses of dissolved organic/​inorganic carbon in crimp-cap glass vials with HgCl₂ and HOSO₂NH₂ and analyses of volatile fatty acids in precombusted glass vials that were transferred to a freezer held at –20°C.

Interstitial water analysis

Interstitial water samples were routinely analyzed for salinity as total dissolved solutes with an RX-5000α refractometer (Atago) and for pH and alkalinity by Gran titration with a Metrohm autotitrator soon after interstitial water was extracted. The volume of titrant used for each analysis was recorded for future reference. Chloride concentration was measured on a 100 µL subsample by titration using silver nitrate (AgNO₃) in a 0.2M sodium nitrate (NaNO₃) solution. International Association for the Physical Sciences of the Ocean (IAPSO) standard seawater was used as a quality control measure by repeated analysis until the relative standard deviation was a maximum of 0.8%.

Sulfate and bromide concentrations were measured by ICS-1500 ion chromatography (Dionex) using subsamples that were diluted 1:100 (10 µL in 990 µL) with Milli-Q water. This dilution provided quality peak detection for chloride, bromide, and sulfate. Chloride data were used only to check the quality of the dilution step. IAPSO standard seawater aliquots (2.5, 5, 7.5, and 10 µL in a total of 1000 µL) were analyzed at the beginning and end of each run as a quality control measure and to monitor potential drift in sensitivity throughout a particular run.

Dissolved phosphate concentration was measured using a colorimetric method with the aid of an UV-2550PC spectrophotometer (Shimadzu) at an absorbance of 885 nm. Because the concentration of phosphate in the analysis solution must be <10 µM, an appropriate aliquot of sample or standard solution (100 or 600 µL) was diluted to 1.1 mL with Milli-Q water (1000 or 500 µL) in a plastic tube. The 2 mL mixed solution (ammonium molybdate, sulfuric acid, ascorbic acid, and potassium antimonyl tartrate) was added to the tube and was well mixed; the tube was capped and kept at room temperature to develop color. A calibration curve was provided from a series of phosphate standard solutions (KH₂PO₄) of 25, 50, 50, 75, 100, 200, and 250 µM with a reproducibility better than 0.3%.

Dissolved ammonium concentration was also determined with the UV-2550PC spectrophotometer at an absorbance of 640 mM. Either 0.1 or 0.05 mL of sample aliquot was diluted with 1 or 2 mL Milli-Q water, 0.5 mL phenol ethanol, 0.5 mL sodium nitroprusside, and 1 mL oxidization solution (sodium hypochlorite and alkaline citrate) in a capped plastic tube and was kept in the dark at room temperature for >3 h to develop color. A calibration curve was provided from a series of ammonium standard solutions (NH₄Cl) of 0.5, 1.0, 2.0, 5.0, and 10 mM with a reproducibility better than 0.3%.

Concentrations of sodium, magnesium, calcium, potassium, strontium, lithium, iron, manganese, barium, silicon, and boron were obtained by ICP-AES using an Ultima2 (Horiba Jobin Yvon) designed with a radial viewing plasma orientation and two Czerny Turner monochrometers for parallel analysis of sample elements and internal standards. The major ions (Na, Mg, Ca, and K) were diluted by a factor of 501 by addition of a 30 µL sample to 15 mL of 0.15N nitric acid spiked with Y (10 ppm Y in a 1% ultrapure double-distilled nitric acid solution). Standardization of the major ions was achieved by successive dilution of IAPSO standard seawater to 100%, 75%, 50%, and 25% relative to the 1:501 primary dilution ratio. Because of the extremely high dilution ratio, no attempts were made to adjust for variable matrix effects. In the case of high (>1 mM) concentrations of Sr and Li in samples, additional dilutions of IAPSO standard seawater were prepared and spiked with a range of Li and Sr primary standards to measure these elements along with the major cations. The minor ions (Sr, Li, Fe, Mn, Ba, Si, and B) were diluted by a factor of 20 (0.5 mL sample with 9.5 mL of the same 10 ppm Y solution described above). Because of the high concentration of matrix salts in the pore water samples at a 1:20 dilution, matrix matching of the calibration standards was necessary. The matrix solution that approximated IAPSO standard seawater major ion concentrations was prepared from the following salts in 1 L of Milli-Q water acidified with 4 mL of ultrapure double-distilled 6N HCl: 27 g NaCl, 3.8 g MgCl, 1.0 g CaCO₃, and 0.75 g KCl. No sulfate was added to the matrix because the pore water sulfate concentrations decreased rapidly in the first few cores. There was some concern that the salts used for the matrix were not trace metal grade, but analysis of the matrix solution revealed that the concentrations of minor elements in the matrix were less than or equal to the lowest standards analyzed and did not significantly affect the signal-to-noise ratio of the measurements. Because the matrix solution was not a true blank, the procedural blank used was a dilution of 1% ultrapure nitric acid in the Y solution and only the slope of the calibration curve was used for quantification. A stock standard solution was prepared from ultrapure primary standards (SPC Science PlasmaCAL) in 1% ultrapure 15N nitric acid. The relative concentrations of elements in the stock standard were adjusted based on results from Legs 131 and 190 because these samples were taken from sediments from similar geological conditions. The stock solution was then diluted in the same 1% nitric acid solution to concentrations of 50%, 25%, 10%, 5%, and 1%. A 1.25 mL aliquot of each stock solution was added to 8.75 mL of matrix to produce a series of standards that could be diluted using the same method as the samples for consistency. A copious supply of stock and matrix solutions was prepared during this expedition for use on the next expedition so that the analytical results of the two expeditions will be comparable. The final matrix-matched 100% standard solution contained the following concentrations of elements: 3000 µM B, 400 µM Li, 1000 µM Si, 50 µM Mn, 50 µM Fe, 400 µM Sr, and 200 µM Ba. Because values of many of these elements in IAPSO standard seawater are either below detection limits (e.g., Fe and Mn) or variable, a 10% matrix matched standard was repeatedly analyzed to calculate the precision of the method.

Several trace elements (V, Mn, Fe, Cu, Zn, Mo, Rb, Cs, Pb, and U) were analyzed using an Agilent 7500ce ICP-MS equipped with an octopole reaction system to reduce polyatomic and double-charge interferences. To calibrate for interferences by the major ions Na, Cl, K, Ca, and S on some of the transition metals (ClO and SOH on V, Na and CaOH on Cu, and S on Zn), solutions were prepared containing these elements at concentrations similar to IAPSO standard seawater values. These solutions were analyzed at the beginning of each run and an interference correlation was applied based on the average counts per second (cps) measured on the standard solutions divided by the abundance of the interfering elements. This ratio was multiplied by the known concentration of the major ions in the samples based on previous analysis, and the result was subtracted from the measured cps of the sample. Before diluting the samples, a 100 µL aliquot of 500 ppb in standard was added to the empty analysis vials. Samples were then diluted into these vials to 3% in 1% HNO₃ (150 µL sample with 4.85 mL 1% HNO₃) based on previous determination of the detection limits and low concentrations of the elements of interest. A primary standard solution was made that matched the maximum range of predicted concentrations based on published results of deep-sea pore fluid compositions in a variety of settings. The composition of the standard is as follows: 20 ppb V; 40 ppb Cu, Mo, Pb, and U; 140 ppb Zn; 500 ppb Rb; and 5 ppb Cs. This primary standard was diluted in 1% ultrapure 15N nitric acid to relative concentrations of 50%, 25%, 10%, 5%, and 1%. These standards were then diluted to 3%, similar to the samples, with the addition of 150 µL of a 560 mM NaCl solution and 4.7 mL of 1% HNO₃ to account for matrix suppression of the plasma ionization efficiency. The 25% standard was diluted this way and analyzed every eight samples throughout every analysis series for precision and in order to correlate results from different analysis dates. Blanks were also analyzed every eight samples, and the detection limits were determined as three times the standard deviation of a procedural blank of Milli-Q water acidified with 4 mL of subboiled 6N HCl per liter. No standard reference material was available on the ship to verify the accuracy of the analysis. Accuracy was determined by analyzing 31 (40%) of the interstitial water samples collected during the expedition using a high-resolution ICP-MS (Finnigan Element 2) at Moss Landing Marine Laboratories (Hulme et al., 2008). Generally, ship- and shore-based analyses agreed for Cu, Mo, Pb, and U. Minor adjustments to the shipboard Rb (10%) and Cs (20%) were necessary. Shipboard V and Zn data were different from shore-based data and standard reference material by a factor of two, possibly indicating an issue with shipboard standards. Shore-based analysis also provides a measure of Y concentration. Shore-based data are shown in tables and figures for both sites (see the “Expedition 315 Site C0001” and “Expedition 315 Site C0002” chapters).

Oxygen and hydrogen isotopes

Interstitial water was also measured for its stable isotopic compositions of oxygen (δ¹⁸O) and hydrogen (δD) as primary shore-based analyses after the expedition. δ¹⁸O and δD values were determined with the mass spectrometer, Delta Plus XP with Gas Bench II (Thermo Finnigan), at New Energy Resources Research Center, Kitami Institute of Technology, Japan. Results were calculated in permil delta notation against the Vienna standard mean ocean water (V-SMOW) with analytical precisions better than 0.1‰ for δ¹⁸O and better than 1‰ for δD.

GRIND method

A method of pore water extraction for sediments with porosities <40% was used during Expedition 315. This method was initially developed by Cranston (1991) and later used by Wheat et al. (1994) to assess pore fluid composition under conditions where it was impossible to extract a sufficient supply for chemical analysis. During whole-round sampling of the uppermost 200 to 400 m CSF, a series of subsamples of ~5 to 10 cm of sediments were taken from samples when there was excess material after pore fluid extraction. These were taken to compare the GRIND results to traditional pore water extraction methods. Samples were initially scraped clean of the outer portions within the glove bag, wrapped in plastic wrap, and sealed in nitrogen-filled plastic bags. These were stored at 4°C for a period of 3 to 5 days.

Following the Cranston (1991) method, Milli-Q water was bubbled with nitrogen gas for a period of 48 h to remove dissolved oxygen. This water was then spiked with 10 ppm In standard to a final concentration of 100 ppb. This concentration was chosen in order to reach a diluted concentration of 1 ppb for ICP-MS analysis at 1% dilution. The purpose of the In standard was to calculate the degree of mixing between the interstitial waters and the distilled water that is added as part of the method. Sealed samples were placed in a nitrogen-filled glove bag along with ball mill cylinders and the In solution. The samples were unsealed, broken into small pieces, and placed in the ball mill cylinders with three ceramic milling balls. Depending on the dryness of the samples and the volume of sample in the cylinders, between 1.5 and 3 mL of In solution was added prior to milling. A 3 mL subsample of the In solution was taken after each set of mills were filled. The milling cylinders were covered within the glove bag and taken immediately to the mill for grinding. The mill was operated at 400 rotations per minute (rpm) for a period of 5 min, and afterward the containers were placed back into the glove bag. The milling cylinders were opened in the glove bag and the ground samples were squeezed using the same methods described above. The volume of In solution added to the sample, the volume recovered after squeezing, the time of sample processing, and the corresponding In solution subsample were recorded for each sample.

Infrared thermal observation

Because the dissociation of gas hydrates is an endothermic reaction, observation of low-temperature anomalies on the surface of the core liner provides a means to identify sections that contain or contained gas hydrate. Therefore, infrared thermal observation of the surface of the core liner was used during ODP Legs 201 and 204 and IODP Expedition 311 to identify the distribution of gas hydrates immediately before core disturbance because of their dissociation.

During Expedition 315, a handheld infrared camera, the ThermaCAM SC640 (FLIR Systems), was used to visualize temperature distributions of the surface of several core liners before cores were sectioned in the core cutting area. The initial purpose of thermal imaging is to rapidly identify gas hydrate–bearing sections for immediate sampling and for stabilizing and storing the gas hydrate in liquid nitrogen for future shore-based analyses. Infrared images were acquired from the cores between 0 and 450 m CSF, which are within the gas hydrate stability zone.

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