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doi:10.2204/iodp.proc.343343T.102.2013

Geochemistry

Pore water chemistry

Interstitial water collection

The following description of geochemical methods is based largely on the same methods reported for Expedition 322 (Expedition 322 Scientists, 2010). The sampling locations and frequency were modified in accordance with the core handling plan described in “Core handling.”

Interstitial water samples were obtained from 15.5 to 31 cm long whole-round sections from cores with >1.4 m of recovery (i.e., more than one section recovered). Samples were collected at a frequency of one per core if recovery was sufficient and interstitial water samples did not conflict with samples needed for visual core description. Sample volume was selected in an attempt to recover the necessary 25 mL of interstitial water to achieve the goals of chemical analyses while minimizing the volume of sample removed from the core.

Whole-round samples were cut and capped as quickly as possible after the core arrived on deck and immediately taken from the core cutting area to be scanned by X-ray CT. A watchdog (shipboard structural geologist) and geochemist viewed the composite X-ray CT scan to select a sample that did not contain potentially important structural features and was suitably free from fracturing and other defects to minimize contamination from drilling fluid. Once approved by the watchdog, the sample was taken to the quality assurance/quality control (QA/QC) laboratory and immediately extruded from the core liner into a nitrogen-flushed glove bag. The exterior of the whole-round sample was thoroughly cleaned with a spatula to remove drilling contamination. The cleaned sediment was placed into a 9 cm diameter Manheim-type titanium squeezer (Manheim and Sayles, 1974) on top of filter paper rinsed with reagent-grade water (18.2 MΩ Millipore water, or “Milli-Q”) placed on two 40 mesh titanium screens. Sediment was squeezed at ambient temperatures and pressures no higher than 25,000 psi to prevent the release of interlayer water from within clay minerals during the squeezing process. The interstitial water (4.1–63 mL) was collected through a 0.2 µm disposable polytetrafluoroethylene (PTFE) filter into a 24 or 60 mL acid-washed plastic syringe attached to the bottom of the squeezer assembly.

Interstitial water aliquots were collected for both shipboard and shore-based analyses (Table T8). High-density polyethylene (HDPE) sample vials for minor and trace element analysis were cleaned by immersion in 55°C 10% trace metal–grade 12M HCl for a minimum of 24 h and were subsequently rinsed in Milli-Q water and dried in a class 100 laminar flow clean hood. Samples for minor and trace element shipboard analyses were acidified with optima-grade 6M HCl before analysis by inductively coupled plasma–atomic emission spectroscopy (ICP-AES) or inductively coupled plasma–mass spectrometry (ICP-MS). Unless otherwise noted, the samples were stored at 4°C after collection. Aliquots for shore-based Li-B-Sr-Pb isotopic and trace element analyses were acidified with 6M HCl and stored in acid-washed 4 mL HDPE bottles. Aliquots for isotopic analyses of oxygen, hydrogen, and dissolved inorganic carbon (DIC) were placed in 2 mL septum screw-lid glass vials; samples for DIC were treated with 10 µL of a saturated HgCl₂ solution at room temperature. Aliquots for nitrogen isotopic analysis were stored in acid-washed 4 mL HDPE bottles with HCl and HgCl₂. Aliquots for sulfur isotopic analysis were placed in septum screw-lid glass vials with ZnCl₂ and frozen at –20°C. Aliquots for carbon isotopic analysis of dissolved organic carbon and acetate were placed in septum screw-lid glass vials with HgCl₂ and sulfamic acid. Aliquots for rare earth element analysis were acidified with optima-grade 6M HCl and stored in acid-washed 8 mL HDPE bottles. After interstitial water extraction was complete, sediment squeeze cakes (24–456 cm³) were divided and sealed in plastic bags for archive material and shore-based analyses. Squeeze cake samples for shore-based microbiological analysis were stored at –80°C. All other squeeze cake samples were chilled at 4°C.

Correction for drilling contamination

In many intervals, the sedimentary material was extensively fractured by drilling disturbance, making some interstitial water whole rounds extremely difficult to clean prior to processing. In these cases, even after thorough cleaning, the interstitial water samples may possibly be contaminated with fluid that circulated in the borehole during drilling operations. In continental margin sediment, the sulfate–methane transition (SMT) zone is generally reached at shallow depths in the sediment section, typically shallower than ~40 mbsf. Below this depth, sulfate generally remains depleted. Thus, the presence of sulfate and methane in a sample collected below the SMT zone may be used as an indication of drilling-induced contamination by comparing the ratio of the sulfate measured in the sample to the sulfate concentration of seawater (28.9 mM). Likewise, concentration of the other ionic species can be corrected based on their concentration in surface seawater using the following equation:

where

• X_corrected = corrected or in situ concentration of the analyte,

• X_measured = measured concentration of the analyte,

• X_sw = analyte concentration in surface seawater (used in drilling),

• f_sw = fraction of seawater introduced into the sample during drilling, and

• f_iw = fraction of in situ interstitial water in the sample.

The fraction of seawater introduced by drilling disturbance and the fraction of in situ interstitial water are computed using Equations 47 and 48:

and

where SO_{4 measured} is the measured sulfate concentration and SO_{4 sw} is the sulfate concentration in surface seawater.

Shipboard analytical results indicate that rather than seawater or drilling-fluid contamination, the presence of sulfate in pore water is derived from mixing with a remote sulfate-rich fluid source (see “Geochemistry” in the “Site C0019” chapter [Expedition 343/343T Scientists, 2013]). When corrections were applied to interstitial waters using sulfate concentrations, in some cases the correction resulted in negative values for magnesium and potassium. Potential contamination by seawater or drilling mud was checked using the perfluorocarbon (PFC) contamination tracer method described in “Microbiology.” Those results indicate that contamination was generally low. Cores 343-C0019E-1R, 4R, 12R, 13R, 14R, 15R, and 19R were not contaminated (Table T14 in the “Site C0019” chapter [Expedition 343/343T Scientists, 2013]). Cores 5R, 6R, 7R, 8R, and 20R showed minor contamination. Core 6R showed the greatest contamination, and caution should be exercised in interpreting interstitial water data from this sample. Given the entirety of the interstitial water data and the results from the PFC contamination tracer method, no contamination corrections were applied to these samples.

Interstitial water analysis

Interstitial water samples were routinely analyzed for refractive index with a RX-5000α refractometer (Atago) immediately after interstitial water extraction. The refractive index was converted to salinity based on repeated analyses of International Association of Physical Sciences of the Oceans (IAPSO) standard seawater. Precision for salinity was <0.1‰. Also, immediately after interstitial water extraction, samples were analyzed for pH and alkalinity by Gran titration with a pH electrode and a Metrohm autotitrator for the two samples where sufficient fluid was available. Alkalinity titrations had a precision of <2%, based on repeated analysis of IAPSO standard seawater. For sample volumes of ≤14 mL, alkalinity and pH were not measured (Table T8).

Sulfate and bromide concentrations were analyzed by ion chromatography (IC) (Dionex ICS-1500) using subsamples that were diluted 1:100 with Milli-Q water. At the beginning and end of each run, several different dilutions of IAPSO standard seawater were analyzed for QC and to determine accuracy. IAPSO standard seawater was analyzed after every seventh sample as a check for instrumental drift and to calculate analytical precision. Precision for the bromide and sulfate analyses was <3% and <0.8%, respectively. Average accuracy of bromide and sulfate analyses was <2% and 1.5%, respectively.

Chlorinity was determined by titration with silver nitrate (AgNO₃). We use the convention “chlorinity” for the titration data because it yields not only dissolved chloride but also all of the other halide elements and bisulfide. The average precision of the chlorinity titrations, expressed as 1σ standard deviation of means of multiple determinations of IAPSO standard seawater, is ≤0.5%.

Dissolved ammonium concentration was measured within 24 h of collecting the interstitial water sample by colorimetry, using an ultraviolet-visible (UV-vis) spectrophotometer (Shimadzu UV-2550) at an absorbance of 640 nm. A 0.1 mL sample aliquot was diluted with 1 mL of Milli-Q water, to which 0.5 mL phenol ethanol, 0.5 mL sodium nitroprusside, and 1 mL oxidizing solution (trisodium citrate and sodium hydroxide) were added in a capped plastic tube (Gieskes et al., 1991). The solution was kept in the dark at room temperature for >3 h to develop color. Precision and accuracy of the ammonium analyses were <2.5% and 3%, respectively.

Dissolved phosphate concentration was also measured by a colorimetric method using the UV-vis spectrophotometer at an absorbance of 885 nm. Because the phosphate concentration in the analysis solution must be <10 µM, appropriate aliquots of sample or standard solution (100 or 600 µL) were diluted to 1.1 mL with Milli-Q water (1000 or 500 µL) in a plastic tube. The mixed solution (ammonium molybdate, sulfuric acid, ascorbic acid, and potassium antimonyl tartrate) was added to the tube (Gieskes et al., 1991), which was capped and kept at room temperature to develop color. Precision and accuracy of the phosphate analyses were <2% and 2%, respectively.

Concentrations of major cations (sodium, potassium, magnesium, and calcium) were analyzed by IC on samples acidified with 0.4% 6M HCl. Samples were diluted by a factor of 200 with Milli-Q water. Each measurement batch included a Milli-Q water blank, seven standards for calibration prepared from a commercially available cation mixed standard (Kanto Chemical Co. Inc., P/N 07197-96), and IAPSO standard seawater (P-series) as a reference material, in addition to the interstitial water samples. The average precision estimated by repeated measurements of IAPSO standard seawater were Na⁺ < 1.1%, K⁺ < 1.2%, Mg²⁺ < 1%, and Ca²⁺ < 1.1%, and the average accuracy of the analyses was Na⁺ < 2.4%, K⁺ < 0.5%, Mg²⁺ < 0.8%, and Ca²⁺ < 0.4%.

The minor elements (boron, barium, iron, lithium, manganese, silica, and strontium) were analyzed by ICP-AES (Horiba Jobin Yvon Ultima2). The interstitial water sample aliquot was diluted by a factor of 20 (0.5 mL sample added to 9.5 mL of 1% ultrapure double-distilled nitric acid solution spiked with 10 ppm yttrium). Because of the high concentration of matrix salts in interstitial water samples at a 1:20 dilution, matrix matching of the calibration standards is necessary to achieve accurate results by ICP-AES. A matrix solution that approximated seawater major ion concentrations was prepared by dissolving the following salts in 1 L of Milli-Q water acidified with 4 mL of optima-grade 6M HCl: 27 g NaCl, 3.8 g MgCl₂, 1.0 g CaCO₃, and 0.75 g KCl. Sulfate was not added to the matrix-matching solution because we expected the interstitial water sulfate concentration to decrease rapidly in the upper few tens of meters below seafloor. Because the matrix solution was not a true blank, the procedural blank used was a dilution of the 1% nitric acid solution in the yttrium 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 the 1% nitric acid solution. The stock solution was then diluted in the same 1% ultrapure 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 solution to produce a series of standards that could be diluted using the same method as the samples for consistency. The final matrix-matched 100% standard solution contained the following concentrations: B = 3000 µM, Li = 400 µM, Si = 1000 µM, Mn = 50 µM, Fe = 50 µM, Sr = 400 µM, and Ba = 200 µM. A standard prepared in the 10% matrix-matching solution was repeatedly analyzed to calculate the precision of the method. The average precision of the minor element analyses were B < 1%, Ba < 1%, Fe < 1.8%, Mn < 2%, Li < 1%, Si < 2%, and Sr < 1.5%, and the average accuracy of the analyses were B < 1.5%, Ba < 2%, Fe < 2%, Mn < 2%, Li < 2%, Si < 3%, and Sr < 4%.

Vanadium, copper, zinc, molybdenum, rubidium, cesium, lead, and uranium were analyzed by ICP-MS (Agilent 7500ce) 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 correction 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. A 100 µL aliquot of 500 parts per billion (ppb) indium standard was added to the empty analysis vials before dilution. Sample aliquots were then diluted with the 1% nitric acid solution to 3% in these vials (150 µL sample with 4.85 mL of 1% HNO₃ solution) based on previous determination of the detection limits and the concentrations of the elements of interest. A primary standard solution that matched the maximum range of predicted concentrations was made based on published results of deep-sea interstitial water compositions in a variety of settings. The composition of the standard is as follows: V = 20 ppb; Cu, Mo, Pb, and U = 40 ppb; Zn = 140 ppb; Rb = 500 ppb; and Cs = 5 ppb. This primary standard was diluted in the 1% nitric acid solution to relative concentrations of 50%, 25%, 10%, 5%, and 1%. These standards were then diluted to 3%, similar to the standard solutions, with the addition of 150 µL of a 560 mM NaCl solution and 4.7 mL of the 1% HNO₃ solution to account for matrix suppression of the plasma ionization efficiency. The 25% standard was diluted accordingly and analyzed every eight samples throughout every analysis series for precision and in order to compare the results from different analysis dates. Blanks were also analyzed every eight samples, and detection limits were determined as three times the standard deviation of a procedural blank of Milli-Q water acidified with 4 mL of optima-grade 6M HCl. The average precision of multiple determinations of the 10% ICP-MS standard was V < 6%, Cu < 14%, Zn < 3%, Mo < 0.8%, Rb < 0.7%, Cs < 12%, Pb < 3%, and U < 0.8%.

Gas chemistry

Safety gas monitoring and H₂ and CO

A 1 cm³ sediment or deposit was collected with a cut-off plastic syringe or a corkscrew, usually from the exposed ends of every section of the retrieved core, and was extruded into a 20 mL glass vial containing 3 mL of Milli-Q water. Then the vial was placed in an oven at 70°C for 30 min. The evolved gases were analyzed using an Agilent 6890N gas chromatograph (GC) equipped with a flame ionization detector (FID) for CH₄ and other hydrocarbons (C₂–C₄) and a GL Science GC4000 GC equipped with a helium ionization detector (HID) for H₂ and CO. The GC-FID system determined the C₁–C₄ hydrocarbon concentration. Chromatographic response on the GC was calibrated against five different authentic standards with variable quantities of low molecular weight hydrocarbons. The GC-HID was brought on board the Chikyu for the JAMSTEC Subsurface Geobiology and Advanced Research (SUGAR) project as a third-party tool. Chromatographic response on the GC was calibrated against the prepared standard gas mixture (10 and 100 ppm of H₂, CH₄, and CO). After the measurement of H₂ and CO concentrations, the vial was again heated at 70°C for 30 min. The evolved gases were analyzed first with the GC-HID and then with the GC-FID.

Organic geochemistry

Total carbon, nitrogen, and sulfur contents of the solid phase

Solid-phase samples were taken at a frequency of 1–3 per core as individual samples for milling and XRF, XRD, carbonate, and CNS analyses. After drying and homogenizing the sample, total carbon, nitrogen, and sulfur concentrations were determined by elemental analysis using a Thermo Finnigan Flash EA 1112 carbon-hydrogen-nitrogen-sulfur (CHNS) analyzer. Calibration was based on the synthetic standard sulfanilamide, which contains 41.81 wt% carbon, 16.27 wt% nitrogen, and 18.62 wt% sulfur. About 20–50 mg of ground sediment was weighed and placed in a tin container for carbon and nitrogen analyses. For sulfur analysis, the same amount of sediment was weighed and put in a tin container with the same amount of V₂O₅ catalyst. Sediment samples were combusted at 1000°C in a stream of oxygen. Nitrogen oxides were reduced to N₂, and the mixture of CO₂, N₂, and SO₂ was separated by gas chromatography and detected by a thermal conductivity detector. The standard deviation of carbon, nitrogen, and sulfur concentrations for the samples is less than ±0.1%. Accuracy for carbon and sulfur analysis was confirmed using two GSJ reference samples.

Inorganic carbon, organic carbon, and carbonate content of the solid phase

In the same set of samples that was used for the analysis of total carbon, nitrogen, and sulfur contents of the solid phase, inorganic carbon concentration was determined using a Coulometrics 5012 CO₂ coulometer. About 10–20 mg of ground sediment was weighed and reacted with 2 M HCl. The liberated CO₂ was titrated, and the change in light transmittance was monitored with a photodetection cell. The weight percentage of calcium carbonate was calculated from the inorganic carbon content, assuming that all the evolved CO₂ was derived from dissolution of calcium carbonate, by the following equation:

No correction was made for the presence of other carbonate minerals. National Institute of Standards and Technology–Standard Reference Material (NIST-SRM) 88b was used to confirm accuracy. Standard deviation for the samples was less than ±0.1 wt%. Total organic carbon contents were calculated by subtraction of inorganic carbon from total carbon contents as determined by elemental analysis.

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