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doi:10.2204/iodp.proc.331.102.2011

Geochemistry

Organic geochemistry

Shipboard organic geochemical analyses included volatile hydrocarbon contents (C₁–C₄); inorganic carbon and carbonate contents; and elemental analyses of total carbon, nitrogen, and sulfur. Procedures used generally follow Pimmel and Claypool (2001).

Safety gas monitoring

A 5 cm³ sediment sample was collected with a cut-off plastic syringe, usually from the exposed end of Section 1, and was extruded into a 20 mL glass vial. The vial was then 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). This system determined the concentration of C₁–C₄ hydrocarbons with an FID. Chromatographic response on the GC was calibrated against five different authentic standards with variable quantities of low molecular weight hydrocarbons.

Headspace H₂ and methane (science gas sample)

A 5 cm³ sediment sample was collected with a cut-off plastic syringe from adjacent to the safety gas sample and as close as possible to the interstitial water sample. The sample was immediately extruded into a 10 mL glass vial containing 3 mL of saturated sodium chloride and one drop of a saturated mercuric chloride solution. The vial was capped with a Teflon-coated butyl rubber stopper. An aliquot of 300–500 µL of headspace gas was drawn from the vial with a plastic syringe and manually injected into a GL Science GC equipped with a helium ionization detector (HID) to determine H₂ and CH₄ concentrations. The GC with HID was supplied by the JAMSTEC SUGAR project as a third-party tool. Chromatographic response on the GC was calibrated against a standard gas mixture (22.7 ppm each of H₂, O₂, N₂, CH₄, and CO).

Inorganic carbon

Inorganic carbon concentrations were determined using a Coulometrics 5012 CO₂ coulometer. About 10–12 mg of vacuum-dried 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 (IC) content, assuming that all evolved CO₂ was derived from dissolution of calcium carbonate, by the following equation based on molecular weight ratio:

All carbonate minerals were treated as CaCO₃. For Expedition 331, repeated measurements of the NIST SRM 88b (dolomitic limestone) reference material produced a mean IC value of 12.74 ± 0.10 wt%, which corresponds to a CaCO₃ composition of 106.1 ± 0.8 wt%.

Elemental analysis

Total carbon, nitrogen, and sulfur concentrations were determined using a Thermo Finnigan Flash EA 1112 CHNS analyzer calibrated using the synthetic standard sulfanilamide, which contains C (41.81 wt%), N (16.27 wt%), and S (18.62 wt%). About 10–20 mg vacuum-dried ground sediment was weighed and placed in a tin container for carbon and nitrogen analyses. For sulfur analysis, the same amount of freeze-dried sediment was weighed and put in a tin container with an equal amount of V₂O₅. 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 GC and detected by a thermal conductivity detector (TCD). Total organic carbon content was calculated by subtraction of inorganic carbon from total carbon. Standard deviation of carbon, nitrogen, and sulfur for the samples is less than ±0.1%.

During Expedition 331, the analytical accuracies of our C and N analyses were confirmed through repeated analyses of Soil NCS reference material, run after every sixth sample, which has a certified composition of 1.755 ± 0.028 wt% for C and 0.195 ± 0.0274 wt% for N. Our observed mean for the Soil NCS reference material was 1.719 ± 0.034 wt% C and 0.1881 ± 0.005 wt% N. The analytical accuracy for our S analyses was determined through repeated analyses of the JMS-1 reference material (from the Geological Society of Japan), run after every sixth sample, which as a certified S composition of 1.32 wt%. Our observed mean for repeated analyses of JMS-1 was 1.23 ± 0.2 wt%. These tests of accuracy include weighing errors.

The reproducibility for the analysis of total carbon (TC) and total nitrogen (TN) when their concentrations were very low was tested using a sample retrieved from Site C0014 (Sample 331-C0014G-17T-4, 40.0–42.0 cm). Repeated analysis of this sample resulted in a mean value of 0.0105 ± 0.0006 wt% (N = 3) for carbon (TC) and 0.0605 ± 0.0008 wt% (N = 3) for nitrogen (TN).

Inorganic geochemistry

Interstitial water collection

We obtained interstitial water typically from 10–20 cm long whole rounds, selected based on core recovery and expedition objectives. Whole rounds were cut and capped, brought into the laboratory, and placed in a nitrogen-filled glove bag, where they were generally scraped to remove contaminated sediment and transferred into a titanium piston-cylinder-type squeezer (Manheim, 1966). The squeezer was removed from the glove bag and placed into a hydraulic press, which was used to compress the sediment to extract the pore water. As the sediment was squeezed, pore water passed through one or two rinsed paper filters mounted on a titanium support screen and into a plastic syringe. Once collected, the pore water was ejected through a 0.45 µm syringe-mounted filter into plastic bottles or glass vials for various analyses. Where appropriate, these containers were cleaned with HCl and rinsed with 18 MΩ water. Samples for analysis by inductively coupled plasma–optical emission spectrometry (ICP-OES) and inductively coupled plasma–mass spectrometry (ICP-MS) were acidified by adding 4 µL of 6 N HCl per mL sample. We note that precipitation occurred regularly during sample processing both before and after filtering. The precipitate was usually fine grained and black and is probably polymetallic sulfide. Occasionally we also saw white flocs forming in the sampling syringe which may be silica or organics; these flocs did not appear to form following sample filtration. Caution should be exercised in interpreting any data that might be affected by such precipitation.

Interstitial water analysis

Refractive index was measured on a 100 µL filtered aliquot using an RX-5000a refractometer (Atago); this measurement can be used to estimate pore water salinity. pH and alkalinity were measured using a pH electrode and Gran titration with 0.1 M HCl using a Metrohm autotitrator. Chlorinity (Cl + Br) was analyzed by titration with silver nitrate using International Association for the Physical Sciences of the Oceans (IAPSO) seawater as a primary standard. This titration is one of the more precise and accurate chemical measurements we made. For any batch of analyses, no samples were run until the operator achieved a precision of ±0.2% on three consecutive replicates of the IAPSO standard, which was then run frequently throughout the analyses. For 133 replicates of IAPSO run during Expedition 331, we achieved a mean daily precision of 0.4% ± 0.3% (1σ), where the standard deviation represents the variation in the mean precision from day to day.

Sulfate, Br, Mg, Ca, K, and Na were measured using a Dionex ICS-1500 ion chromatograph. Dissolved phosphate, silicon, and ammonium were measured using standard colorimetric methods on a Shimadzu UV-2550 ultraviolet (UV)-visible spectrophotometer. The colorimetric method for Si measures only monomeric silica. We had anticipated measuring Si by ICP-OES shipboard using a Horiba Jobin Yvon Ultima2 ICP-AES, but this instrument was not operational during Expedition 331. Aliquots for Si were not prediluted, so the concentrations we report should be considered minima because of the possibility of polymerization. A preliminary comparison with Si analyses done postcruise by ICP-OES indicates this problem may have been minimal. However, for 10 samples from Site C0014 and all five from Site C0015, ICP-OES yielded lower concentrations than colorimetry in an approximately fixed ratio that likely resulted from either a computational or analytical error. One additional ICP-OES sample from Site C0014, which was part of the group of data that was not used, actually had a higher Si value than the colorimetric analyses. However, the colorimetric analysis of this particular sample is questionable as it was notably low (0.09 mM) compared to the other samples above and below (i.e., >1 mM) and may have suffered from a dilution error. Because of its consistency with other ICP-OES data that appear to suffer from an analytical artifact, we have deleted this value (noted in Table T8 in Expedition 331 Scientists, 2011a). Although we present both data sets in the site chapter tables, we have removed these 16 ICP-OES analyses, as marked. This error appears to apply only to the Si data. In any case, our interpretations are sufficiently robust that they are unaffected by any remaining inaccuracies in the ICP-OES data.

Because the ICP-OES was not operational during Expedition 331, concentrations of strontium, lithium, iron, manganese, barium, silicon, and boron were determined postcruise, on the Chikyu, once the instrument was repaired. Although we used the National Research Council Canada Seawater Reference Materials for Trace Metals—CASS-4 and NASS-5 (North Atlantic Surface Seawater)—as quality controls for these analyses, these standard reference materials have low signal intensities for Ba, Fe, Mn, and Si relative to our samples and thus may not provide a practical assessment of our data quality for these elements. For B, Li, and Sr, the average values we obtained for NASS-5 are 354 ± 8 µM for B, 36.4 ± 3.3 µM for Li, and 73 ± 2 µM for Sr. For CASS-4 the values are 363 ± 15 µM for B, 37 ± 2 µM for Li, and 75 ± 2 µM for Sr. Unfortunately these reference materials do not have certified values for these particular analytes; however, based on their salinity (NASS-5 = 30.4; CASS-4 = 30.7), we would have expected B, Li, and Sr values to be ~370 µM for B, ~23.5 µM for Li, and ~82 µM for Sr for these standards. Data for the elements Zn, Mo, Rb, Cs, and U are reported here and were measured using an Agilent 7500ce ICP-MS equipped with an octopole reaction system and indium internal normalization standard. For quality control we again measured CASS-4 and NASS-5. The average values we obtained for NASS-5 are 80 ± 40 nM for Zn, 97 ± 3 nM for Mo, 1.16 ± 0.03 µM for Rb, 1.81 ± 0.14 nM for Cs, and 10.1 ± 0.8 nM for U. Our average values for CASS-4 are 78 ± 15 nM for Zn, 100 ± 4 nM for Mo, 1.18 ± 0.03 µM for Rb, 1.8 ± 0.2 nM for Cs, and 10.3 ± 0.8 nM for U. These seawater reference materials are not ideal for quality control because our pore waters often differ significantly from seawater in both major and trace element content. Detection limits are defined as three times the standard deviation of the detection blank determined during the sample runs.

Interstitial waters were analyzed for dissolved inorganic carbon (DIC) on a UIC coulometer (CM5012). These samples were stored at 4°C in 2 mL amber glass vials. For analysis, 500 µL of pore water was placed in a sample vial and purged with N₂ for 1 min to remove headspace gas. After purging, 0.5 mL of 10% phosphoric acid was added to the sample. We used 0.025 and 0.25 M Na₂CO₃ solution (Wako chemical) as standards and analyzed a certified reference material for oceanic CO₂ measurements supplied by Dr. Andrew Dickson of Scripps Institute of Oceanography (San Diego, USA), batch 99, as a quality control solution.

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