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doi:10.2204/iodp.proc.322.102.2010

Inorganic geochemistry

Interstitial water collection

Interstitial water samples were obtained from 19 to 57 cm long whole-round sections from cores with >1.4 m of recovery. Samples were collected at a frequency of one per core if recovery was sufficient. Core length ranged between 2.2 and 9.5 m and core recovery varied from 0% to 110%, as listed in the lithology section of each site chapter.

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 (a shipboard structural geologist) viewed the composite X-ray CT scan to determine if there were structures that might warrant description and sampling from a split core. Once approved, the sediment 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, and the cleaned sediment was placed into a 9 cm diameter Manheim-type titanium squeezer (Manheim, 1966) on top of a reagent-grade water (18.2 MΩ Millipore water) rinsed filter paper that was placed on two to four 320 mesh stainless steel screens. Sediments were squeezed at ambient temperatures and pressures no higher than 25,000 psi to prevent the release of interlayer water of clay minerals during the squeezing process. The interstitial water was collected through a 0.2 µM disposable polytetrafluoroethylene (PTFE) filter into a 60 mL acid-washed plastic syringe that was attached to the bottom of the squeezer assembly.

Interstitial water aliquots were collected for both shipboard and shore-based analyses (Table T12). High-density polyethylene (HDPE) sample vials 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 in Milli-Q water and dried in a class 100 laminar flow clean hood. Samples for minor and trace element shipboard analysis were acidified with optima-grade 6N HCl at least 24 h before analysis by inductively coupled plasma–atomic emission spectroscopy (ICP-AES) or inductively coupled plasma–mass spectrometry (ICP-MS). A 0.5 mL sample was collected in acid-washed minicentrifuge tubes for onboard analyses of dissolved iron and sulfide. Aliquots for shore-based isotopic and transition metals analyses, as well as for I and ¹²⁹I measurements, were stored in acid-washed HDPE bottles. Additional aliquots for analyses of oxygen, hydrogen, dissolved inorganic carbon (DIC), lithium, and chloride isotopes 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 shore-based characterization of volatile fatty acids were collected in septum screw-lid glass vials and immediately frozen at –20°C. After interstitial water extraction was complete, sediment squeeze cakes were divided (15–180 cm³) and sealed in plastic bags for archive and shore-based analyses (Table T13). Sediment samples for archive and shore-based organic geochemical analyses were stored in a –20°C freezer; all other squeeze cake samples were stored at room temperature.

Correction for drilling contamination

In many intervals, the sedimentary material was extensively fractured by drilling disturbance (see "Inorganic geochemistry" in the "Site C0011" chapter), making some interstitial water whole rounds extremely difficult to clean prior to processing. In these cases, even after thorough cleaning, the interstitial water samples were still contaminated with fluid that had circulated in the borehole during drilling operations. In continental margin sediments, the sulfate–methane transition (SMT) zone is generally reached at shallow depths in the sediment section, typically shallower than ~40 m. Below this depth, sulfate generally remains depleted. Thus, the presence of sulfate and methane in a sample collected below the SMT may be used as an indication of drilling-induced contamination. For these cases, the sulfate concentrations measured in the samples were used to estimate the amount of drilling fluid introduced to the sample by taking 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 were 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 41 and 42:

and

Both the measured and corrected interstitial water chemical concentration data are presented in the site chapters.

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. Alkalinity titrations had a precision of <2%, based on repeated analysis of IAPSO standard seawater.

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 quality control 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 was <2% and 1.5%, respectively.

Chlorinity was determined via 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 with 1.1 mL of 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.

Analyses of bisulfide/acid volatile sulfide (AVS) and ferrous iron were conducted within ~12 h after a subsample of interstitial water (200–1500 µL) was collected and stored in an anaerobic chamber under 98.2% N₂:H₂. For bisulfide/AVS analysis, an aliquot of interstitial water (>100 µL) was treated with either Hach proprietary sulfide reagents or 1N zinc acetate (5 µL/100 µL sample or standard reference solution), depending on the length of expected delay (due to core processing workflow and limited personnel available to help with spectrophotometry) before analysis could be performed. Aliquots of interstitial water for ferrous iron analyses (≥100 µL/100 µL sample or standard reference solution) were treated with 100X Ferrozine reagent in HEPES (4-[2-hydroxyethyl]piperazine-1-ethanesulfonic acid) buffer (1 µL) at pH 7. Analyses of reference standards stored anaerobically demonstrated the relative stability of both bisulfide and ferrous iron with respect to oxidation over 1–2 days.

Modifications of well-established spectrophotometric assays (Cline, 1969; Stookey, 1970) were used with a BioRad SmartSpec Plus UV-vis spectrophotometer capable of detecting nanomolar to micromolar concentrations of the two species of interest. Colorimetric assays measured absorbance at 562 nm for Fe(II) and 665 nm for S(II–). Absorbance for bisulfide was calibrated against Milli-Q washed Na₂S·9H₂O (25 parts per million [ppm] bisulfide standard measured by titration with thiosulfate and iodine reaction). Ferrozine-complexed ferrous iron absorbance was calibrated against a standard of ferrous ammonium sulfate hexahydrate in hydrochloric acid solution (1000 ppm) reacted with Ferrozine-HEPES buffer as above. Analyses of these reference standards stored anaerobically demonstrated the relative stability of both bisulfide and ferrous ion with respect to oxidation over 1–2 days. Colorimetric analyses required ~6–8 min total per sample once the suite of reference standard calibrations was performed (generally once per complete analytical run).

Concentrations of major cations (sodium, potassium, magnesium, and calcium) were analyzed by IC (Dionex ICS-1500) on samples acidified with 0.4% 6M HCl. Samples were diluted by a factor of 200 with ultrapure water (18.2 MΩ·cm). Each measurement batch included an ultrapure 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 1/200 IAPSO standard seawater were Na⁺ < 1.1%, K⁺ < 1.2%, Mg²⁺ < 1%, and Ca²⁺ < 1.1%, and the average accuracy of the analyses were Na⁺ < 2.4%, K⁺ < 0.5%, Mg²⁺ < 0.8%, and Ca²⁺ < 0.4%. Because of the poor precision and accuracy of the sodium determination, which is typical of Na analysis by IC, Na concentration was computed by charge balance, where

Sodium concentration determined by charge balance and by IC is tabulated in each of the site chapters, but only sodium concentration calculated by charge balance is plotted in the figures.

The minor elements (B, Ba, Fe, Li, Mn, Si, and Sr) 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 6N HCl: 27 g NaCl, 3.8 g MgCl, 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 20 m CSF, based on prior drilling results on this margin (Shipboard Scientific Party, 2001b; Tobin et al., 2009). Because the matrix solution was not a true blank, the procedural blank used was a dilution of the 1% nitric acid solution 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 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 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 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 standards, 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 6N HCl per liter. 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%.

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