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doi:10.2204/iodp.proc.314315316.132.2009

Inorganic geochemistry

Interstitial water collection

Interstitial water samples were obtained from 9 to 43 cm long whole-round sections from cores with >1 m of recovery. These whole-round samples were cut and capped immediately after the core arrived on deck and then were scanned using X-ray CT. In general at each site, three whole rounds were collected from the first core and two whole rounds were collected per 9.5 m to the depth of the sulfate-methane transition (SMT). Below this depth, samples were collected at a frequency of one per 9.5 m, if recovery was sufficient, with additional samples collected across lithologic and structural boundaries (i.e., deformed zones, splay faults, and the décollement). Within the gas hydrate stability zone, the entire core was scanned with an infrared (IR) camera to detect low-temperature anomalies, which are indicators of the potential occurrence of gas hydrates. When there was evidence of gas hydrate, an interstitial water whole-round sample was immediately cut from the core, taken to the QA/QC laboratory, and pushed out of the core liner, and if there was gas hydrate present, the pieces were stored in liquid nitrogen. If there was no visible hydrate, the section was cleaned thoroughly and squeezed. The differences in Cl concentration between the background profile and the hydrate section were used to compute the pore space gas hydrate occupancy.

Typical whole-round samples were immediately taken from the catwalk to the CT laboratory to be scanned for important lithologic boundaries and structures. The watchdog viewed the composite scan to determine if there were significant structures in the sample. The watchdog only rejected an interstitial water whole round if there was a clear structure unique to the core; otherwise, the sediment was taken to the QA/QC laboratory and immediately extruded from the core liner into a nitrogen-flushed glove bag. The exterior of the whole-round sample was then thoroughly cleaned of drilling contamination with a spatula, and the clean parts were placed into a Manheim-type titanium squeezer (Manheim, 1966) on top of two Millipore 18.2 MΩ·cm Type 1 ultrapure (Milli-Q) water rinsed filter papers placed on two to four 320 mesh stainless steel screens. Two sizes of Ti squeezers were used during Expedition 316; one with an inner diameter of 5.5 cm and another with an inner diameter of 9 cm. The sediments were squeezed at ambient temperatures and pressures of up to 20,000 lb for the small Ti squeezers and up to 32,000 lb for the larger Ti squeezers to ensure that the interlayer water of clay minerals was not released during the squeezing process. This interstitial water was collected through the filters into a 60 mL acid-washed plastic syringe attached at the bottom of the squeezer assembly. After squeezing, the water was filtered through a 0.45 µm disposable polytetrafluoroethylene (PTFE) filter into sample vials. Interstitial water aliquots were collected for shipboard analyses and will be supplemented by future shore-based analyses (Table T11). 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 prior to inductively coupled plasma–atomic emission spectroscopy (ICP-AES) or inductively coupled plasma–mass spectrometry (ICP-MS) analysis. Aliquots for shore-based isotopic, transition metals, and U series analyses were stored in detergent- or acid-washed HDPE bottles. Additional aliquots for analyses of dissolved organic carbon and volatile fatty acids were placed in crimp-cap glass vials with HgCl₂ and HOSO₂NH₂ and in precombusted glass vials (stored at –20°C), respectively. Samples for dissolved inorganic carbon were stored in crimp-glass vials with H₂Cl₂ at room temperature. After pore fluid extraction was complete, the sediment squeeze cakes were immediately divided (10–20 cm³) and stored in vacuum-sealed plastic bags for shore-based analyses (Table T12). Sediment samples for shore-based microbiological and organic geochemical analyses were stored in a –20°C freezer.

Correction for drilling contamination

In some intervals, the sedimentary material can be extremely fractured and brecciated (see “Inorganic geochemistry” in the “Expedition 316 Site C0004” chapter), making some interstitial water whole rounds extremely difficult to clean prior to processing. In these cases, even after being thoroughly cleaned, the interstitial water samples were still contaminated with fluid that was circulated in the borehole during drilling operations. This is not the case with cores collected using HPCS but does occur with cores collected using ESCS and RCB methods. In continental margin sediments, the SMT is generally reached at shallow depths in the sediment section, typically shallower than ~40 m. Below this depth, sulfate generally remains depleted. During Expedition 316, we cored using the HPCS through the SMT; ESCS and RCB coring were not used until much deeper in the sediment section. Thus, the presence of sulfate and methane in a sample collected below the SMT is a clear indication of drilling-induced contamination. For these cases, the SO₄ concentration measured in the sample was used to estimate the amount of drill fluid introduced to the sample by taking the ratio of the sulfate measured in the sample to the SO₄ concentration of surface seawater. The other analyses were corrected based on their concentration in surface seawater using the following equations:

where

• X_measured = measured concentration of the analyte.
• X_sw = element concentration in surface seawater (used in drilling).
• X_corrected = corrected or in situ concentration of the analyte.
• f_sw = fraction of seawater introduced into the sample during drilling.
• f_pf = fraction of in situ pore fluid in the sample.

The fraction of seawater introduced by drilling and the fraction of in situ pore fluids are computed using Equations 21 and 22:

and

We collected a sample of the drilling fluid used during coring operations and analyzed it for all of the species measured shipboard. When a sample contained sulfate above the ion chromatograph (IC) detection limit, the sulfate concentration of the drilling fluid was used to compute the amount of fluid added to the contaminated sample, which was then used to correct the other species analyzed. Both the measured and corrected pore fluid chemical concentration data are presented in the site chapters.

Interstitial water analysis

Interstitial water samples were routinely analyzed for pH and alkalinity by Gran titration with a pH electrode and a Metrohm autotitrator and for refractive index with a RX-5000α refractometer (Atago) immediately after pore fluid 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‰ and the average precision for alkalinity analyses was <2%. The precision of the alkalinity titrations was monitored by repeated analysis of IAPSO standard seawater.

Sulfate and bromide concentrations were analyzed by ion chromatography (Dionex ICS-1500 IC) using subsamples that were diluted 1:100 with Milli-Q water. During squeezing, the first 4 mL of pore fluid was immediately transferred to a shipboard sample vial and analyzed for alkalinity. A 0.1 mL aliquot was taken from the vial and placed into a separate vial, and 20 µL of 4M cadmium nitrate (Cd[NO₃]₂) was added to precipitate the sulfide, thus leaving only sulfate in solution. The fixed samples were also diluted 100× with Milli-Q water. The nitrate in the fixed samples was used as an internal standard to increase the analytical precision of the sulfate analyses. Thus, two separate batches of analyses were run on the IC, one for dissolved Br concentration and a separate run for SO₄ concentration. At the beginning and end of each run, several different dilutions of IAPSO standard seawater were analyzed as a quality control measure and to determine accuracy, and IAPSO standard seawater was analyzed after every seven samples 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.

Sulfate concentration was used as an indicator of drill water contamination of the interstitial water whole rounds. Below the SMT, sulfate is totally depleted and values should be below the detection limit of the IC method. Any sulfate measured in the interstitial water samples taken below the SMT is a clear indication of drilling fluid contamination.

Chlorinity was determined via titration with silver nitrate (AgNO₃). We used the convention “chlorinity” for the titration data because it yields not only dissolved chloride but also bromide and iodide. Because we analyzed bromide concentration shipboard by IC, the IC bromide concentration was subtracted from titrated chlorinity to compute true chloride concentration. Iodide was not measured shipboard during Expedition 316, but iodide concentration is generally low, within the error of the chloride data. In the site chapter geochemistry data tables, both the chlorinity data (from titration) and calculated chloride concentration (less IC bromide) are presented. The average precision of the chlorinity titrations, expressed as 1σ standard deviation of means of multiple determinations of IAPSO standard seawater, is ≤0.2%.

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. Dissolved phosphate concentration was also measured by 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 ammonium analyses were <2.5% and 3%, respectively; precision and accuracy of the phosphate analyses were <2% and 2%, respectively.

Concentrations of major cations and minor elements (magnesium, calcium, sodium, potassium, strontium, lithium, iron, barium, silica, and boron) were analyzed by ICP-AES (Horiba Jobin Yvon Ultima2) designed for simultaneous analysis of sample elements and an internal standard. The major cations (Mg, Ca, Na, and K) were diluted by a factor of 501 by adding a 30 µL aliquot of sample to 15 mL of 1% nitric acid (ultrapure double-distilled nitric acid solution) spiked with yttrium (10 ppm Y). Standardization of major cations was achieved by successive dilution of IAPSO standard seawater to 100%, 75%, 50%, and 25% relative to the 1:501 primary dilution ratio. Analytical precision based on repeated analyses of the 50% dilution standard is Ca < 0.6%, Mg < 1.5%, Na < 2.5%, and K < 1.5%. Average accuracies of the analyses based on repeated analyses of two unknowns (30% and 60% IAPSO standard seawater) were Ca < 2%, Mg < 2%, Na < 4%, and K < 2.5%. Because of the poor precision and accuracy of the sodium determination, which is typical of Na analysis by ICP-AES, Na concentration was computed by charge balance, where

Na concentration determined by charge balance as well as by ICP-AES is tabulated in each of the site chapters, but only sodium concentration calculated by charge balance is plotted in the figures.

To analyze the minor elements (B, Ba, Fe, Li, Mn, Si, and Sr), the interstitial water sample aliquot was diluted by a factor of 20 (0.5 mL sample added to 9.5 mL of the same 10 ppm Y solution described above). Because of the high concentration of matrix salts in the 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 IAPSO standard seawater major ion concentrations was prepared from 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 pore fluid sulfate concentration decreased rapidly in the first few cores. 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 relative concentrations of elements in the stock standard were adjusted based on results from Legs 131 and 190, as well as Expedition 315, during which samples were taken from sediments from similar formations. The stock solution was then diluted in the same 1% ultrapure nitric acid solution to concentrations of 50%, 25%, 10%, 5%, and 1%. 1.25 mL 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 of elements: B = 3000 µM, Li = 400 µM, Si = 1000 µM, Mn = 50 µM, Fe = 50 µM, Sr = 400 µM, and Ba = 200 µM. Because values of many of these elements in IAPSO standard seawater are either below detection limits (e.g., Fe and Mn) or variable, 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 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. A 100 µL aliquot of 500 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 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: 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 correlate 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%.

Shore-based analyses

Because of the paucity of argon during the expedition, the analyses of the trace metals (V, Cu, Zn, Mo, Rb, Cs, U, and Pb) were suspended after Site C0006. Interstitial water samples from all sites were subsequently analyzed on shore for these trace metals on a Finnigan Element 2 high-resolution ICP-MS at Moss Landing Marine Laboratories (Moss Landing, California). There was no reference material available on the ship to verify the accuracy of the ICP-MS analyses. The accuracy of the shipboard analyses was determined by comparing the shore-based results for Sites C0004 and C0006 with the data generated during the expedition. In general, the shore-based and shipboard analyses agreed for Cu, Mo, Pb, and U. There were significant offsets between the analyses of Rb and Cs of 10% and 20%, respectively. The shipboard V and Zn data were different from the shore-based data and standard reference material by a factor of 2, likely because of the poor quality of the shipboard standards. As a result of the large discrepancies between the data produced on shore and at sea, only the shore-based trace metal data are presented in this report. Yttrium was not determined during the expedition but was determined in the shore-based laboratory and is included in all of the data tables.

Shore-based ICP-MS analyses followed the methodology outlined in Hulme et al. (2008). Briefly, Rb, Mo, Cs, Y, U, and Pb concentrations were analyzed by dilution mass spectrometry. The samples were diluted to 1% in a solution of 10 mL/L that was made from diluting subboiled, concentrated (14.7N) nitric acid (optima grade) in Milli-Q water. Standard solutions were prepared from Claritas PPT standards in 1% HNO₃. The average percent error of the analyses ranged between 0.7% for Rb and 8.2% for Mo. V, Cu, and Zn were determined by standard addition on 10% dilutions of three 200 µL aliquots dissolved in 1% HNO₃. The accuracy of the standard addition analyses was determined by comparing the measured values of Pacific deep water for V (Wheat et al., 2002) and National Research Council of Canada (NASS-4) standard reference material for Cu and Zn. The average precisions of the analyses are as follows: V < 4%, Cu < 22%, and Zn < 18%.

Interstitial water was also analyzed for stable isotopic composition of oxygen (δ¹⁸O) and hydrogen (D/H) on shore after the expedition. The δ¹⁸O and D/H ratios were measured via a Delta Plus XP stable isotope mass spectrometer with a Gas Bench I (Thermo Finnigan), at New Energy Resources Research Center, Kitami Institute of Technology, Japan. The data are presented in per mil (‰) notation in reference to Vienna standard mean ocean water (VSMOW) with analytical precisions better than 0.5‰ for δ¹⁸O and 1‰ for D/H.

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