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doi:10.2204/iodp.proc.333.102.2012

Inorganic geochemistry

Interstitial water collection

Interstitial water samples were obtained from 19 to 61 cm long whole-round sections. Samples were collected at a frequency of one per core when possible. 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) 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 sample preparation laboratory and immediately extruded from the core liner into a nitrogen-flushed glove bag. Because of core expansion, the sediment could not, at times, be extruded from the core liner manually. In this situation the liner only was carefully split in the core splitting room, taking care to not disturb the sediment. Once extruded, the exterior of the whole-round sample was thoroughly cleaned with a spatula to remove drilling or liner splitting contamination, and the cleaned sediment was placed into a 9 cm diameter Manheim-type titanium squeezer (Manheim, 1966) on top of two pieces of filter paper rinsed with reagent-grade water (18.2 MΩ Millipore water) placed on two 40 mesh titanium screens. If needed, the mesh edges were wrapped with sealing tape and/or a second stack of two filter papers was placed below the mesh. Sediments were squeezed at ambient temperatures and pressures no higher than 2500 psi (17.2 MPa) to limit membrane filtration effects in clays and the release of smectite interlayer water. The interstitial water was collected into a 60 mL acid-washed plastic syringe that was attached to the bottom of the squeezer assembly. The water was then passed through a 0.45 µM disposable polytetrafluoroethylene (PTFE) filter as aliquots were distributed.

Interstitial water aliquots were collected for both shipboard and shore-based analyses (Table T7). 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 12 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 spectrometry (ICP-AES), inductively coupled plasma–mass spectrometry (ICP-MS), or major cation ion chromatography (IC). Aliquots for shore-based isotopic and transition metals analyses 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. Following treatment, all interstitial water samples were stored in a 4°C refrigerator. After interstitial water extraction was complete, sediment squeeze cakes were divided and sealed in plastic bags for archive and shore-based analyses (Table T8). Sediment samples for archive and some shore-based geochemical analyses were stored in a 4°C refrigerator; 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 C0018” chapter [Expedition 333 Scientists, 2012c]), 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. This is not the case with cores collected using HPCS but does occur with cores collected using EPCS, ESCS, and RCB. In continental margin sediments, the sulfate–methane transition (SMT) zone is generally reached at shallow depths in the sediment section, typically shallower than ~40 mbsf. 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 = element concentration in surface sea-water (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 the equations below:

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 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 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 better than 0.7%, based on repeated analysis of a standard 50 µM NaHCO₃ solution.

Sulfate and bromide concentrations were analyzed by 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 <5% and <3%, respectively. Average accuracy of bromide and sulfate was <0.0002% and <2%, 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.

Dissolved ammonium concentration was measured by colorimetry within 24 h of collecting the interstitial water sample, using an ultraviolet-visible (UV-vis) spectrophotometer (Shimadzu UV-2550) at an absorbance of 640 nm. A 0.05 or 0.1 mL sample aliquot was diluted with 1–4 mL of Milli-Q water depending on interstitial water volume, 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 of the ammonium analyses was <2% based on triplicate measurements of a standard solution.

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 1000 µL) were diluted with 1 or 0.5 mL of Milli-Q water in a plastic tube depending on the volume of interstitial water available. The mixed color development 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 of the phosphate analyses was <3%.

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). Each measurement batch included an ultrapure water blank, seven standards for calibration prepared from a commercially available cation mixed standard (Kanto Chemical Co., Inc., 07197-96), and IAPSO standard seawater (P-series) as a reference material, in addition to the interstitial water samples. IAPSO dilutions of 25%, 50%, 75%, and 100% are applied as the calibration standard. The average precision estimated by repeated measurements of IAPSO standard seawater were Na⁺ < 3%, K⁺ < 1.5%, Mg²⁺ < 1.3%, and Ca²⁺ < 1.3%, and the average accuracy of the analyses were Na⁺ < 0.2%, K⁺ < 1%, Mg²⁺ < 0.2%, and Ca²⁺ < 0.08%. Because of potentially 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% high-purity 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 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 20 m CSF, based on prior drilling results on this margin (Shipboard Scientific Party, 2001; Tobin et al., 2009, Underwood et al. 2009). Because the matrix solution was not a true blank, the procedural blank used was a dilution of 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 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 < 6%, Ba < 6%, Fe < 9%, Mn < 10%, Li < 7%, Si < 7%, and Sr < 7.

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. A 100 µL aliquot of 500 parts per billion (ppb) indium internal standard was added to the empty analysis vials before dilution. Sample aliquots were then diluted with 1% nitric acid solution to 10% in these vials (500 µL sample, 500 µL internal standard, and 4.0 mL of 0.15M 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 1% nitric acid solution to relative concentrations of 50%, 25%, 10%, 5%, and 1%. The standards were diluted with the addition of 500 µL of a 560 mM NaCl solution, 500 µL of an internal standard, and 3.5 mL of 0.15M 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 blank solution prepared with 500 µL internal standard and 4.5 mL of 0.15M HNO₃ solution. The average precision of multiple determinations of the 25% ICP-MS standard was V < 3.3%, Cu < 13.4%, Zn < 3.0%, Mo < 2.9%, Rb < 1.5%, Cs < 1.2%, Pb < 6.9%, and U < 17.7.

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