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doi:10.2204/iodp.proc.320321.102.2010

Geochemistry

The geochemistry program included characterization of volatile gases, interstitial water composition, sedimentary inorganic geochemistry including inorganic carbon, and sedimentary organic carbon. These analyses were carried out to satisfy routine shipboard safety and pollution prevention requirements, to characterize interstitial waters and sediment geochemistry for shipboard interpretation, and to provide sampling frameworks for shore-based research.

Sediment gases sampling and analysis

The organic geochemistry program monitored the compositions and concentrations of volatile hydrocarbons (C₁–C₆) and other gases (i.e., O₂ and N₂) in the sediments from headspace gas samples at typical intervals of one per core. The IODP gas sampling protocol for pollution prevention and safety as required by IODP safety regulations was modified to better constrain the concentrations of dissolved gases. The routine headspace procedure involved placing ~5 cm³ of sediment sample in a 21.5 cm³ glass serum vial that was sealed with a septum and metal crimp cap and heated at 70°C for 30 min. A 5 cm³ volume of gas from the headspace in the vial was removed with a glass syringe for analysis by gas chromatography.

Headspace gas samples were analyzed using an Agilent 6890 gas chromatograph equipped with a 2.4 m × 3.2 mm stainless steel column packed with 100–120 mesh HayeSep R and a flame ionization detector. This instrument quickly measures concentrations of methane (C₁), ethane (C₂), ethene (C₂=), propane (C₃), and propene (C₃=). The gas syringe was directly connected to the gas chromatograph via a 1 cm³ sample loop. Helium was used as the carrier gas, and the gas chromatograph oven temperature was programmed to ramp at 30°C/min from 90° to 100°C, 15°C/min from 100° to 110°C, and remain at 110°C for 4.5 min before ramping at 50°C/min to 150°C with a final holding time of 1.8 min. Data were collected and evaluated with an Agilent Chemstation data-handling program. Chromatographic response was calibrated against preanalyzed standards. Gas void samples were not taken during Expedition 320/321 because gas voids were not present in the core.

Interstitial water sampling and chemistry

Interstitial waters were extracted from 5 or 10 cm long whole-round sections that were cut and capped immediately after core retrieval on deck. In one hole at each site, two samples per core were taken from the upper 50 to 60 m CSF and from every core thereafter to total depth. When possible, a whole-round section at least 10 cm long was taken near (~1 m above) the sediment/basement contact. Occasionally, samples from more than one hole were treated as constituting a single depth profile ("splice") using CCSF-A as the depth reference if possible. Before squeezing, samples were removed from the core liner and the outer surfaces were carefully scraped off with spatulas to minimize potential contamination by the coring process. Whole rounds were placed into a titanium and steel squeezing device and squeezed at ambient temperature with a hydraulic press at pressures typically up to 20 MPa and occasionally as high as 40 MPa, if needed. Interstitial water samples were collected in acid-cleaned plastic syringes, filtered through 0.45 µm Whatman polyethersulfone disposable filters, and stored in plastic sample tubes for shipboard analyses or archived in either glass ampoules or plastic vials for shore-based analysis.

Interstitial waters were also sampled at higher resolution (up to every 10 cm) across selected lithologic transitions or in zones of pronounced interstitial chemical gradients with Rhizon samplers. These were originally developed as soil moisture samplers for root zones and they consist of thin tubes of hydrophilic porous polymer that pull water from sediment under vacuum. Rhizon samplers have recently been applied to sampling of sediment interstitial water, including during Expedition 302 (Dickens et al., 2007). Rhizon CSS-F 5 cm Core Solution Samplers (Rhizosphere Research Products) were soaked in distilled water for ~1 h before use. Rhizon samplers were wiped clean of excess water and carefully inserted through holes either drilled in the end cap or drilled through the core liner at a 55° angle so that the 5 cm porous tube was in contact with presumably undisturbed sediment away from the core liner on either side. Rhizon samples collected through the end cap were taken before the core was run through the STMSL and samples collected through the core liner were taken after the core was run through the STMSL. Acid-washed 10 mL syringes were attached to each Rhizon sampler, pulled to generate vacuum, and held open with wooden spacers. The first ~1 mL of water was discarded, with the rest collected and split for shore-based and shipboard analyses. Collection of 10–12 mL typically took 20–40 min.

Interstitial water analyses followed the procedures outlined by Gieskes et al. (1991), Murray et al. (2000), and user manuals for the new shipboard instrumentation with modifications as indicated (see "Geochemistry" in each site chapter). Interstitial water samples were analyzed for salinity with a handheld refractometer, for pH and alkalinity by Gran titration with a Brinkman pH electrode and Metrohm autotitrator, for Cl^– concentrations by titration, and for SO₄^2– concentrations by ion chromatography with a Dionex ICS-3000 ion chromatograph. H₄SiO₄ and HPO₄^2– concentrations were analyzed by an OI Analytical Discrete Analyzer (DA3500) spectrophotometer unit during Expedition 320. H₄SiO₄ concentrations were analyzed by inductively coupled plasma–atomic emission spectroscopy (ICP-AES) with minor elements (see below) during Expedition 321.

Major and minor elements were determined by ICP-AES with the newly installed Teledyne Prodigy high-dispersion ICP-AES. ICP-AES techniques for the major cations Na⁺, Mg²⁺, Ca²⁺, and K⁺ used dilutions of International Association for the Physical Sciences of the Ocean (IAPSO) standard seawater as calibration standards. Standards and samples were diluted 1:5 with distilled water and then diluted 1:10 with a 2.5% HNO₃ (by volume) solution with Sc at 10 ppm as an internal standard during Expedition 320 and with Y at 10 ppm as an internal standard during Expedition 321. Sc was used as an internal standard during Expedition 320 because we did not have sufficient Y standard solution, which was acquired for Expedition 321. For Site U1336, major cations Mg²⁺, Ca²⁺, and K⁺ were determined by ion chromatography on the 1:200 dilutions used for sulfate determinations. These values were later confirmed by ICP-AES. ICP-AES techniques for the minor elements Mn²⁺, Fe²⁺, B, Sr²⁺, Ba²⁺, and Li⁺ were modified from those described by Murray et al. (2000) by preparing calibration standards in an acidified (2.5% HNO₃, by volume) sodium chloride matrix (35 g NaCl/L) and by using 2.5% HNO₃ solution with Sc or Y at 10 ppm as an internal standard. Standards and acidified interstitial water samples were diluted for each analytical run to 1:10 at Sites U1331–U1333 and 1:5 at Sites U1334–U1338. For Sites U1331–U1335 (Expedition 320), we applied an internal drift correction using Sc for major element analyses. Interferences on both Sc lines occurred in the standard calibration for minor elements, so we did not apply an internal drift correction using Sc for minor element analyses. During Expedition 321, drift of the instrument during even long (>100 samples) runs was only on the order of a few percent and was element (wavelength line) specific. Therefore drift correction was made when necessary for both major and minor elements using the factor from a drift monitor solution (middle value standard solution) that was analyzed every 5 or 10 samples. The measured Y intensity was still useful for checking whether sample dilutions had been conducted successfully.

IAPSO standard seawater was used for calibration and quality control purposes when applicable. To assess precision of the ICP-AES technique for elements not present in IAPSO seawater at high enough concentrations and to facilitate the direct comparison of data generated during both PEAT expeditions, some interstitial water samples, including three samples analyzed during Expedition 320, were analyzed multiple times. These data and those from repeat measurements of IAPSO seawater are summarized in Table T8. Agreement is generally good between the data generated by the different expeditions except for silicic acid, which was determined by different techniques during the different expeditions (DA3500 during Expedition 320 and ICP-AES during Expedition 321). This inconsistency resulted from the failure of the DA3500 during Expedition 321 and will require shore-based measurements to be resolved. Na⁺ was also determined by charge balance, neglecting contributions by ammonium and bromide. Chemical data for interstitial water are reported in molar concentration units in each site report.

Bulk sediment geochemistry

Major and minor elements

Bulk sediment samples were taken for the determination of selected major, minor, and trace element concentrations; calcium carbonate contents; and organic carbon contents. Samples were regularly taken at a frequency of one per section (every 1.5 m) adjacent to physical property samples (typically within 5 cm). Sample intervals were occasionally adjusted depending on the prevailing lithology, and not all samples were analyzed for every element in order to best approach the scientific questions during the available time and with the available resources. Taking sediment composition samples adjacent to physical property samples is most effective for using measured analytical data (e.g., calcium carbonate content) to calibrate higher resolution multisensor track data and for the calculation of mass accumulation rates. Samples were freeze-dried for 12–24 h and ground in a mortar for subsequent analyses.

Major, minor, and trace element analyses of solid bulk samples were carried out following modifications of the methods described in Murray et al. (2000) and following procedures applied during Leg 199 (Quintin et al., 2002; Shipboard Scientific Party, 2002). Approximately 100 mg of sample (or standard) powder was added to 400 mg of a LiBO₂ flux (preweighed onshore) in a platinum vial. As a wetting agent, 10 µL of a 0.172 mM LiBr solution was added. Sample and flux were mixed in the platinum vial and melted with a NT-2100 Bead Sampler at ~1050°C for ~10 min. After cooling, the sample glass was dissolved in 50 mL 10% HNO₃ (trace metal grade). Five milliliters of this sample solution was diluted with 35 mL 2.5% HNO₃, resulting in the dilution of the freeze-dried rock powder by a factor of 4000 before analysis. Na, Mg, Al, Si, P, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Cu, Sr, Y, Zr, and Ba were measured with the ICP-AES. Calibration was achieved by analyzing solutions of international powdered rock standards subjected to the same sample preparation (Table T9). Instrumental drift was externally monitored by repeated analyses of drift standards. Reproducibility was monitored by repeated measurement of limestone standards treated as samples (Table T10).

Sedimentary inorganic and organic carbon

Inorganic carbon (IC) concentrations were determined using a Coulometrics 5011 carbon dioxide coulometer. One carbonate determination was performed typically for each 1.5 m section of core. Samples of ~10 mg of freeze-dried ground sediment were reacted with 2N HCl. The liberated CO₂ was backtitrated to a colorimetric end point. Calcium carbonate content, as weight percent, was calculated from the IC content with the assumption that all IC is present as calcium carbonate:

wt% CaCO₃ = wt% IC × 8.33

Reproducibility was determined by replicate measurements of selected samples and standards treated as samples, with typical absolute standard deviations of 0.3–0.4 wt% on sample and standard duplicates.

Total carbon (TC) content was determined using a Thermo Electron Flash EA 1112 element analyzer equipped with a Thermo Electron packed column CHNS/NCS (polytetrafluoroethylene; length = 2 m; diameter = 6 × 5) and thermal conductivity detector (TCD) on a subset of the samples used for inorganic carbon determinations. Aliquots of 10 mg of freeze-dried ground sediment in tin cups were combusted at 1800°C in a stream of oxygen. Nitrogen oxides were reduced to N₂, and the mixture of N₂, CO₂, H₂O, and SO₂ gases was separated by gas chromatograph and detection performed by the TCD. The gas chromatograph oven temperature was set at 65°C. H₂ values are not useful because they represent hydrogen from both organic matter and (clay) minerals. All measurements were calibrated by comparison to pure sulfanilamide as a standard. The reproducibility of TC measurements was determined to be ±0.03 – ±0.06 wt% (1σ; N = 5), with a typical detection limit of 0.03 wt%. Contents of total organic carbon (TOC), as weight percent, were calculated as the difference between TC and IC,

wt% TOC = wt% TC – wt% IC.

The "acidification method" of TOC analysis was also applied to examine improving analytical precision of TOC in high-CaCO₃–low-TOC sediments. Freeze-dried samples (~30 mg) in precombusted silver capsules were treated with small aliquots (10 µL) of 2N HCl at room temperature to remove CaCO₃; samples were treated with repeated aliquots until no further reaction was visible and then dried. TOC concentration was determined using a Thermo Electron Flash EA 1112 element analyzer for TC, calibrated using solutions that contain 0.071–0.281 mg of cysteine hydrochloride. During Expedition 321 the mean TOC value measured for National Institute of Standards and Technology (NIST) 1646a estuarine Sediment reference material was 0.87 ± 0.01 wt% (1σ; N = 10).

Bulk carbonate geochemistry

The trace element content of bulk carbonate leachates was measured using the ICP-AES for selected samples from Sites U1336–U1338. About 20 mg of freeze-dried powdered sample was weighed, placed in acid-cleaned centrifuge tubes, and rinsed with 10 mL of 18.2 MΩ H₂O buffered to pH 10 with ammonium hydroxide (100 µL of 30% assay per liter of water) to remove any salts using agitation with a vortex stirrer followed by a wrist-action shaker for 20 min. Following rinsing, samples were centrifuged at 40,000 rpm for 10 min, and the supernatant was discarded. Approximately 3 mL of 18.2 MΩ H₂O was then added to each tube to rinse away the buffer before leaching. Samples were briefly vortex-stirred before centrifugation and the removal of all liquid with a disposable plastic pipet. Samples were then leached with 0.5 mL of 1M acetic acid (HOAc) adjusted to pH 5 with sodium acetate (NaOAc) while being ultrasonicated at room temperature for 60–120 min. After ultrasonication until the absence of visible air bubbles, samples were left overnight before the addition of 4.5 mL of 18.2 MΩ H₂O and centrifugation at 40,000 rpm for 10 min. Between 1 and 4 mL (average = 1.5 mL) of supernatant was transferred to a clean centrifuge tube and diluted to 400 ppm Ca using the CaCO₃ weight percent data for each sample. Samples were further diluted for analysis with Y-spiked 2% HNO₃ to a Ca concentration of 100 ppm. Calibration was achieved with matrix-matched standard solutions prepared from single-element solutions. Aliquots of the NIST 1C (National Bureau of Standards 1C) limestone reference material were treated as samples for quality control purposes and results are summarized in Table T11. Reproducibility of the technique is generally better than 2% relative standard deviation and values compare well to the only comparable published values from Kryc et al. (2003). Unfortunately, Sr was present at concentrations too low in the NIST 1C standard to be measured by the technique employed here, and Mg/Ca data are not available from the Kryc et al. (2003) study, as those authors used a MgCl₂ soak prior to carbonate leaching.

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