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doi:10.2204/iodp.proc.339.102.2013

Geochemistry

The shipboard geochemistry program for Expedition 339 included characterization of (1) volatile gases, (2) interstitial water composition, and (3) sedimentary inorganic geochemistry including inorganic carbon, sedimentary organic carbon, and C/N ratios. 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 samples for shore-based research. Nonstandard sampling and analyses were also carried out to address some of the specific research goals of this expedition, namely high-resolution sampling and analysis of interstitial water for stable isotopes and chlorinity.

Sediment gases sampling and analysis

The organic geochemistry program monitored the compositions and concentrations of volatile hydrocarbons (C₁–C₆) and other gases in the sediments from headspace gas samples at intervals of typically one per core. 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. After heating, a 5 cm³ volume of gas from the headspace in the vial was removed with a glass syringe for analysis by gas chromatography. A second gas sampling procedure was used for gas pockets or expansion voids that appeared in the core while it was still in the core liner. A device with a heavy-duty needle was used to penetrate the core liner, and an attached syringe was employed to collect the gas.

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 80/100 mesh HayeSep R and a flame ionization detector. The 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 through a 1 cm³ sample loop. Helium was used as the carrier gas, and the gas chromatograph oven temperature was programmed to start with an oven temperature of 80°C held for 8.25 min before ramping at 40°C/min to 150°C, with a final holding time of 5 min. Data were collected and evaluated with an Agilent Chemstation data-handling program. Chromatographic response was calibrated against known standards.

Interstitial water sampling

Routine interstitial water was extracted from 5–15 cm long whole-round sediment sections that were cut and capped immediately after core retrieval on deck. Standard whole rounds are 5 cm long, but as porosity decreased downhole, the size of the whole-round sections was increased to enable extraction of the needed ~30 mL of water to split between shipboard and shore-based analyses. Samples were taken from near the bottom of each core for the upper 150 m and at intervals of every third core thereafter to total depth, with modifications as indicated (see “Geochemistry” in each site chapter). Samples from more than one hole were treated as constituting a single depth profile (“splice”) using CSF-A as the depth reference if possible.

High-resolution interstitial water sampling was done at Sites U1385, U1386, U1389, and U1391. For high-resolution interstitial water sampling, small plugs of sediment (~10 cm³) were taken in the upper 150 m from one of the holes from the bottom of each section using a 60 mL syringe, excluding the section from which the whole round came (Fig. F12). A 25 mm diameter wire was inserted through two holes drilled at the end of the syringe in order to facilitate clean removal of the sediment. When the syringe was completely inserted into the core (and full of sediment), the syringe was rotated before removal to cut the sample cleanly from the section. Sediment plugs were taken on the catwalk, before running the core through the STMSL. This sampling technique was used to obtain high-resolution interstitial water samples while minimizing impact on the continuity of the core. No acetone was used to seal the end caps of the cut cores until after all interstitial water had been extracted because organic solvents can interfere with the spectroscopic analysis of stable water isotopes.

At Site U1385, interstitial water was also sampled using Rhizon samplers (Rhizosphere Research Products), consisting of a hydrophilic porous polymer tube (Seeberg-Elverfeldt et al., 2005). Two methods were employed at this site in order to assess and compare their performance in five areas:

1. Contamination of interstitial water chemistry,

2. Amount of disturbance to the sediment and stratigraphy,

3. Amount of water recovered,

4. Ease of implementation, and

5. Efficiency/disruption to core flow.

Rhizon samplers were carefully inserted through holes drilled in the core liner. Syringes were attached to each Rhizon sampler with a Luer-lock, pulled to generate vacuum, and held open with wooden spacers. Samplers were left in place during the core temperature equilibration (~3 h). We found that emptying the syringes and re-pulling vacuum on the Rhizon samplers at intervals of ~30 min to 1 h increased their yield. The Rhizon samplers were used in sets of three spaced 3 cm apart at the center of each section (i.e., 75 cm). Water from all three samplers was combined into one sample and shaken to mix before analysis and splitting. Contrary to the methods used on previous expeditions, the Rhizon samplers were used dry in order to avoid sample contamination from presoaking. In qualitative tests done during Expedition 339, we found that flow rate through the Rhizon samplers did not depend on presoaking. Further, stable water isotope measurements were sensitive to the isotopic values of the solution in which the Rhizon samplers were presoaked, even when the first few milliliters were discarded from the syringe during sampling. That is, after an initial sampling period using a presoaked Rhizon, the syringe was detached from the Rhizon, a few milliliters of water were discarded, and then the syringe was reattached and a “fresh” sample was taken. The isotopic measurements of this “fresh” sample were different than those of the sample taken with a dry Rhizon sampler. Because of the low total water volume recovery, the presoaking fluid cannot be flushed completely from the Rhizon sampler in order to recover an uncontaminated measurement.

In the shipboard laboratory, whole-round sediment samples were removed from the core liner and the outside surfaces (~1 cm) of the sediment samples were carefully scraped off with spatulas to minimize potential contamination with drill fluid. The drill fluid used was surface seawater, which had significant sulfate concentration at all sites; therefore, contamination of samples below the sulfate reduction zone was inferred when there were small deviations from 0 in the sulfate profile. Scraped whole-round sediment samples were placed into a Manheim titanium squeezer and squeezed at ambient temperature with a Carver hydraulic press (Manheim and Sayles, 1974), reaching pressures typically up to 200 MPa and occasionally as high as 300 MPa, if needed. Interstitial water samples discharged from the squeezer were passed through 0.45 µm polyethersulfone membrane filters, collected in plastic syringes, and stored in plastic sample tubes for shipboard analyses or archived in flame-sealed glass ampules for shore-based analyses. Samples used for analysis by inductively coupled plasma–atomic emission spectrometer (ICP-AES) were acidified with HNO₃ in order to prevent precipitation of element complexes.

Interstitial water chemistry

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 digital refractometer made by Index Instruments Ltd., for pH and alkalinity by Gran titration with a Brinkman pH electrode and Metrohm 794 Basic Titrino autotitrator, for Cl^– concentrations by titration against silver nitrate using a Metrohm 785 DMP Titrino Autotitration system, and for SO₄^2– concentrations by ion chromatography with a Dionex ICS-3000 ion chromatograph. Alkalinity is reported throughout as milliequivalences per liter (meq/L), which means the equivalent millimols of hydrogen ion added (in the form of HCl) to bring 1 L of sample to the reaction’s final endpoint. Measurements by the Dionex were made on autodilutions of 1/200 (v/v) sample to deionized water. For Cl^– titrations, 0.3–0.5 mL samples of interstitial water were diluted with 30 mL of dilute nitric acid to keep precipitated flocculent well separated, which increased the probability of contact between Cl^– and Ag⁺. Dissolved ammonium concentration was determined spectrophotometrically (Gieskes et al., 1991) using an Agilent Cary 100 ultraviolet-visible light (UV-VIS) spectrophotometer.

Major and minor elements were determined with the 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. For major element analysis on the ICP-AES, standards and acidified samples were diluted 1:100 (v/v) with a 2% HNO₃ (by volume) solution (matrix) with Y at 10 ppm as an internal standard. Calibration for the minor elements Mn²⁺, Fe²⁺, B, Si, Sr²⁺, Ba²⁺, and Li⁺ was done with dilutions of a multielement synthetic standard solution (composed of single-element standards). Acidified samples measured for minor elements on the ICP-AES were diluted 1:10 (v/v) with the same matrix used for the major element analysis. Drift correction was made for both major and minor elements using the factor from a drift monitor solution (high-value standard solution) that was analyzed every five samples. The ICP-AES autosampler and analysis chamber were rinsed with a 3% (by volume) HNO₃ solution between samples. Major cations Mg²⁺, Ca²⁺, K⁺, and Na⁺ were also determined by ion chromatography on the same 1:200 dilutions used for sulfate determinations. Because values measured on both the ion chromatograph and the ICP-AES agreed well at Sites U1385–U1387, the major cations were only measured on the ion chromatograph for Sites U1388–U1391.

Typical precision of digital refractometer measurements is 0.1% (1‰). On the ion chromatograph, typical internal precision (standard deviation/average measured value) from eight measurements of a standard for Na⁺, K⁺, Mg²⁺, Ca²⁺, and SO₄^2– was, respectively, <0.5%, <1%, <1%, <2%, and <1.5%. Replicates of IAPSO alkalinity measurements throughout the expedition were usually within 2% of the theoretical value ([measured – theoretical]/theoretical), with a maximum measured error of 5%. Chloride standard and sample replicate maximum variation using titration was generally <0.8% (maximum difference/average) and often as low as or lower than 0.2%, particularly for the higher concentration samples. Internal precision on the ICP-AES measurements (four replicates) of minor major and minor elements was <3%. Typical external precision for major elements measured on the ICP-AES was <3% (relative standard deviation [RSD]). The spectrophotometer internal precision (RSD) was 0.01% and the external precision was <0.5%.

Chemical data for interstitial water are reported in molar concentration units in each site report. No water samples were weighed over the course of the expedition; all measured dilutions were done volumetrically, either by autodilution or pipetting.

Water isotope analysis

Oxygen and hydrogen isotope measurements of interstitial water were made for the first time aboard the JOIDES Resolution by cavity ringdown laser spectroscopy (CRDS). CRDS is a time-based measurement system that uses a laser to quantify spectral absorption lines unique to H₂¹⁶O, H₂¹⁸O, and HD¹⁶O in an optical cavity (Gupta et al., 2009). The equipment consisted of an L1102-i Picarro water isotope analyzer manufactured in July 2009 (serial number 202-HBDS033; 200-CPVU-HBQ33), an A0211 high-precision vaporizer manufactured in August 2011 (serial number VAP 292), and a CTC HTC-Pal liquid autosampler (serial number 142552). The Picarro L1102-I measures δ¹⁸O, δD, and total H₂O concentration simultaneously. The bench-top instrument is field deployable, and shock and vibration tests indicate the ship environment should have no impact on instrument performance. Guaranteed precision for liquid water using the Picarro L1102-i with autosampler injection is <0.1‰ for δ¹⁸O and <0.5‰ for δD. Guaranteed drift is less than ±0.3‰ for δ¹⁸O and less than ±0.9‰ for δD. Precision and drift are defined based on the standard deviation and range (maximum – minimum) of the average values for 12 injections of the same water sample (tap water) measured 12 times, which is equivalent to 144 injections averaged in blocks of 12.

During Expedition 339, approximately 500 µL of filtered interstitial water was loaded in a 2 mL glass vial with septum top and placed in the autosampler. Each water sample was injected into the vaporizer nine times. Memory effects from previous samples were avoided by rejecting the first three results and averaging the final six injections. An internal seawater standard (SPIT) was analyzed between each unknown sample to correct for drift. Each value measured on an unknown sample was normalized to the mean of the two adjacent standards. Analysis of each sample, consisting of nine injections, took 90 min. Three hours per sample is required if one includes the time needed to measure bracketing standards. The vaporizer septum was changed regularly, after no more than 300 injections. Considerable salt buildup occurred in the vaporizer, which necessitated periodic cleaning. We used two vaporizers so that one could be cleaned while the other was analyzing samples.

The instrument was calibrated using three working standards from the University of Cambridge (United Kingdom) with known values: Delta (δ¹⁸O = –27.6; δD = –213.5), Botty (δ¹⁸O = –7.65; δD = –52.6), and either standard mean ocean water (SMOW) or SPIT (δ¹⁸O = 0; δD = 0). The δ¹⁸O and δD of SPIT is indistinguishable from SMOW within analytical error. Because the Picarro analyzer is extremely linear, it is only necessary to use three calibration standards. The calibration line was determined by subtracting the measured values of SPIT from each of the standards and deriving a regression equation forced through the origin. Over the duration of the expedition, the slope of the δ¹⁸O regression varied between 1.051 and 1.083 (average = 1.067), whereas the δD slope varied from 1.129 to 1.160 (average = 1.147). Measured δ¹⁸O and δD were corrected to Vienna SMOW in parts per thousand (‰) by multiplying the SPIT-normalized value by the slope of the calibration line. Because organic compounds can cause spectroscopic interference in CRDS and affect isotopic results, we processed the data using Picarro’s ChemCorrect software that identifies irregularities caused by hydrocarbons. Despite significant amounts of methane in headspace samples, interstitial water samples were not flagged as being contaminated by the ChemCorrect software, suggesting that methane gas was lost during the interstitial water sampling and squeezing process.

Figures F13 and F14 show the measured δ¹⁸O and δD of the SPIT standard measured for approximately a 1.5 month period. The measured δ¹⁸O and δD values vary considerably, with a range of 3‰ and 13.6‰, respectively. Shifts in values are expected between runs as instrument conditions vary, but jumps also occurred in the middle of some runs. The cause of this atypical behavior could not be determined onboard. As shown in Figure F14, the times of the shifts did not correspond to power outages (PO) or auto-sampler septum changes (S). The shifts in isotopic value were corrected by normalizing to the mean of adjacent values. In the normalized data, the shifts largely disappeared except for samples straddling the boundaries of the shift. Care was taken to eliminate any sample data run near one of these shifts. The standard deviation (1σ) of the normalized SPIT values over the 1.5 month period was ±0.2‰ for δ¹⁸O and ±0.6‰ for δD. At Site U1386, we replicated seven samples (10% of the samples taken), and the mean difference of the absolute values of replicate analyses was 0.05‰ for δ¹⁸O and ±0.4‰ for δD. Additional replication will occur postcruise to determine the reproducibility of the measurements.

Bulk sediment geochemistry

Sedimentary inorganic and organic carbon

Inorganic carbon concentrations were determined using a Coulometrics 5011 carbon dioxide coulometer. Samples of ~10 mg of freeze-dried ground sediment were reacted with 2N HCl. The liberated CO₂ was back-titrated to a colorimetric end point. Calcium carbonate content, as weight percent, was calculated from the inorganic carbon (IC) content with the assumption that all inorganic carbon is present as calcium carbonate,

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 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 mm × 5 mm) 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 chromatography and detection performed by the TCD. The gas chromatograph oven temperature was set at 65°C. H₂ values represent hydrogen from both organic matter and (clay) minerals. All measurements were calibrated by comparison to pure sulfanilamide as a standard. The reproducibility of total carbon measurements was determined to be from ±0.03 to ±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 total carbon (TC) and inorganic carbon,

The “acidification method” of TOC analysis was not applied for direct analysis. 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; however, the available silver capsules disintegrated, resulting in sample loss.

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