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doi:10.2204/iodp.proc.340.102.2013

Geochemistry

Interstitial water collection

In general we attempted to take one or two samples from each 10 m core for pore water analyses; however, many of the collected cores were deemed inappropriate for pore water sampling because of sediment lithology (e.g., sand, debris, etc.). Pore fluid samples were extracted by squeezing whole-round sections. Once the core was on the catwalk, a 10–15 cm length of whole-round core was cut from the end of one to two sections per core. The whole rounds were transferred to the chemistry laboratory for squeezing. Whole-round sections were processed in a nitrogen-filled glove bag for sediment transfer into the squeezing apparatus. A hydraulic press was then used to squeeze the fluids from the sediment (e.g., Manheim, 1966). During squeezing, pore fluid was passed though one or two paper filters placed over titanium support screens. Following extraction, pore fluids were filtered through 0.5 µm syringe filters into various sample containers. Samples for inductively coupled plasma–optical emission spectrometry (ICP-OES) were acidified by adding ~10 µL of trace metal grade HNO₃. In addition to the standard shipboard analyses (below), additional subsamples were collected as available water allowed. These samples were likewise acidified for shore-based analyses with Optima Grade HCl.

Interstitial water analysis

Salinity was estimated by measuring the sample refractive index. This measurement was done following sample processing on a ~100 µL sample using an Index Instruments Limited digital refractometer or by a handheld refractometer. In general, these two approaches appeared comparable. International Association for the Physical Sciences of the Oceans (IAPSO) standard seawater salinity values from both techniques yielded a value of 37 ppt, and there is a reasonable uncertainty of ±1 ppt for the handheld refractometer. Given these uncertainties, salinity data should be treated with caution. pH was measured with a combination glass electrode immediately prior to performing the alkalinity titration, and alkalinity was measured via Gran titration using a Metrohm autotitrator on ~3 mL of sample.

Sulfate, Cl, Mg, Ca, K, and Na were measured at sea using a Dionex ICS-3000 ion chromatography system. However, we do not report those data in this report, as the instrumentation produced highly erratic results. Figure F5 provides two comparison profiles of the shipboard and shore-based data. Although not all data exhibit this level of disagreement, these profiles demonstrate the general symptom that the shipboard data were less precise. The shore-based results presented here for total sulfur (ΣS), Na, Mg, Ca, and K were analyzed at Oregon State University. Instrumentation used was a Leeman Labs Prodigy ICP-OES. Emission lines were measured in radial view for all elements with the exception of ΣS, which was analyzed in axial mode. It is worth noting that the ICP sulfur measurement is not strictly speaking sulfate, as this instrumentation does not discriminate among the different species. However, for practical purposes, the dominant S species in the majority of our samples is sulfate. Thus, for clarity we refer to the ICP sulfur determination as ΣS. Reported uncertainties for the major cations and ΣS are derived in one of three ways. The first method utilizes (1) the regression uncertainty, which is calculated as the regression standard error, and (2) the uncertainty calculated from the standard deviation of three replicate analyses from each sample tube. These two factors are combined as the square root of the sum of squares. For the second and third assessments of uncertainty, we report the mean and standard deviation of a set of samples that were run either on the same or separate days. Samples using these latter two assessments are identified as such within the individual site pore fluid data tables. We note that the coefficient of variation for these analyses typically lies between ~1% and 2% for each of the major cations reported here, which is somewhat higher than our other measures of precision. For Sample 340-U1395B-10H-2, the duplicate run produced particularly high values for each of the analytes, and we suspect that the second analysis may have suffered from sample evaporation and the reanalyzed value is not included in our analyses here. To further assess precision, we also ran a set of IAPSO samples as unknowns. For 21 determinations (over three separate run dates), the mean and 1σ standard deviation for Na = 467.5 ± 2.5 mM, Mg = 53.8 ± 0.3 mM, K = 10.0 ± 0.1 mM, Ca = 10.4 ± 0.1 mM, and ΣS = 28.4 ± 0.3 mM. Chloride data reported here were analyzed on board the ship by the silver nitrate titration method using IAPSO standard seawater as a quality control reference. Dissolved ammonium was measured using standard colorimetric methods (Gieskes et al., 1991) on an Agilent Technologies Cary Series ultraviolet/visible light spectrophotometer.

Solid-phase geochemistry

For solid-phase analyses, a 5 cm³ plug sample was taken for carbonate, carbon-hydrogen-nitrogen analyzer (CHN), and XRD analyses. Samples were freeze-dried for ~24 h and ground by hand using an agate mortar and pestle in preparation for solid-phase analyses.

Inorganic carbon concentrations were determined using a Coulometrics Inc. CO₂ coulometer. Approximately 10–100 mg of dried, ground sediment was reacted with 2 M HCl, and the liberated CO₂ was titrated to an end point determined using a photodetection cell. CaCO₃ weight percentage was calculated from the inorganic carbon content, assuming that all evolved CO₂ was derived from calcium carbonate as follows:

where IC = inorganic carbon. This approach does not discriminate among carbonate minerals; thus, all the evolved CO₂ is treated as though it is derived from CaCO₃.

Total carbon and nitrogen were determined using a ThermoElectron Corporation FlashEA 1112 CHNS elemental analyzer with calibration using the Thermo Soil Reference Material NC standard (338 40025 Lot N12A). Sample analyses were performed on ~10–20 mg of dried, ground sediment. Total organic carbon content was estimated as the difference between inorganic carbon and total carbon.

XRD analyses were done using a Bruker D-4 Endeavor diffractometer with a Vantec-1 detector using Ni-filtered CuKα radiation. Rapid mineral identifications (e.g., carbonates, hydrothermal alteration minerals, magmatic silicates, and oxides) were performed using EVA 13 software (Diffrac plus evaluation package). For Expedition 340 the generator voltage was 35 kV with a generator current of 40 mA. The scan was continuous from 4° to 70°2θ with a step size of 0.0174°, scan speed of 1 s/step, and a divergence slit of 0.6 mm. Mineral identification was performed using EVA software, with complementary mineral identification then performed using the Mineral Library of EVA Software.

Four main types of mineralogical associations were defined on the basis of XRD observations, and reference spectra given in Figure F6, F7, and F8 were used to rapidly investigate the data set. The first is a carbonate type (Type I) with minor or no volcanic minerals (Fig. F6). This type mainly consists of calcite and Mg-rich calcite with the possibility of some aragonite. Relative abundances of the different carbonates may be directly estimated from the 30°–40°2θ zone of the spectra (Fig. F9). Clay minerals (smectites, kaolinite, and glauconite) are often present in low amounts (mainly represented as bulges at low 2θ values). Illite and chlorite were not identified in the collected samples. The second is a volcanic type (Type II) with minor carbonate components (Fig. F7). This type contains plagioclase, amphibole (Hb), and pyroxenes (mainly orthopyroxene). Sometimes plagioclase is present alone. Quartz is a ubiquitous mineral phase in Sites U1397–U1400 but is absent in Sites U1393–U1396. A mixed type of mineral association (Type III, not represented) is a mixture of the previous types that were present in similar amounts. Finally, a clay minerals type (Type IV) is relatively rare and consists of the dominant clay minerals smectite, kaolinite, glauconite, and rarely halloysite or dolomite (Fig. F8). Also of note were numerous mineral phases such as orthopyroxene, pyrite, and Fe-Ti oxides, present in significant amounts as evidenced by visual observations, which are difficult to identify in the general spectra overview and necessitate longer investigations for identification in XRD spectra.

Microbiological sampling

Microbiological samples were only taken at Sites U1394–U1396 for shore-based analyses. The Operations Superintendent was notified prior to the intent to take microbiological samples, and a bag of fluorescent microspheres was added to the core barrel. As the APC was advanced, the bag burst and the microspheres were carried around the margins of the sediment core by circulating surface seawater. The penetration distance of microspheres into the interior portions of the core provided a measure of potential contamination by seawater prokaryotes (House et al., 2003).

Approximately nine microbiology samples were taken from each of these three sites. A 10 cm section of whole-round core was transferred to the N₂ bag for sampling. The exposed end of the core was scraped away using a sterile sampling spatula. A total of 3–4 cm³ of core material from the center of the core was then transferred to a sterile centrifuge tube and stored in the –80°C freezer. A further 20 cm³ of sediment was taken from the center of the core and transferred to Pyrex bottles and sealed with a rubber stopper. These samples were stored in a cold room or refrigerator at 0°C. Approximately 3–4 cm³ of sediment was also placed in a centrifuge tube for complementary geochemical analyses to the microbiology.

Headspace hydrocarbon gases

We analyzed one sample per core for headspace hydrocarbon gases as part of standard IODP shipboard safety monitoring (Kvenvolden and McDonald, 1986). For these analyses a ~3–5 cm³ sample was collected after the core was brought on deck. This sample was typically extracted from the exposed end of Section 1 of each core. The sample was placed into a 20 mL glass vial, which was in turn placed in an oven at 70°C for 30 min. Hydrocarbon gases (C1–C3) were analyzed using an Agilent/HP 6890 Series II gas chromatograph (GC3) equipped with a flame ionization detector at 250°C. The column was a 2.4 m × 3.2 mm stainless steel column packed with 100/120 mesh HayeSep R.

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