GEOCHEMISTRY

Gas Analyses

Concentrations of light hydrocarbon gases methane (C1), ethane (C2), and propane (C3) were monitored for safety and pollution prevention. The C1/C2 ratio is particularly important for indicating potential petroleum occurrences; sediments rich in organic carbon (Corg) commonly have a ratio of >1000, whereas values <200 may indicate potential petroleum generation related to increasing depth and temperature (Pimmel and Claypool, 2001).

The standard procedure described by Pimmel and Claypool (2001) was followed for sampling headspace gases in each core. Immediately after core retrieval on deck, a ~5-cm3 sediment sample was collected using a borer tool, placed in a 20-cm3 glass serum vial, and sealed immediately in the laboratory with a septum and metal crimp cap. For consolidated or lithified samples, chips of material were placed in the vial and sealed. Prior to gas analyses, the vial was heated at 70°C for 30 min.

For volatile hydrocarbon analysis, a 5-cm3 subsample of the headspace gas was extracted from each vial using a 5-cm3 glass gas syringe and injected into a Hewlett-Packard (HP) 6890 gas chromatograph (GC3). The GC3 is equipped with a sample loop, an 8 ft x 1/8 in stainless steel column packed with divinylbenzene and N-vinyl-2-pyrollidinone (HayeSep R), a flame ionization detector, and an electropneumatic control (EPC) system. Helium was used as the carrier gas. HP Chemstation software was used for data acquisition and processing. Chromatographic responses were calibrated using commercial standards (Scotty II Analyzed Gases, Scott Specialty Gas Co.), and the results were reported in parts per million by volume (ppmv [µL/L]).

Interstitial Water Sampling and Chemistry

Shipboard interstitial water analyses were performed on 5- to 10-cm-long whole-round sections that were cut immediately after the core arrived on deck. In most cases, one whole-round section was taken from the lower third of each core. To avoid the destruction of critical intervals, whole-round sections were not removed from cores adjacent to such intervals as determined by shipboard biostratigraphy. Details of the sampling resolution are described in the individual site chapters of this volume. After extrusion from the core liner, the surface of each whole-round section was scraped with a spatula to remove potential contamination. Interstitial waters were collected using a titanium squeezer, modified after the standard ODP stainless steel squeezer of Manheim and Sayles (1974). Pressure up to 205 MPa (30,000 psi) was applied using a hydraulic press. Pore waters were passed through prewashed Whatman number 1 filters fitted above a titanium screen and subsequently extruded through a 0.45-µm polycarbonate filter into a plastic syringe attached to the bottom of the squeezer assembly. Samples for shipboard analysis were stored in plastic vials pending analysis. Aliquots for future shore-based analyses were placed in glass ampules or plastic tubes and heat-sealed.

Interstitial water samples were routinely analyzed for salinity as total dissolved solids with a Goldberg optical handheld refractometer. The pH and alkalinity were determined by Gran titration with a Brinkmann pH electrode and a Metrohm autotitrator. Dissolved chloride was determined by titration with AgCl. Sulfate (SO42–) content was quantified by ion chromatograph. Lithium (Li+), boron (H3BO3), sodium (Na+), magnesium (Mg2+), silicon (Si(OH)4), potassium (K+), calcium (Ca2+), manganese (Mn2+), iron (Fe2+), zinc (Zn2+), strontium (Sr2+), and barium (Ba2+) concentrations were determined by inductively coupled plasma–atomic emission spectroscopy (ICP-AES) following the general procedure outlined by Murray et al. (2000). In preparation for analysis by ICP-AES, aliquots of interstitial waters were acidified with nitric acid (HNO3). These acidified samples were then diluted to a ratio of 1:10 for the analysis of minor elements and to a ratio of 1:50 for the analysis of major elements (Na, Mg, K, and Ca), with 2.5% HNO3 doped with 10 ppm of Y for internal standardization.

The chemical data for interstitial waters are reported in molar units. The reproducibility of results, determined by multiple analyses of International Association for the Physical Sciences of the Ocean (IAPSO) standard seawater (alkalinity, Cl, Na+, Mg2+, K+, and Ca2+), a synthetic seawater standard (trace element ICP-AES determinations), or through the use of a calibration curve (SO42–), is available in Table T6. In addition, one interstitial water Sample 208-1262A-15H-5, 140–150 cm, was analyzed repeatedly for both major and minor elements throughout the duration of shipboard ICP-AES analyses and the reproducibility for this consistency standard is given in Table T6. The consistency standard was chosen because it contained representative concentrations of all elements analyzed and because a substantial volume was available.

Inductively Coupled Plasma–Atomic Emission Spectroscopy

The range of concentrations to be included in the suite of synthetic seawater standards was determined following consideration of concentration ranges in seawater (IAPSO certified values or Burton, 1996) and the Li, Si, Ca, and Sr data presented for interstitial water samples from Deep Sea Drilling Project Leg 74 (Gieskes et al., 1984). Li, Si, and especially Sr were found to be enriched in interstitial water samples from Leg 74 with respect to seawater. This was also the case for Ca, which was found to be as concentrated as 76.5 mM in material from the bottom of Hole 528 compared with a seawater concentration of 10.55 mM. Therefore, it was necessary to dope the IAPSO seawater used to calibrate ICP-AES analyses of the major elements (Mg, Na, K, and Ca) with CaCO3 to ensure that the potential range of Ca concentrations in the interstitial water samples would be bracketed. To assess the potential contribution of contaminants in the CaCO3 powder, a blank of CaCO3 powder was dissolved in 2.5% nitric acid (the matrix to be used in the dilution of standards and samples). Salinity is correlated to total dissolved solids and is likely to affect the intensities measured by the ICP-AES, so care was taken to ensure the calibration standards were salinity matched to the samples. Specifically, for the calibration of Mg, K, and Ca analyses, the IAPSO seawater was diluted with 2.5% HNO3 and a NaCl solution to ensure constant salinity of 35 g/kg. For the calibration of Na, it was necessary to prepare dilutions of the IAPSO seawater with 2.5% HNO3 to obtain standards of differing Na concentration.

The synthetic seawater standards for standardization of the trace and minor element signals were doped with NaCl to maintain a salinity of 35 g/kg, matching that of the pore water samples. Nitric acid doped with NaCl to 35 g/kg was run as a blank for the synthetic seawater standards, and plain nitric acid blanks were used as the blank for the samples. The trace elements that were usually analyzed during previous legs (Li, B, Si, Mn, Fe, Sr, and Ba) were included in the synthetic seawater standards. Zn at concentrations between 0.1 and 4.6 µM was also added to quantify the abundance of Zn in the interstitial water samples. Zn pore water data was collected for Sites 1262–1265 but failed to display any significant downhole trends and was characterized by stochastic noise associated with a large and variable blank. Thus, shipboard pore water Zn analyses were considered unreliable and were discontinued. The concentrations of Mn and Fe in the synthetic standards were adjusted to be above detection limits but at very low concentrations to ensure that if natural samples were encountered with low Fe and Mn contents, they would be bracketed by standard calibration points. Even so, the range of Mn and Fe concentrations encountered in Leg 208 pore waters was large and in some samples the concentrations were about twice that of the most concentrated standard. However, as the calibration curves were linear (giving r2 values of >0.99), the calibrations were considered robust enough to quantify unknowns above their range.

Sediment Elemental Analysis

Freeze-dried and powdered squeeze cake sediment samples with CaCO3 contents >80% from Sites 1263 and 1264 were leached using dilute (~0.1%) nitric acid in an attempt to dissolve the carbonate fraction. The leachate was analyzed for the same ions as the interstitial water samples using ICP-AES with the exception of Li, Si, and Zn, which were present below blank levels. The reproducibility of multiple analyses of a synthetic carbonate standard is presented in Table T7. In addition, six separate splits of interval 208-1264A-14H-5, 140–150 cm, were analyzed with the other samples, and the reproducibility of this consistency standard is given in Table T7. The Ca2+ content of the leachate was intended to be used for calculating CaCO3 content by assuming that all Ca2+ was derived from CaCO3. However, the resulting Ca concentrations for all samples vary between ~48% and 53% Ca, indicating excess Ca was leached from a source other than the carbonate fraction (CaCO3 consists of only 40% Ca). Therefore, the data cannot be used to calculate CaCO3 content and results should be viewed as representing the carbonate plus other fractions with respect to the other elements analyzed.

Sediment Carbonate Analysis

The weight percentage of inorganic carbon (IC) in bulk sediment samples was determined using a Coulometrics 5011 CO2 coulometer equipped with a System 140 carbonate analyzer. A total of ~10–12 mg of freeze-dried, ground sediment was reacted with 2-N HCl to liberate CO2. The change in light transmittance monitored by a photodetection cell controlled the CO2 titration. The percentage of carbonate was calculated from the IC content using the following equation:

CaCO3 (wt%) = IC (wt%) x 8.33.

This method assumes that all of the CO2 evolved was derived from dissolution of calcium carbonate. No corrections were made for other carbonate minerals.

Sediment Organic Carbon Analysis

Total carbon (TC) content was determined using a Carlo Erba 1500 CHNS analyzer, which combusts sediment samples in tin cups with an oxidant (V2O5) at 1000°C in a stream of oxygen. Nitrogen oxides were reduced to N2; the mixture of N2, CO2, H2O, and SO2 gases was separated by gas chromatography; and detection was performed by a thermal conductivity detector. The H2 value is not useful because it represents hydrogen derived from organic matter as well as water bound to clay minerals.

The analytical procedure employed a new combustion column for each sample batch. An aliquot of 5–15 mg of freeze-dried, crushed sediment with ~10 mg of V2O5 oxidant was combusted for each sample. All measurements were calibrated by comparison to a pure sulfanilamide standard. The amount of total organic carbon (TOC) was calculated as the difference between TC and IC (determined from coulometry) using the following equation:

TOC (wt%) = TC (wt%) – IC (wt%).

Sediment Solvent-Extractable Components and Gas Chromatography–Mass Selective Detector

Solvent-extractable organic constituents were examined from selected samples to assess the characteristics of the organic matter present. The focus of these analyses was to qualitatively characterize organic matter, identify hydrocarbons and other components present, and determine their relative abundance. No quantitative determination of the concentrations of individual constituents was made.

Extraction procedures were as follows: ~10–50 g of sediment was extracted ultrasonically using CH2Cl2 (~80–100 mL) for 1 hr. A 12-hr extraction was performed on samples from some sites for characterization of heavier compounds. The extract was reduced to dryness under N2 and then transferred in hexane to a column packed with 0.9–1.4 g of silica gel (deactivated with 5% of nanopure water) in a disposable pipette to remove polar constituents and recover hydrocarbon and ketone fractions by successive elution with hexane (2.5 mL) and CH2Cl2 (4 mL). Each eluate was taken to near dryness under N2 and transferred in hexane (20–50 µL) to a vial (with small-volume insert) for analysis by gas chromatography with a mass selective detector (GC-MSD). The hexane eluate predominantly contains aliphatic hydrocarbons, and the CH2Cl2 eluate contains aromatic hydrocarbons and ketones.

An HP 5973 GC-MSD system consisting of an HP 6890 GC with an MSD and an HP 7683 automatic liquid sampler generated mass chromatograms, a total ion chromatogram for each eluate, and mass spectra for compounds detected. The GC is equipped with an EPC system, a split-splitless injector, and an HP capillary column (5% phenylmethylsiloxane; 30 m x 0.25 µm; HP-5) programmed from 70° to 130°C at 20°C/min, then at 4°C/min to 320°C, and held isothermally at 320°C for 10 min. Helium is used as the carrier gas. The transfer line is set at 280°C, and the source of the MSD is set at 230°C. The MSD scanned from m/z (mass to charge ratio) 28 to 700. The HP MS Chemstation software was used for data acquisition and processing. The identity of individual hydrocarbons was determined from mass spectral characteristics and GC retention times by comparison with the literature.

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