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doi:10.2204/iodp.proc.301.105.2005

Biogeochemistry

Interstitial water samples

Shipboard IW samples were obtained from 20–40 cm long whole-round intervals cut on the catwalk, capped, and taken to the laboratory for immediate processing. Details on sampling resolution are described in the individual site chapters of this volume. When there were too many IW intervals to process immediately, the capped whole-round intervals were stored temporarily in the refrigerator.

Processing of sediment for IW sampling in the laboratory was carried out in a nitrogen-flushed glovebag. After extrusion from the core liner, the outer layer of each whole-round interval was carefully scraped with a spatula to remove potential contamination from drill water (surface seawater). Remaining sediment was then placed into a titanium squeezer, modified after the standard stainless steel squeezer of Manheim and Sayles (1974). The piston was positioned on top of the squeezer, and the entire unit was removed from the glove bag and placed on the hydraulic press. Pressures of up to 76 MPa were applied in the squeezer, calculated based on the measured hydraulic press pressure and the ratio of the piston areas of the hydraulic press and the squeezer. Interstitial water was passed through a prewashed Whatman number 1 filter above a titanium screen, filtered through a 0.45 µm Gelman polysulfone disposable filter, and subsequently extruded either into a 50 mL gas-tight glass syringe or into a precleaned (10% HCl) 50 mL plastic syringe. Both types of syringes were attached to the bottom of the squeezer assembly via a three-way plastic valve. The 50 mL glass syringe was used to collect the first fraction of IW for analyses of organic components. This procedure was implemented in order to prevent the release of organic contaminants from plastic syringes and the loss of ephemeral constituents during the squeezing process. After collection of IW, the syringe was removed to dispense aliquots for shipboard and shore-based analyses.

Samples were stored in acid-cleaned plastic vials pending shipboard analyses. Aliquots for future shore-based trace metal and elemental analyses were placed in triple acid-washed plastic vials and acidified with 4 mL of subboiled 6N HCl per liter of sample. Samples for organic and carbon isotopic analyses were placed in precombusted glass vials with polytetrafluoroethylene-coated screw caps.

Interstitial water analyses

Interstitial water samples were analyzed routinely according to standard shipboard procedures (Gieskes et al., 1991). The pH was determined by ion-selective electrode. Alkalinity was determined by Gran titration with a Metrohm autotitrator.

Dissolved total carbon (TC), total inorganic carbon (IC), and total organic carbon (TOC) were measured using a Shimadzu TOC analyzer TOC-5000. Aliquots of 0.5 to 1.0 mL of IW were diluted manually to 5 mL. Solutions were then injected with the Shimadzu ASI-5000A autosampler using volumes of 26 µL and 33 µL for TC and IC, respectively. The total carbon was combusted at 680°C in the TC combustion tube filled with Pt catalyst to become CO₂. For the inorganic carbon analysis, only the IC component of the sample was converted to CO₂ into a reaction vessel containing a solution acidified with H₃PO₄. The CO₂ was then detected by a nondispersive infrared gas analyzer. The amount of total organic carbon was calculated as the difference between total carbon and inorganic carbon.

Concentrations of sulfate and of the major cations calcium, magnesium, potassium, and sodium were determined by ion chromatography after manual dilution of IW samples (200×). The instrumental setup involved either a Dionex AS4 column for anion exchange or a Dionex CS12A column for cation exchange on a Dionex DX-120 ion chromatograph equipped with a Spectrophysics autosampler. Chloride analysis was carried out by potentiometric titration using silver nitrate and a Mettler Toledo DL25 titrator equipped with a silver ring electrode (Mettler/Toledo ME 89599) and 2M KNO₃ electrode filling solution. All quantifications were based on comparison with International Association of the Physical Sciences of the Ocean (IAPSO) standard seawater.

Dissolved phosphate and ammonium concentrations were determined by spectrophotometric methods using a Milton Roy Spectronic 301 spectrophotometer. In order to account for the limited volume of IW samples, these analyses were carried out on samples that had already been titrated for alkalinity and were thus acidified, degassed, and in a pH range appropriate for colorimetric determination of phosphate by the phosphomolybdate blue method.

Selected major, minor, and trace elements were analyzed using the JY2000 ICP-AES. Concentrations of Ba, B, Fe, Li, Mn, Si, and Sr were determined following the procedures outlined by Murray et al. (2000). For these analyses, the shipboard "Master" ICP standard (Murray et al., 2000; modified by M. Delaney) was expanded so that Si concentrations could be determined. In preparation for analysis by ICP-AES, aliquots of IW were acidified with nitric acid and diluted tenfold with nanopure water for minor elements. Analytical blanks were prepared identically by analyzing deionized water, which was acidified to matrix match the samples.

Gas analyses

Concentrations of the light hydrocarbon gases (methane, ethane, propane, and propene) were monitored for safety and pollution prevention. Sampling of headspace gases followed the standard procedures described by Kvenvolden and McDonald (1986). Upon core retrieval, a 3 mL sediment sample was collected with a syringe or borer tool from a freshly exposed end of a core section. After withdrawing the syringe, the plunger was advanced slightly to extrude a small amount of sediment. This excess was shaved off with a flat spatula flush with the end of the syringe barrel to provide an accurate determination of the sediment volume within the syringe. The sample was then extruded into a 20 mL glass serum vial and sealed immediately with a septum and metal crimp cap. For consolidated or lithified samples, chips of material were placed in a vial and sealed. Prior to gas analysis, vials were heated to 60°C for 20 min.

For gas chromatographic analyses, a 5 mL subsample of headspace gas was extracted from the vial using a standard gas syringe. Concentrations of methane, ethane, ethene, propane, and propene were analyzed using a Hewlett Packard 6890 Plus gas chromatograph (GC) equipped with a 25 µL sample loop, an 8 ft × ⅛ inch stainless steel column packed with HayeSep R (80–100 mesh), and a flame ionization detector. The carrier gas was helium, and the GC oven was programmed from 100°C (5 min hold) to 140°C (4.5 min hold) at a rate of 50°C/min. Data were collected using a Hewlett-Packard 3365 chromatography data processing program. Chromatographic responses were calibrated using commercial standards (analyzed gases from Scott Specialty Gas Co.) and the results reported in parts per million by volume (ppmv [µL/L]).

The concentration of methane in interstitial water was derived from the headspace concentration by the following equation:

where

V_H = volume of the sample vial headspace,

V_S = volume of the whole sediment sample,

χ_M = molar fraction of methane in the headspace gas (obtained from GC analysis),

P_atm = pressure in the vial headspace (assumed to be the measured atmospheric pressure when the vials were sealed),

R = the universal gas constant,

T = temperature of the vial headspace in degrees Kelvin, and

ϕ = sediment porosity (determined either from moisture and density measurements on adjacent samples or from porosity estimates derived from gamma ray attenuation [GRA] data representative of the sampled interval).

Quantities of methane that remain undetected because of dissolution in the aqueous phase are minimal (e.g., Duan et al., 1992) and are not accounted for. The internal volume of the employed headspace vials was measured beforehand and was determined to be 21 mL. This volume was taken as constant in calculations of gas concentrations.

Sediments

Sediment samples were analyzed according to the standard methodology employed during previous ODP legs. IC concentration was determined by titration using a Coulometrics 5011 CO₂ coulometer. Approximately 10 mg (±10%) of freeze-dried, ground sediment was weighed using a gimbaled Cahn C-31 microbalance and then reacted with 2N HCl to release CO₂. Purified air used as the carrier gas was passed through a KOH solution to remove CO₂. Sample gas was then scrubbed of SO₂ through a 3% AgNO₃ solution before being transferred into a coulometric (cathode) cell filled with a monoethanolamine (ME) proprietary solution used as the colorimetric solution. In this cell, the CO₂ was quantitatively absorbed, where it reacted with the ME solution to form a titratable acid, causing the blue color to fade. A photodetector monitored the color change as percent transmittance (%T). As %T increased, a titration current was automatically activated to generate a base from an adjacent anode cell at a rate proportional to %T. The endpoint of the titration was determined when %T returned to the original setpoint of 29. Calcium carbonate, expressed as weight percent, was calculated from the IC content, assuming that all evolved CO₂ was derived from dissolution of CaCO₃, by the following equation:

No correction was made for the presence of other carbonate minerals.

TC, nitrogen, and sulfur concentrations were determined using a Carlo Erba 1500 CHNS elemental analyzer. Approximately 10 mg of freeze-dried, ground sediment was weighed again, using the Cahn C-31 microbalance, and combusted at 1000°C in a stream of oxygen. Nitrogen oxides were reduced to nitrogen, and the mixture of carbon dioxide, nitrogen, and sulfur dioxide was separated by gas chromatography and detected by thermal conductivity detector (TCD). All measurements were calibrated by comparison to a pure sulfanilamide standard. The amount of TOC was calculated as the difference between TC and IC (determined from coulometry).

In addition to the TOC concentration, elemental analysis yields the C/N atomic ratio, which can be used to help identify sources of organic matter (fresh marine C/N = 6–8; degraded marine C/N = 8–20; terrestrial C/N = 20). Rock-Eval pyrolysis analysis was not carried out because data are generally unreliable for samples containing <0.5 wt% TOC.

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