Shipboard interstitial water analyses were performed on 5- to 10-cm whole-round sections cut immediately after the core was sectioned on deck. At Holes 1088B and 1093A, relatively closely spaced samples were taken for interstitial water analyses from the bottom of each section (except the last section) for the first 60 mbsf, one sample per core was taken to ~100 mbsf, and then one sample every other core to depth. At other sites, samples were usually taken from the fourth section from the top of each core, away from any possible drilling disturbance (see site chapters for details).
Interstitial water samples were collected with titanium squeezers that are modified versions of the standard ODP stainless steel squeezer of Manheim and Sayles (1974). Each whole round was carefully scraped free of the outer rind with a stainless steel spatula, then squeezed through one or two Whatman No. 1 filters prerinsed in high-purity water, and then through a 0.45-µm Gelman polysulfone disposable filter into a 50-mL plastic syringe. Interstitial waters were extruded by applying pressures up to 40,000 lb using a Carver Laboratory Press (Model 2702). After collection of 40 to 50 mL of interstitial water, the syringe was removed, a fresh 0.45-µm Gelman filter was attached, and aliquots were dispensed into plastic vials for shipboard analyses and into acid-washed plastic vials and 5-mL glass ampoules (flame sealed) for future shore-based work.
Interstitial waters were routinely analyzed for salinity, as total dissolved solids, with a Goldberg optical handheld refractometer (Reichart). Alkalinity and pH were measured immediately after squeezing by Gran titration with a Metrohm autotitrator and a Brinkmann pH electrode, respectively. Chloride was measured by titration with AgNO3.
Na, K, Mg, Ca, Cl, and SO4 were measured by ion chromatography on 1:200 diluted aliquots in nanopure water using a Dionex DX-100. In general, the results obtained for some elements from this technique are less accurate than alternate methods, including titration for Cl and charge balance calculations for Na. However, the relative trends are usually similar and can serve as a second check of the results generated by other methods. The precision of results measured by ion chromatography was generally within 3%-5%. The Cl and Na measurements obtained by ion chromatography are not reported here.
Silica, ammonia, and phosphate were determined by colorimetic methods using a Milton Roy Spectronic spectrophotometer with a 1-cm cell and sample introduction by Mister Sipper. The chemical methods employed follow those of Gieskes et al. (1991). Sodium was estimated by charge balance where total cation charge equals total anion charge. For most of these analyses, the International Association of Physical Sci-ences Organizations (IAPSO) seawater standard was used for standardization.
Strontium, lithium, iron, and manganese were determined using a Varian Spectra AA-20 atomic absorption spectrophotometer. Samples for iron and manganese were acidified with 50 µL triple-distilled HCl per 5 mL immediately after collection. Standards were matched in matrix composition to the samples. Lithium and manganese standards and samples were determined on 1:5 diluted aliquots in nanopure water and 0.1 N HCl, respectively. Strontium was determined on 1:10 diluted aliquots, and iron was determined without dilution. Lithium was determined by emission using an air-acetylene flame. Strontium, manganese, and iron were determined by atomic absorption using an air-acetylene flame. Strontium and manganese utilized lanthanum chloride as an ionization suppressant. The precision of these techniques is approximately <1%-2% for lithium and <4% for strontium, manganese, and iron.
The iron measurements should be interpreted with caution primarily for two reasons: (1) it was not possible to squeeze sediment samples under oxygen-free conditions and still maintain timely processing of interstitial waters through the chemistry laboratory; thus, some dissolved Fe+2 may have oxidized, been trapped on filters, or been lost to the walls of the syringe; and (2) some colloidal iron oxides are able to pass through 0.45-µM filters. The first effect would cause Fe+2 concentrations to be underestimated, whereas the second could result in an overestimation. Despite these potential problems, the iron data reported here are probably at least qualitative representations of the true in situ iron concentrations.
The shipboard organic geochemistry program for Leg 177 included the following: (1) real-time monitoring of volatile hydrocarbon gases; (2) measurement of the inorganic carbon concentrations to determine the amount of carbonate in the sediments; (3) elemental analyses of total carbon, total nitrogen, and total sulfur; and (4) preliminary characterization of organic matter. All methods and instruments used during Leg 177 are described below. Additional details are available in Emeis and Kvenvolden (1986). These analyses were conducted as part of the routine shipboard safety requirements, and to provide information for preliminary site summaries and shore-based organic geochemical research.
For safety considerations, the concentrations of methane (C1), ethane (C2), and propane (C3) gases in the sediments were measured at frequencies of generally one per core. The headspace method was used throughout the cruise. Gases released by the sediment after core recovery were analyzed by gas chromatography (GC) using the following technique. Immediately after retrieval on deck, a calibrated cork borer was used to obtain a measured volume of sediment from the top of one section for each core. The sediment, with a typical volume of ~5 cm3, was placed in a 21.5-cm3 glass serum vial that was sealed with a septum and metal crimp cap. When consolidated or lithified samples were encountered, chips of material were placed in the vial and sealed. Before gas analysis, the vial was heated to 60ºC for 30 min. A 5-cm3 volume of the headspace gas was extracted from each vial using a standard glass syringe. Vacu-tainer samples were not taken during this leg because gas voids were not present in the core. The collected gas was analyzed by a gas chromatograph (Hewlett Packard 5890 II Plus) equipped with a 60 m × 0.32 mm DB-1 capillary column and a frame ionization detector. Helium was used as a carrier gas and a Hewlett Packard Chemstation was used for data acquisition and processing. Chromatographic response was calibrated against pre-analyzed standards; gas contents are reported in parts per million by volume.
Inorganic carbon is determined using a Coulometric 5011 carbon dioxide coulometer. A sample of ~10 mg of freeze-dried, ground sediment was reacted with 2N HCl. The liberated CO2 was back-titrated to a colorimetric end point. The percentage of carbonate is calculated from the inorganic carbon (IC) content with the assumption that all inorganic carbon is present as calcium carbonate:
%CaCO3 = %IC × 8.33. (1)
The total carbon (TC), total nitrogen (TN), and total sulfur contents of the sediment are determined using a Carlo Erba Model NA1500 carbon-nitrogen-sulfur analyzer. A sample of ~6 mg of freeze-dried, ground sediment was combusted at 1000ºC in a stream of oxygen. Helium was used as a carrier gas, the oxygen was removed, and the combustion products were reduced. The reduced gases were separated by GC and quantified with a thermal conductivity detector. Contents of total organic carbon (TOC) were calculated as the difference between TC and IC:
TOC = TC - IC. (2)
The origin of the organic matter in the sediments can be characterized using organic carbon/nitrogen (C/N) ratios. Organic matter atomic C/N ratios were calculated from the TOC and TN concentrations. The average C/N ratio of marine zoo- and phytoplankton is between 5 and 8, whereas fresh higher land plants have ratios between 25 and 35 (e.g., Emerson and Hedges, 1988; Meyers, 1994). Pyrolysis analyses were not performed because TOC contents were generally low in the Leg 177 sediments.