INORGANIC GEOCHEMISTRY

Interstitial Water Sampling and Chemistry

Shipboard interstitial water analyses for Leg 181 were performed on 5-10-cm whole-round sediment sections, which were cut and capped immediately after the core arrived on deck. Specific details of the sampling resolution are described in the individual site chapters. Before squeezing, samples were removed from the core liner and the outside surface of each whole-round section was carefully scraped off with a spatula to minimize potential contamination. Whole rounds were placed into a titanium and stainless steel squeezing device and squeezed at ambient temperature by applying pressure up to 40,000 lb (~4150 psi = 2.918 x 106 kg/m2) with a hydraulic press (Manheim and Sayles, 1974).

Interstitial water was extruded through a prewashed Whatman no. 1 filter, fitted above a titanium screen. All interstitial water samples were filtered through 0.45-Ám Gelman polysulfone disposable filters and collected into acid-washed (10% HCl) 50-mL plastic syringes. The interstitial water samples were subsequently refiltered through 0.45-Ám Gelman polysulfone disposable filters. The samples were stored in plastic vials for shipboard analyses or archived in glass ampoules and/or heat-sealed in acid-washed plastic tubes for shore-based analyses.

Interstitial water analyses followed the procedures outlined by Gieskes et al. (1991). Interstitial water samples were routinely analyzed for salinity as total dissolved solids (g/kg) with a Goldberg optical hand-held refractometer made by Reichart. Alkalinity was measured by Gran titration with a Brinkmann pH electrode and a Metrohm autotitrator, and pH was measured on the NBS scale as part of the alkalinity titration. It should be noted that pH measurements obtained in this fashion are not always reliable given that the algorithm employed for pH measurement before the start of the alkalinity titration is adversely influenced by degassing. Dissolved chloride (Cl-) was determined by titration with AgNO3. Dissolved SiO2 (H4SiO4), ammonium (NH4+), and phosphorous (PO43-) concentrations were determined by spectrophotometric methods with a Milton Roy Spectronic 301 spectrophotometer (Gieskes et al., 1991). As a result of the shortage of fresh phenol, the reproducibility of NH4+ measurements decreased during the cruise. International Association of Physical Sciences Organizations (IAPSO) standard seawater was used for calibrating most techniques. The reproducibility of these analyses, determined by repeated measurements of standards, are alkalinity = <1.5%; Cl- = <0.3%; H4SiO4 = <3%; and PO43-, and NH4+ = ~3%.

Sodium (Na+), potassium (K+), magnesium (Mg2+), calcium (Ca2+), chloride (Cl-), and sulfate (SO42-) concentrations were measured by ion chromatography on 1/200 diluted aliquots in nannopure water using a Dionex DX-100. In general, the results obtained from this technique for some elements are less accurate than alternative methods, such as titration for Cl- and charge balance calculations for Na+. However, the relative trends are usually similar and can serve as a second check of results generated by the other methods. The precision of results measured by ion chromatography was generally within 3%-5%. The Cl- and Na+ measurements by ion chromatography are not reported here.

Lithium (Li+) and strontium (Sr2+) concentrations were quantified using flame atomic emission (AES) and absorption (AAS) spectrometry, respectively, on a Varian SpectrAA-20. Tenfold dilutions of the interstitial water samples were used for lithium and five- to twentyfold dilutions were employed for strontium analyses. Air-acetylene (Li+) and nitrous oxide-acetylene (Sr2+) flames were used for these analyses. Standards for all flame AAS/AES techniques were matrix matched as closely as possible to the samples (Li+ and Sr2+). A more detailed description of analytical methods and standards used can be found in ODP Technical Note 15 (Gieskes et al., 1991). Standard deviations (1) were better than 2% for Li+, and better than 3%-4% for Sr2+. Sodium (Na+) was estimated by charge balance on the assumption that total cation charge equals total anion charge ( [cation charge] = [anion charge]). The chemical data for interstitial water are reported in molar concentration units in each site report.

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