Procedural details for core processing and the analytical approaches, with the exception of the sulfur methodologies discussed below, are provided in Sigurdsson, Leckie, Acton, et al. (1997). In brief, sulfate was determined by ion chromatography. Dissolved cations were measured using flame atomic absorption spectrometry. Alkalinity was determined by Gran titration, and inorganic carbon (CaCO3) was quantified via coulometric titration. Total carbon and sulfur were measured using a Carlo Erba CNS analyzer; total organic carbon (TOC) was calculated as the difference between total and inorganic carbon. Silica, ammonium, and phosphate were quantified spectrophotometrically. Headspace gases were evaluated using gas chromatography equipped with both flame ionization and thermal conductivity detectors.
Concentrations of total
reduced inorganic sulfur were measured using the chromium reduction method
described by Canfield et al. (1986). Based on visual evaluations of sediment
samples and discrete ash layers, it is likely that most, if not all, of this
Cr-reducible sulfur represents pyrite. Isotopic compositions of bulk
"pyrite" sulfur for selected ash samples were measured on Ag2S
precipitates of the sulfide liberated during chromium reduction (Lyons, 1997).
Aliquots of the Ag2S were combusted in the presence of cupric oxide
under vacuum for a quantitative conversion to sulfur dioxide and analyzed via
mass spectrometry. Sulfur isotope data are expressed as per mil (
)
deviations from the sulfur isotope composition of the troilite phase of the
Cañon Diablo meteorite (CDT) using the conventional delta (
34S)
notation. Sulfur isotope results are generally reproducible within ± 0.1
to 0.2. Sulfur yields
via the chromium reduction method are typically 96% or better using pyrite
standards.
