METHODS

Sedimentary Petrography

Textural, compositional, and morphological features of carbonate precipitates were studied and described on hand specimens, slabs, and polished thin sections using light and scanning electron microscopy (CamScan SEM) at the GEOMAR Research Center for Marine Geosciences. The CamScan SEM was equipped with an energy dispersive X-ray spectrographic analyzer (EDAX) that allowed us to obtain qualitative elemental analyses of selected mineral grains.

Carbonate Content and X-ray Diffraction Analysis

Bulk mineralogy and the relative abundance of different carbonate minerals in each sample, were investigated by X-ray diffraction (XRD) using a Philips PW 1820 X-ray diffractometer at the GEOMAR Research Center for Marine Geosciences and a Philips Norelco 1720 X-ray diffractometer at the University of North Carolina at Chapel Hill, both generating monochromatic Cu_k radiation. Samples were crushed with an agate mortar and pestle, mixed with an internal standard (-Al₂O₃), and prepared as randomly oriented powder slides. Scans were run from 20º to 60º 2 at a scanning speed of 0.01º 2/s. The relative proportions of different carbonate minerals were quantified on the basis of the (104) peak areas of calcite, Mg-calcite and dolomite, and the (111) peak area of aragonite using prepared calibration curves. For details about the calibration, see Greinert (in press). The position of the (104) peak was used to determine Mg content of carbonate minerals (Goldsmith et al., 1961; Lumsden, 1979). Calcite with less than 5 mol% MgCO₃ is considered low-Mg calcite (LMC), and all other calcite compositions are referred to as high-Mg calcite (HMC) after Burton and Walter (1987). Carbon and carbonate contents (expressed as weight percent [wt%] CaCO₃) of both carbonate precipitates and background sediment samples were determined using a NA-1500 Carlo-Erba element analyzer at the GEOMAR Research Center for Marine Geosciences and a Coulometrics Model 5011 CO₂ Coulometer at the University of North Carolina at Chapel Hill.

Electron Microprobe Analyses

Carbon coated, polished thin sections were made from selected authigenic carbonates and examined by electron microprobe analyses to provide detailed mineral chemistry of carbonate precipitates. All analyses were performed using a Cameca SX 50 electron microprobe at the GEOMAR Research Center for Marine Geosciences, which is equipped with a wavelength dispersive system. During a typical analysis, concentrations of Ca, Mg, Mn, Fe, Sr, Ba, C, O, Al, and P were determined. Counts were collected for 20 s using an accelerating voltage of 15 kV and a beam current of 10 nA in scanning mode (magnification 25,000). Natural carbonate minerals, feldspars, and glasses were used as standards. Matrix corrections were made using standard ZAF techniques.

Stable Isotope Analyses

Samples for oxygen and carbon isotope analyses were extracted from the surfaces of polished slabs using a hand-held microdrill. The CO₂ for analysis was obtained by reacting samples with 100% orthophosphoric acid in vacuo at 75ºC (Carbo Kiel CO₂ preparation device). The purified CO₂ gas was analyzed isotopically in a Finnigan MAT 252 mass spectrometer at the Geological Institute Erlangen. Results are expressed in notation relative to the Peedee belemnite (PDB) standard. Precision is on the order of 0.1 for both oxygen and carbon. The ¹⁸O values for dolomite have been corrected following the method proposed by Rosenbaum and Sheppard (1986). Pore-water oxygen isotope data are from Borowski et al. (1997).

Stable oxygen and carbon measurements on bulk sediments were measured on a Finnigan Mat 251 Ratio Mass Spectrometer at the North Carolina State Stable Isotope Laboratory. Samples were roasted under vacuum at 325ºC for 1 hr to remove volatile organics and then reacted at 75ºC for 10 min with 100% orthophosphoric acid in side-arm reaction vessels. The CO₂ was cryogenically separated for analysis. Because many of these samples contained sulfides, an additional cryogenic separation was done at ~ -130ºC to insure that any SO₂ produced during the digestion procedure would be separated from CO₂ (Rodriguez et al., Chap. 30, this volume).

Strontium Isotope Analysis

The ⁸⁷Sr/⁸⁶Sr composition of 15 carbonate and six pore-water samples was determined using a VG-Sector 54 Thermal Ionization Mass Spectrometer at the Department of Geology, University of North Carolina at Chapel Hill. Samples for ⁸⁷Sr/⁸⁶Sr analysis were microdrilled adjacent to samples for ¹³C and ¹³O determination. Samples were dissolved in 500-µL 1-M ultra-pure acetic acid, centrifuged, and then the decantate was evaporated dry. Samples were then redissolved in 250 µL of 5-N HNO₃ and loaded into columns containing Eichrom Sr-specific resin. Details of the procedure are given in Paull et al. (1995a). Values of ⁸⁷Sr/⁸⁶Sr are reported relative to a ⁸⁷Sr/⁸⁶Sr ratio of 0.71025 for SRM-987. Replicate analyses of SRM indicate uncertainties of ±0.000011 (1). For pore-water ⁸⁷Sr/⁸⁶Sr analyses, see Paull et al. (1995a).