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doi:10.2204/iodp.proc.311.204.2009

Sampling and analytical methods

During Expedition 311, >500 samples of standard sediments (SED), usually one per section (~75–77 cm) regardless of facies sand or clay, and 85 samples (CARB) of semi-indurated sediments (<10% carbonate cement and mostly lighter color than the surrounding sediments), small concretions, or hard rock pieces were collected from several sites.

Some of the samples collected, inferred as carbonate nodules, were removed from the CARB sample set, as they contained no carbonate. These anomalous nodules were interpreted as dropstones. On the contrary, some SED samples were added to the CARB sample set because further investigations (XRD, smear slide observations, and examination of 50–250 μm and >250 μm sieved fractions) indicate that their carbonate fraction was mainly authigenic (not detrital or biogenic).

Samples were dried and ground in an agate mortar for carbonate content measurement using a ManoCalcimeter Mélières (MCM) apparatus derived from the carbonate bomb technique first described by Müller and Gastner (1971).

Total carbonate content (in weight percent) was calculated from CO₂ volume evolved from the reaction of 100 mg of finely crushed sediment with 8 N HCl. The MCM apparatus is calibrated at 100% so that 1 mM of CO₂ corresponds to 100 mg of calcite or aragonite. For other carbonates with different molecular weights, 100 mg does not correspond to 1 mM of CO₂, and values must be corrected (e.g., percent dolomite = MCM/1.0855 and percent siderite = MCM/0.8639) to estimate the real weight percentages.

Analyses were performed at Muséum National d’Histoire Naturelle (MNHN), Paris (France). The run time was 5–15 min for each sample, depending on carbonate composition (dolomite-rich samples react more slowly).

Because of the low carbonate values observed in the first SED samples analyzed (Site U1328), the quantity of sediment powder used for carbonate determination was increased to 200 mg for Sites U1325–U1327 and U1329 sediments in order to improve measurement accuracy.

To visualize the carbonate content fluctuations measured on the MCM apparatus, uncorrected values were used. These values are equivalent to the CaCO₃ weight percent calculated from inorganic carbon determined with the Coulometrics 5011 coulometer on board the JOIDES Resolution, as described by Pimmel and Claypool (2001). Therefore, the shipboard data published in the site chapters and the shore-based MCM data can be used together to compare downhole carbonate fluctuations within and between holes.

All SED and CARB samples were analyzed by XRD to identify the minerals present and to give an estimation of the mineralogical composition of the carbonate fraction. XRD analyses of randomly oriented powders were performed using a Siemens D500 instrument (CuKα Ni filtered radiation) at MNHN. Instrument parameters were set to 40 kV accelerating voltage and 30 mA current. Scans were run from 22°θ to 64°2θ with a step size of 0.02°2θ and counting time of 1 s for standard sediments or 2 s for authigenic carbonates. If necessary, the position of the peaks was corrected by reference to the main quartz peak present in all samples.

Identification of the main minerals present in the samples was determined with Eva 8.0 software and the International Centre for Diffraction Data (ICCD) Powder Diffraction File 2 database, release 2002.

The Mac Diff 4.2.5 program by Rainer Petschik (servermac.geologie.uni-frankfurt.de/Staff/Homepages/Petschick/RainerE.html) was used to resolve the composite peaks by a peak-fitting technique and to measure the peak surface of carbonates.

Microfacies (smear slides or thin sections) were examined to determine the morphology and probable origin of the carbonates (detrital, biogenous, or authigenic). For the micritic fine-grained facies, scanning electron microscopy (SEM) images of selected samples (mainly from Sites U1328 and U1327) were used to illustrate crystal morphologies, and energy dispersive spectrometer (EDS) analyses was used to confirm the semiquantitative composition of the carbonates.

Semiquantitative carbonate percentages are given for carbonate-rich samples based on the surface of the main peak of each mineral corrected by arbitrary factors (I/Icor values given in the ICCD database for pure calcite, dolomite, siderite, and rhodochrosite) and on the MCM value for absolute weight percent. The relative error of the weight percent of a given carbonate phase is roughly estimated at ±5% but may be higher because

• Authigenic calcite and dolomite have highly variable characteristics, as reflected by the wide range of d₁₀₄ values (Fig. F2);

• Siderite (d₁₀₄ ≈ 2.796 Å) and rhodochrosite (d₁₀₄ ≈ 2.850 Å), as determined from XRD, may correspond to other complex carbonate phases (Fe, Mg, Ca, and sometimes Mn in EDS analyses);

• I/Icor values for nonstoichiometric calcites or dolomites are not known; and

• Feldspars s.l. (Ca-albite, microcline, anorthite, etc.) or amphiboles (hornblende), which are sometimes abundant, have secondary peaks (around 2.99, 2.94, and 2.90 Å) superposed over the main carbonate peaks.

The d₁₀₄ values of calcite were used to calculate their content in mol% MgCO₃ using a linear interpolation between interplanar spacings of stoichiometric dolomite (d₁₀₄ = 2.886) and stoichiometric calcite (d₁₀₄ = 3.035), as experimentally observed by Goldsmith and Graf (1958). This method was not used to estimate dolomite composition, as the position of the d₁₀₄ peak depends on the incorporation of Mg²⁺, Ca²⁺, Mn²⁺, and Fe²⁺, and our microprobe analysis (EDS spectra) show that Fe is sometimes present in the dolomite crystal lattice.

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