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doi:10.2204/iodp.proc.317.204.2015

Methodology

Indurated intervals of Expedition 317 cores were sampled shipboard for use in this study. Thirty sample billets thought to contain carbonate cements were selected for analysis from the indurated intervals and split with a saw at California State University Northridge (CSUN; USA). A portion of each billet was impregnated with blue-dyed epoxy, and then a thin section was prepared to a 30 ?m thickness and polished, prepared without the traditional coverslip to facilitate chemical analysis via scanning electron microscope (SEM). A second portion weighing ~2 g was cleaved off of the remaining sample using a hand-screwed steel guillotine and ground to a fine powder using an agate mortar and pestle. The resulting powder was split to be analyzed by powder X-ray diffraction (XRD) and isotopic analysis. Table T1 lists the location and depth for each sample along with the analysis techniques employed for each.

Thin section imaging and point counts

Documentation of the macro texture of each sample was achieved by imaging the entire thin section on a standard flatbed scanner. The thin sections were scanned in transmitted light at a resolution of 3200 dots per inch, which resulted in a final image with dimensions of 3366 pixels wide × 5605 pixels high. In addition, the microtexture of each sample was documented using polarized light microscopy (visible transmitted light, with and without polarizing filters). These images assisted in describing the overall character of the samples in terms of components, matrix, porosity, and cement content (see COREIMAGES in “Supplementary material”). Estimations of pore size and abundance of bioclastic material were made using percentage diagrams (Table T2).

More detailed compositional analysis was provided through point-counting the 30 thin sections using a Leitz Labrolux 12 Pol petrographic microscope fitted with an automated stage and counter. Three-hundred points were counted per section, and counted categories included various sand, matrix, cement and, porosity (Table T3).

X-ray diffraction analysis

XRD was carried out at the Getty Conservation Institute in Los Angeles, California (USA) using a Siemens D5005 diffractometer. Approximately 200 mg of finely ground sample was placed on a zero-background quartz plate and flattened to an approximate uniform thickness. Scans were performed using the following conditions within the Bruker XRD Commander software: locked coupled scan, 40 kV, 30 mA, scan from 10° to 60°2θ, step size 0.01°2θ, and scan speed of 1 s/step. The resulting raw files were imported into the Bruker DiffracPlus software package and transformed using a fourier noise filter. Mineral peak identification was confirmed using an embedded JCP2 PDF database file. Slight differences in the thickness of the individual powdered samples were noted as a shift in the position of the optimal quartz peak position at 26.66°2θ and were corrected by adjusting the X-offset of each pattern. The diffraction peak position and relative intensity, measured as counts per second for quartz and the identified carbonate minerals (calcite, Mg calcite, and dolomite), were then calculated using the DiffracPlus software (Table T4). Methodology for determining quantitative percentages of components was not available with this software package and are not reported here.

Scanning electron microscopy

Backscattered electron (BSE) imaging and energy dispersive X-ray spectroscopy (EDS) mapping were performed on all 30 thin section samples to obtain elemental composition and distribution of the cements within the samples (see COREIMAGES in “Supplementary material”; Table T5). The analysis was carried out at the Getty Conservation Institute on a Philips-FEI XL30 environmental scanning electron microscope with field emission gun (ESEM-FEG). The samples were imaged uncoated with a chamber pressure of 0.8 torr, beam voltage of 20 keV, and spot size of 3. X-ray EDS mapping was performed with an Oxford X-Max 80 mm² detector and collected and processed with the Oxford INCA software package. To better facilitate the comparison of cemented zones within the samples, area spectra of these zones were reconstructed from the elemental map data set within the INCA software. Three separate areas representing the cemented zone were selected in each map. The three resulting spectra were manually evaluated for maximum peak height in counts, divided by the accumulation time, and averaged yielding a value in counts per second.

Tabulated results of elemental analysis performed on samples that are imaged uncoated while in low-vacuum mode of an environmental SEM need to be carefully considered because of the physics of the beam interaction with the sample. Numerical results are generally considered to be qualitative because of deflection of the electron beam as it passes through water vapor in the chamber, which can lead to X-ray signal generated from outside of the region of interest. Relative peak intensities and peak ratios are usually more informative, such as the Ca/Mg ratio, which when combined with XRD data can confirm mineral identification.

Isotopic analysis

Previous studies have shown that using δ¹³C and δ¹⁸O values can aid in determining the origin (i.e., marine, brackish, or freshwater) of authigenic carbonate cements (Malone et al., 2002). The 30 study samples were evaluated using optical microscopy and SEM imaging to determine their suitability for isotopic study. Only samples with minimal evidence of bioclastic carbonate fragments were submitted for isotopic analysis. This resulted in 15 of the original 30 samples being submitted to California State University, Long Beach, for analysis of the gas isotope ratios of ¹³C/¹²C and ¹⁸O/¹⁶O. Sample analysis was carried out using a Thermo-Finnigan Gas Bench II online gas preparation and introduction system coupled with a Finnigan DELTAplus XP stable isotope mass spectrometer. Approximately 200 μg of each sample, as well as three different standards (NBS-18, NBS-19, and in-house Standard A), were placed in individual sample vials and heated to 72°C. The headspace of each vial was flushed with helium for 5 min, followed by automated dropwise addition of concentrated phosphoric acid. The samples were then allowed to react for a minimum of 2 h for the evolved CO₂ gas to reach equilibration within the vial. Measurement of the ¹³C/¹²C and ¹⁸O/¹⁶O ratios was then performed sequentially for each standard and sample, during which 10 doses of each were evaluated. Generated peaks were evaluated for their consistent peak height and shape, and values deemed to be nonrepresentative were rejected. The resulting δ¹³C and δ¹⁸O values were then corrected for linearity and converted to their relative Peedee belemnite (PDB) values using a calibration curve created from the three standards. These results are presented in Table T6.

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