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Sample preparation

Most of the samples analyzed in this study came from “clusters” that included companion specimens for shipboard bulk-powder XRD, X-ray fluorescence, moisture and density, and carbon-carbonate. The clusters were positioned next to sampling intervals for whole rounds, including those extracted for interstitial water. Shipboard XRD scans provided estimates of the relative abundance of total clay minerals, quartz, feldspar, and calcite (see the “Site C0018” chapter [Expedition 333 Scientists, 2012b], and the “Site C0021” chapter [Strasser et al., 2014b]).

Clay-size fractions were isolated for XRD analyses by air-drying and gentle hand-crushing of the mud with mortar and pestle, after which specimens were immersed in 3% H2O2 for at least 24 h to digest organic matter. To prevent flocculation, ~250 mL of Na-hexametaphosphate solution (4 g/1000 mL distilled H2O) was added, and beakers were inserted into an ultrasonic bath for several minutes to promote disaggregation. That step (and additional soaking) was repeated until visual inspection indicated complete disaggregation. Washing consisted of two passes through a centrifuge (8200 rpm for 25 min; ~6000 g) with resuspension in distilled deionized water after each pass. After transferring the suspended sediment to a 60 mL plastic bottle, each specimen was resuspended by vigorous shaking and a 2 min application of an ultrasonic cell probe. A centrifuge was used to separate the clay-size splits (<2 µm equivalent spherical settling diameter; 1000 rpm for 2.4 min; ~320 g). Preparation of oriented clay aggregates followed the filter-peel method (Moore and Reynolds, 1989) using 0.45 µm filter membranes. A closed vapor chamber was used to saturate clay aggregates with ethylene glycol heated to 60°C for at least 24 h prior to XRD analysis.

X-ray diffraction

Two X-ray diffractometers were used to analyze clay-size specimens. When the project began, the XRD laboratory at the University of Missouri was equipped with a Scintag Pad V X-ray diffractometer with CuKα radiation (1.54 Å) and Ni filter. Those scans of oriented clay aggregates were run at 40 kV and 30 mA over a scanning range of 3° to 26.5°2θ, a rate of 1°2θ/min, and a step size of 0.01°2θ. Slits were 0.5 mm (divergence) and 0.2 mm (receiving). The Department of Geological Sciences shut down that facility before NanTroSEIZE research was finished, so the remaining samples (including all samples from Site C0021) were analyzed at the New Mexico Bureau of Geology and Mineral Resources using a Panalytical X’Pert Pro diffractometer with Cu anode. We ran those continuous scans at generator settings of 45 kV and 40 mA over an angular range of 3° to 26.5°2θ, with scan step time of 1.6 s and step size of 0.01°2θ. Slits are 1.0 mm (divergence) and 0.1 mm (receiving) and the sample holder spinning. MacDiff software (version 4.2.5) was utilized to draw a baseline of intensity, smooth counts, correct peak positions offset by misalignment of the detector (using the quartz [100] peak at 20.95°2θ; d-value = 4.24 Å), and calculate integrated peak area (total counts) and peak width at half height (Δ°2θ).

Calculations of mineral abundance

To achieve the best possible accuracy, XRD methods require calibration with internal standards, use of single-line reference intensity ratios, and some fairly elaborate sample preparation steps to create optimal random particle orientations (e.g., ?rodo? et al., 2001; Omotoso et al., 2006). Given the unusually large number of samples throughout the NanTroSEIZE project, our priority has been to obtain reliable semiquantitative accuracy with optimal efficiency. To accomplish that for the clay-size fraction, standard mineral mixtures (smectite + illite + chlorite + quartz) were analyzed as described by Underwood et al. (2003), and a matrix of normalization factors was computed using singular value decomposition (SVD). We record the integrated areas of a broad smectite (001) peak centered at ~5.3°2θ (d-value = 16.5 Å), the illite (001) peak at ~8.9°2θ (d-value = 9.9 Å), the composite chlorite (002) + kaolinite (001) peak at 12.5°2θ (d-value = 7.06 Å), and the quartz (100) peak at 20.85°2θ (d-value = 4.26 Å). Because of differences in X-ray tubes and diffractometers, it was necessary to solve for three sets of normalization factors (Table T1) and then employ them during computations. Average errors using this method are 3.9% for smectite, 1.0% for illite, 1.9% for chlorite, and 1.6% for quartz. The chlorite (002) and kaolinite (001) peaks overlap almost completely, so a refined version of the Biscaye (1964) method was used to discriminate kaolinite (002) from chlorite (004), as documented by Guo and Underwood (2011). The average error of accuracy for the chlorite/kaolinite ratio is 2.6%. That ratio was used to compute individual mineral percentages from the SVD weight percent of undifferentiated chlorite (002) + kaolinite (001).

To calculate the abundance of individual clay minerals in the bulk sediment, each relative weight percent value among the clay minerals (where smectite + illite + chlorite + kaolinite = 100%) was multiplied by the weight percent of total clay minerals within the bulk powder (where total clay minerals + quartz + feldspar + calcite = 100%), as determined by shipboard XRD analyses of collocated specimens (Expedition 333 Scientists, 2012b; also see the “Site C0021” chapter [Strasser et al., 2014b]). To facilitate direct comparisons with other published data sets from the region, tabulations include the weighted peak-area percentages for smectite, illite, and undifferentiated chlorite + kaolinite using Biscaye (1965) weighting factors (1 × smectite, 4 × illite, and 2 × chlorite + kaolinite). Errors of accuracy using that method can be substantially greater (±10% or more) as compared to the errors using SVD factors (Underwood et al., 2003).

For documentation of clay diagenesis, the saddle/peak method of Rettke (1981) was used to calculate percent expandability of smectite and illite/smectite (I/S) mixed-layer clay. This method is sensitive to the proportions of discrete illite (I) versus I/S mixed-layer clay; the curve for 1:1 mixtures of I and I/S provides the best match for the range of Nankai specimens. Values of the illite crystallinity (Kübler) index are reported here as peak width at half height (Δ°2θ) for the (001) reflection; the illite peak typically narrows as levels of thermal maturity increase.