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Visual core descriptions

We followed conventional ODP and IODP procedures for recording sedimentologic information on a section-by-section basis using VCD forms (Mazzullo and Graham, 1988; Mazzullo et al., 1988). Core descriptions were transferred to section-scale templates using the J-CORES database software and then converted to core-scale descriptions using Strater software.

The previous drilling record in the Japan Trench area suggested we would encounter hemipelagic to pelagic siliciclastic sediment with various admixtures of siliceous microfossils and volcaniclastic material (Scientific Party, 1980). Accordingly, we adopted a sedimentologic classification scheme specific for this area that could be applied with reasonable accuracy by shipboard personnel.

Expedition 343 sedimentary classification focuses on siliciclastic, siliceous microfossil, and volcaniclastic components. Pebbles, fossils, concretions, and nodules were logged as lithologic accessories except where pebbles (>2 mm) make up a significant component of a bed, in which case the bed was logged as gravel. Classification schemes for siliciclastic, siliceous microfossil, and volcaniclastic sediment are presented in Figure F7. Similar to the methodology of the Expedition 308 Scientists (2006), we modified the classification of Shepard (1954) to describe the texture of the <0.2 mm siliciclastic components of the sediment, with the division between sand and silt at 64 µm and between silt and clay at 4 µm. These classifications are driven by the principal component, siliceous microfossils or volcaniclastic sediment, with modification as necessary by a secondary component. In our modified classification scheme, sediment identified as ashy mud, ashy clay, ashy silt, and ashy silty sand are grouped into the ashy mud log unit. Likewise, sediment identified as siliceous mud, siliceous clay, siliceous silt, and siliceous silty sand are grouped into the siliceous mud log unit. Slope deposits may be mixed because of mass transport; where such synsedimentary deposits were unambiguously identified in the core, the intervals were logged as “sedimentary breccia.”

Consistency between operators and across the two work shifts was maintained throughout the operation. The initial core was examined by all X-ray CT, sedimentology, and structural geology scientists, and agreement was reached as to the initial descriptive framework. At each shift change, the departing scientists reported their findings from that day and detailed new features and interpretations to be adopted in the future. The working nomenclature was then updated at the beginning of each shift following the meeting.

The graphic lithology column on each VCD form plots to scale all beds that are at least 2 cm thick. Interlayers that are <2 cm thick are identified as laminae and are represented by graphic symbols in the sedimentary structures column instead of lithologic units in the graphic lithology column. Figure F8 displays all the graphic patterns used to represent the sedimentary lithologies and sedimentary structures encountered during Expedition 343. Also shown are the symbols for internal sedimentary structures and accessories, soft-sediment deformation structures, bioturbation, and severity of core disturbance in sedimentary rock (see “Structural geology” for a description of drilling disturbance). The graphic lithology column follows the sedimentary classification above. Bioturbation intensity in deposits was noted as light (bedding preserved), moderate (bedding disturbed), or heavy (bedding obliterated).

The description of lithology necessarily overlapped with the structural description, particularly in cases where deformation fabrics (either sedimentary, tectonic, or drilling induced) formed an essential defining characteristic of the rock type. See “Structural geology” for a discussion on the terminology for deformation fabrics that may be primary or secondary (e.g., breccia texture and scaly fabrics).

Smear slides

The archive half of the core was sampled near the top and base of each core, as well as at regular intervals and at changes in lithology or areas of particular interest for smear slide analysis. Smear slides are useful for identifying and reporting basic sediment attributes (texture and composition), but the results are not quantitative. We estimated the abundances of biogenic, volcaniclastic, and siliciclastic constituents with the help of a visual comparison chart (Rothwell, 1989), recognizing that smear slide analysis involves multiple sources of error. Sand-sized particles may be underrepresented because of loss in smear slide preparation, and it is difficult to quantitatively estimate the grain size of fine fractions. Thus, it would be misleading to report values as absolute percentages. Instead, our descriptive results are reported as visual percentage estimates with values grouped into the following range categories:

  • D = dominant (>60%).

  • A = abundant (>30%–60%).

  • C = common (>5%–30%).

  • P = present (>1%–5%).

  • R = rare (0.1%–1%).

  • T = trace (<0.1%).

The relative abundance of major components was also validated by XRD (see “X-ray diffraction”).

The sample location for each smear slide was entered into the J-CORES database with a sample code of “SS” using the Samples application. The position of each sample is shown on the VCD slide editor column of the VCD application. We tabulated data in an Excel spreadsheet because data entry into the J-CORES database is prohibitively time consuming and the program will not accept ranges of values for individual compositional categories.

X-ray diffraction

We completed routine bulk powder XRD analyses using a Philips PANalytical CubiX PRO (PW3800) diffractometer. The principal goal was to identify mineral phases in the samples and estimate spatial trends in sediment composition. Most of the samples were selected from intervals adjacent to whole-round samples, and most are part of sampling clusters with associated physical properties and carbonate analysis. A few additional samples were collected periodically from where less frequent or unexpected lithologies were encountered, such as volcanic ash. Samples were freeze-dried, crushed with a ball mill, and mounted as randomly oriented bulk powder. The instrument settings were as follows:

  • Generator = 40 kV and 45 mA.
  • Tube anode = Cu.
  • Wavelength = 1.54184 Å (CuKα).
  • Step spacing = 0.01°2θ.
  • Rate = 1.0 s/step.
  • Slits = automatic.
  • Measuring diameter = 10 mm.
  • Scanning range = 2°2θ to 60°2θ.

We used Panalytical X’Pert software for data processing. Bulk powder XRD data were used to assist with core-log integration.

The modal mineralogy of the sediment was estimated from bulk powder XRD spectra using a least-squares best-fit approach. Reference spectra for curve matching were restricted to four spectra (quartz, plagioclase, calcite, and total clay), following the methods employed during IODP Expedition 316 (Expedition 316 Scientists, 2009a). These reference spectra were selected and calibrated to the known sediment compositions for Expedition 316, and the applicability of the same reference spectra for Expedition 343 samples is untested. Therefore, we report only general approximations of the relative proportional mineralogy. These approximations also do not allow for detection of other known components of the sediment, such as glass or alkali feldspar from volcanic sources or opaline biogenic silica, and therefore should be considered to be upper bounds.

X-ray fluorescence

Whole-rock quantitative XRF spectrometry analysis was undertaken on core samples for the major elements. Samples of 10 cm3 were taken from the working half of the core next to samples for XRD and carbonate analysis. Samples were freeze-dried, crushed with a tungsten ball mill, and fused with 4.5 g of SmeltA12 flux for 7 min at 1150°C to create glass beads. Loss on ignition was measured using weight changes by heating at 1000°C for 3 h. Analyses were performed on the wavelength dispersive XRF spectrometer Supermini (Rigaku) equipped with a 200 W Pd anode X-ray tube at 50 kV and 4 mA.

Major elements were measured using the fused-glass bead method and are presented as weight percent oxide proportions (Na2O, MgO, Al2O3, SiO2, P2O5, K2O, CaO, TiO2, MnO, and Fe2O3). Processed data were uploaded into the J-CORES database as a comma-delimited data file. Data are reported as total counts on the peak and as semiquantitative oxide weight percents.

X-ray fluorescence scanning

Whole-rock semiquantitative XRF spectrometry analysis was undertaken for major elements and trace elements on core material (Na, Mg, Al, Si, P, S, K, Ca, Ti, V, Cr, Mn, Fe, Cu, Ni, Zn, and Zr) in small targeted areas of the core, focusing on unconformities, veins, cemented and altered intervals, or shear surfaces. Semiquantitative element scanning of core sections was undertaken using the onboard JEOL TATSCAN-F2 energy dispersive spectrometry (EDS)-based split-core scanner equipped with a cryogenic Si semiconductor and a 76 mm beryllium window (Sakamoto et al., 2006). The target for the X-ray generation is rhodium, which can generate X-rays with five times higher intensity compared to the standard EDS-based scanner. A rock standard (JSd-2) of the National Institute of Advanced Industrial Science and Technology (Geological Survey of Japan [GSJ]) was used as the reference material for semiquantitative analysis. Table T6 lists the results and standard deviations for the standard sample.

The diameter of the collimator that detects the incident X-ray beam is 0.8 mm, and the penetration depth on the sample is ~1.1 mm. Although the distance between the sample surface and detector critically affects the intensity of X-rays, XRF scanning is a highly useful, albeit nonroutine, element of shipboard analysis.

Split-core digital photography

The Geotek MSCL-I (with Geoscan IV line-scan imaging camera) was used to capture continuous digital imagery of the archive half for analysis and description. The line-scan system for split-core imaging reduces the optical distortion from the lens or variations of lighting downhole. The camera is a three charge–coupled device system where a light reflected from the sample surface passes through the lens and is split into three paths (red, green, and blue) by a splitter beam in the camera. The camera provides 16-bit red-green-blue (RGB) color TIFF formatted images with a spatial resolution of 100 pixels/cm.

The MSCL-I was used shortly after core splitting in an effort to avoid time-dependent color changes resulting from sediment drying and oxidation. Cores were prepared by careful removal of bumps and dust on the split core face. Camera calibration was performed using Spectralon (SRS-99; Labsphere), a high diffuse reflectance material for white calibration, and a gray card (miniature gray-scale card) consisting of white, 18% gray, and black. A QC measurement was performed before each sample, after the white/black calibration, and after 24 h of routine work.

Working information about the measurements was logged in the measurement log sheet (MLS). These data included core/type/section, date, operator, aperture, curated length, section length, section length + 7 cm, the J-CORES database operator, and shared folder. Scanned images and data were uploaded to the J-CORES database so that image files could be observed through the Composite Log Viewer. Available files included the original high-resolution image with gray-scale and ruler, as well as reduced JPEG images cropped to show only the section-half surfaces. All measurement data, calibration data, setup files, and MLSs were archived to the shared folder on board.