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doi:10.2204/iodp.proc.331.102.2011

Lithostratigraphy

X-ray computed tomography

X-ray CT imaging provides information about lithological features without the need for core splitting. Images were used to assist in deciding locations for obtaining whole-round samples and also to provide a guide to core loggers of which lithologies had been removed by whole-round sampling. We followed the methodology documented in the “cookbook” (X-ray CT Scanner, version 3.00, 31 July 2007) prepared by CDEX/Japan Agency for Marine Earth Science and Technology (JAMSTEC), as well as methods used during previous expeditions (e.g., IODP Expedition 319). The X-ray CT instrument on the Chikyu is a GE Yokogawa Medical Systems LightSpeed Ultra 16 capable of generating sixteen 0.625 mm thick slice images every 0.5 s, the time for one revolution of the X-ray source around the sample. The protocol is based on GE Healthcare (2006), Mees et al. (2003), and Nakano et al. (2000). Data generated for each core consist of core-axis-normal planes of X-ray attenuation values with dimensions of 512 × 512 pixels. These data were stored as Digital Imaging and Communication in Medicine (DICOM) formatted files.

Visual core descriptions

We described working halves of cores on a section-by-section basis to document lithological information together with descriptions of hydrothermal alteration and mineralization (Mazzullo and Graham, 1988). Lithological features were observed visually and recorded in section-scale templates using the shipboard database software J-CORES. Along with alteration and sulfide mineralization (see “Petrology”), lithological descriptions were then included on the section-scale VCDs using Strater software. The Lithology column on each VCD plots all beds that are ≥2 cm thick to scale. Interlayers <2 cm thick are identified as laminae in the Structures/Veins column.

The classification scheme for clastic lithology follows Mazzullo et al. (1988). Texture (defined by the relative proportions of sand, silt, and clay) follows the classification of Shepard (1954). We did not use separate patterns for more heavily indurated examples of the same lithologies (e.g., silty clay versus silty claystone) because the distinction is somewhat arbitrary.

Graphic patterns for all lithologies encountered during Expedition 331 are summarized in Figure F3. Also shown are symbols for sedimentary structures, soft-sediment deformation structures, severity of core disturbance, and features. Note that sulfide-rich sediments were coded as “massive sulfide sediment,” “sulfidic sand,” or “sulfidic mud” in the Lithology column and further described using petrographic nomenclature described in “Petrology.”

Particle size analysis

Particle size analysis can improve the characterization of fine-grained biogenic sediments. Particle size variations can also be used to evaluate lithological/textural controls and effects of microbial activity on biogenic sediments. Grain size analysis is a classic sedimentological tool commonly used for the study of siliciclastic deposits, where particle sizes reflect the processes that generated the clasts, including weathering, erosion, transport, and sedimentation. Despite the dominant application for siliciclastic sediments, grain size studies of fine-grained biogenic sediments have been widely performed (Paull et al., 1988; McCave et al., 1995; Stuut et al., 2002).

We used the multiwavelength laser particle analyzer (LS; Beckman Coulter, model LS13320) to measure the grain size of core sediments. Laser particle analysis is based on the principle that particles of a given size diffract light through a given angle, which increases with decreasing particle size. A parallel beam of monochromatic light is passed through a suspension, and the diffraction light is focused onto a multielement ring detector. The detector senses angular distribution of scattered light intensity (Syvitski, 1991; McCave et al., 1995). The laser particle analyzer can provide rapid, automated, and precise measurement of sediment grain size ranging from 0.375 μm to 2 mm in size in ~10 min, including a rinse stage. The LS used is equipped with a micro volume module (MVM) system, and the autosampler allows the user to load a run of 30 samples to be processed automatically. The accompanying software calculates the grain size distribution of the sample and appropriate statistics. Data may be exported to external programs.

When measuring the particle size distribution of siliclastic sediments, care must be taken to inhibit the formation of aggregates from true sedimentary particles (Ramaswamy and Rao, 2006) unless the analysis of aggregates formed from biogenic/authigenic components is intended. In the case of sediments, the formation of aggregates is most notable within the finer (smaller grain size) fractions when free organic phases or iron oxide precipitates tightly bind sedimentary particles into aggregates (Syvitski, 1991). Furthermore, organic particles and aggregates may have shapes, geometries, and other surface properties that interfere with routine laser particle size analysis of siliclastic sediments. During this expedition, an aliquot of a 0.5 g subsample was used for measuring grain size. When necessary, organic matter was removed by oxidizing with 10% v/v H₂O₂, whereas interference from other nonsiliclastic biogenic/authigenic components, including carbonates and phosphates, was reduced by acidifying samples with 10% v/v HCl at room temperature.

Grain size distributions are displayed as phi (ϕ) values, which is a base two logarithmic scale commonly utilized to represent grain size information for a sediment distribution. Logarithmic phi values (in base two) are calculated from particle diameter size measures in millimeters as follows:

where d = particle size in millimeters.

A comparison of grain sizes in millimeters, phi units, and the Wentworth grain size classification is provided in Table T1.

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