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doi:10.2204/iodp.proc.342.102.2014

Lithostratigraphy

The lithostratigraphy of sediments recovered during Expedition 342 was determined by a combination of core description, smear slide and thin section analyses, digital color imaging, spectrophotometry, and visual color determination. The methods employed are adapted from those used during Ocean Drilling Program (ODP) Leg 199 (Lyle, Wilson, Janecek, et al., 2002) and IODP Expedition 320/321 (Pälike, Lyle, Nishi, Raffi, Gamage, Klaus, and the Expedition 320/321 Scientists, 2010).

Visual core descriptions

VCDs of the archive half of the split cores provide a visual summary of lithostratigraphic, biostratigraphic, magnetostratigraphic, and physical property data obtained during shipboard analyses. IODP VCDs are equivalent to the “barrel sheets” used during the Deep Sea Drilling Program (DSDP) and ODP. Lithostratigraphic data for VCDs were recorded digitally in real time using the DESClogik software (version 3.32.5.1). Prior to drilling operations, a spreadsheet template with seven tabs was constructed in Tabular Data Capture and customized for Expedition 342 (Fig. F1). The tabs were used to record the following information:

1. Drilling disturbance,

2. General (lithologic core description),

3. Detail (higher level detail than general core description),

4. Smear slides (quantified texture and relative abundance of biogenic and lithologic components),

5. Core summary (written description of major lithologic findings by core),

6. Stratigraphic unit, and

7. Age.

DESClogik also includes a graphic display mode of core data (e.g., aligned digital images of section halves and various measurement data) that can be used to aid core description.

During Expedition 342, the Strater software package was used to compile the VCDs for each core. Site, hole, and core number are given at the top of the VCD together with a summary core description (Fig. F2). The written description for each core contains a succinct overview of major and minor lithologies, their Munsell colors, and notable features such as sedimentary structures and major disturbances resulting from the coring process (Fig. F3). Core depth below seafloor (recorded in meters), core length (recorded in centimeters), section breaks, and lithostratigraphic unit are indicated along the left side of the digital color image of the core and Graphic lithology column. Columns to the right of the Graphic lithology column include age and data collected by the WRMSL and SHMSL (see “Physical properties”). These include natural gamma radiation (NGR), lightness (L*) and color (a* and b*) as determined by color reflectance, and corrected magnetic susceptibility. Columns to the right of these data show drilling disturbance, biostratigraphy (nannofossils, planktonic foraminifers, and radiolarian biozones), drilling disturbance and intensity, sedimentary structures, lithologic accessories, and shipboard sampling.

Digital color image

The SHIL imaged the flat face of the archive half of the split cores using a line-scan camera. Sediment cores were split and the archive half scraped with the edge of a glass slide to provide a “clean” surface for imaging. The cleaned, flat face of the archive section halves were imaged as soon as possible after splitting to minimize color changes that occur through oxidation and drying. Images are taken at an interval of 10 lines/mm. Camera height is adjusted so that image pixels are square. Light is provided by three pairs of Advanced Illumination high-current focused LED line lights with fully adjustable angles to the lens axis. Note that compression of line-scanned images into compiled stacks (like the core image shown in the VCDs) may result in visual artifacts (e.g., the false appearance of lamination).

Sediment classification

Lithologic names consist of a principal name based on composition, degree of lithification, and/or texture as determined from visual examination and smear slide or thin section observations. For a mixture of components, the principal name is preceded by major modifiers (in order of increasing abundance) that refer to components making up ≥25% of the sediment (Fig. F4). Minor components that represent between 10% and 25% of the sediment follow the principal name (after “with”) in order of increasing abundance. For example, an unconsolidated sediment containing 30% nannofossils, 25% clay minerals, 20% foraminifers, 15% quartz silt, and 10% manganese nodules would be described as a clayey nannofossil ooze with manganese nodules, quartz silt, and foraminifers. Our naming conventions mostly follow the ODP sediment classification scheme of Mazzullo et al. (1988). However, the mixed sediment class is replaced by a more descriptive set of terms (Fig. F4).

Sediments were divided into lithostratigraphic units on the basis of composition and abundance of different grain types estimated from visual examination of the core, smear slides, thin sections, shipboard measurements of carbonate content, and shipboard XRD analyses. When encountered, pebble-sized fragments and larger particles (e.g., dropstones, pumice, etc.) are noted in each VCD. Size divisions for grains are those of Wentworth (1922).

Terms that describe lithification vary depending upon the dominant composition, as described below:

1. Sediments derived predominantly from calcareous pelagic organisms (e.g., calcareous nannofossils and foraminifers): the lithification terms “ooze,” “chalk,” and “limestone” reflect whether the sediment can be deformed with a finger (ooze), can be scratched easily by a fingernail (chalk), or is unable to be scratched with a fingernail (limestone).

2. Sediments derived predominantly from siliceous microfossils (e.g., diatoms and radiolarians): the lithification terms “ooze,” “diatomite/radiolarite,” “porcellanite,” and “chert” reflect whether the sediment can be deformed with a finger (ooze), cannot be easily deformed manually (diatomite/radiolarite), or displays a glassy luster (chert). We use the term “porcellanite” as defined by Keene (1975) to describe a siliceous limestone/claystone that has a dull luster and is less hard and compact than chert. Porcellanite may contain a mix of opal, quartz, clay minerals, and carbonate. Note that the terms “porcellanite” and “chert” do not imply crystallinity of the silica.

3. Sediments derived predominantly from siliciclastic material: if the sediment can be deformed easily with a finger, no lithification term is added and the sediment is named for the dominant grain size. For more consolidated material, the lithification suffix “-stone” is appended to the dominant size classification (e.g., “clay” versus “claystone”).

Each recovered lithology is shown in the Graphic lithology column on the VCD (Fig. F2). For intervals composed of more than one lithology, symbols are arranged within the column from left to right in order of their relative abundance. Graphic lithologies are used for all components that comprise 25% or more of the total sediment. The width of each pattern in the column approximates the relative abundance of that component. For instance, for nannofossil clay, the left half of the column would show the “clay” graphic lithology and the right half of the column would show the “nannofossil” graphic lithology. For nannofossil ooze with clay, the leftmost 75% of the column would show “nannofossil ooze” and the rightmost 25% of the column would show “clay.” For nannofossil clay with radiolarians, the Graphic lithology column would show (from left to right) 50% clay, 25% nannofossils, and 25% radiolarians.

Smear slide descriptions

Smear slide samples were taken by toothpick sampling of the section halves to define the lithologies. Visual percentage estimates of biogenic, nonbiogenic, and textural features were made from each slide. Biogenic components were divided into major microfossil groups (e.g., nannofossils, radiolarians, and foraminifers). Basic mineralogies were identified. In order to quantify the components observed in the smear slides, we used the following categories:

• 0 = absent (0%).

• P = present (<1%).

• F = few (1%–10%).

• C = common (>10%–25%).

• A = abundant (>25%–50%).

• VA = very abundant (>50%).

Spectrophotometry and visual color determination

Spectrophotometry and magnetic susceptibility of the archive section halves were measured with the SHMSL. The SHMSL also takes measurements on empty intervals and in places where the core surface is well below the level of the core liner, resulting in spurious measurements. Spurious measurements can also result from small cracks, sediment disturbance caused by the drilling process, or plastic section dividers. These data points are to be edited out of the data set by the user. Additional detailed information about measurement and interpretation of spectral data can be found in Balsam et al. (1997, 1998) and Balsam and Damuth (2000).

Reflectance of visible light from the archive halves of sediment cores was measured using an Ocean Optics USB4000 spectrophotometer mounted on the automated SHMSL. Freshly split cores were covered with clear plastic wrap and placed on the SHMSL. Measurements were taken at 1–5 cm spacing to provide a high-resolution stratigraphic record of color variation for visible wavelengths. Each measurement was recorded in 2 nm wide spectral bands from 400 to 900 nm.

In addition to the digital color image captured by the SHIL, VCDs include a description of sediment color and the corresponding hue, value, and chroma data as determined qualitatively using Munsell Soil Color Charts for each major and minor lithology (Munsell Color Company, 1994). These data were recorded immediately after cores were split to avoid color changes associated with drying and redox reactions.

Sedimentary structures

Sedimentary structures formed by natural processes (i.e., not a result of drilling disturbance) are represented on the VCD with symbols in the Structures column (see Fig. F3 for the definition of structures and symbols). Structures formed by both biogenic and physical processes are included. An estimate of bioturbation intensity is indicated on the left side of the Biostratigraphy column. Bioturbation intensity is classified as

• Absent, for laminated sediments;

• Slight, for sediments with still-visible horizontal bedding;

• Moderate, for sediments with obvious burrows;

• Heavy, for sediments with a nearly uniform appearance and rare burrows; and

• Complete, for completely uniform (homogeneous) sediments with no obvious burrows or sedimentary layers.

When identifiable, ichnofossils such as Zoophycos, Chondrites, and Planolites burrows were reported in the lithologic description. Sedimentary structure types are indicated on the right side of the Structures column (e.g., traction structures, soft-sediment deformation structures, etc.). All contacts between lithologies are gradational unless otherwise specified.

Drilling disturbance

Sediment disturbance resulting from the coring process is illustrated in the Drilling disturbance column on the VCD (e.g., fall-in, flow-in, biscuits, and drilling breccia) (Fig. F2). If the feature extends over an interval, the symbol appears centered on a vertical line to denote the extent of occurrence. Blank regions indicate an absence of drilling disturbance. Disturbance intensity follows the following subjective scheme:

• Slight

• Slight to moderate

• Moderate

• Moderate to high

• High

• Destroyed

Shipboard sampling

VCDs display the interval where sample material was taken for shipboard analysis (all whole rounds and all samples taken to aid core description). Whole rounds consist of samples taken for interstitial water and micropaleontology samples. Samples taken to aid core description include toothpick samples that were analyzed for smear slides, thin section billets, and discrete samples for mineralogical XRD analysis (Fig. F2). Typically, three or four smear slides were made per core, but more samples were selected in cores showing a high degree of lithologic variability. Interstitial water samples were taken at designated intervals, and a micropaleontology sample was obtained from the core catcher of most cores. XRD samples were taken only where needed to assess the lithologic components.

X-ray diffraction analyses

Bulk sample XRD analyses were performed using a Bruker D-4 Endeavor X-ray diffractometer with a Vantec detector using Ni-filtered CuKα radiation. Instrument settings were

• Voltage = 40 kV.

• Current = 40 mA.

• Goniometer scan = 2°–70°2θ (air-dried samples).

• Step size = 0.01°2θ.

• Scan speed = 1.2°2θ/min.

• Count time = 0.5 s.

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