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doi:10.2204/iodp.proc.341.102.2014

Lithostratigraphy

The primary lithostratigraphic procedures used during Expedition 341 included visual core description, sediment classification, digital color imaging, XRD, and smear slide preparation and description. Color spectrophotometry and point source magnetic susceptibility data acquired by the Lithostratigraphy group during core description are described in detail in “Physical properties.” Percent carbonate, percent organic (CHNS), major element, and trace element geochemical measurements on sediment and sedimentary rocks collected by the Lithostratigraphy group are described in detail in “Geochemistry.”

Core preparation

The technique applied for splitting cores into working and archive halves (either using a piano wire or a saw) affected the appearance of the split core surface. In soft sediment, the wire tended to drag mud across the split core surface, thereby obscuring the clasts, which may have resulted in an emphasis on muddier lithologies in the visual core descriptions (VCDs). Cutting by saw allowed for the identification of sedimentary structures, clast distribution, and clast composition at much greater detail. However, intervals of low density (e.g., diatom-rich mud and ooze) were often disturbed by water introduced during sawing. Splitting method choices were made by considering the physical properties data (density and magnetic susceptibility) determined on whole-round cores, and the saw was used when gamma ray attenuation (GRA) bulk density and magnetic susceptibility values both were high, indicating clast-rich, indurated lithologies.

Prior to core description and high-resolution digital color imaging, the quality of the split core surface of the archive half of each core was assessed, and when necessary (e.g., the surface was irregular or smeared), the split core surface was scraped lightly with a flexible plastic nonmagnetic card. Cleaned split core sections were then described in conjunction with measurements by the SHIL, discussed below, and SHMSL (see “Physical properties”).

Visual core descriptions

Macroscopic and smear slide descriptions of each section (nominally 0–150 cm long) were recorded on handwritten visual core description forms (Fig. F1). All handwritten forms were digitally preserved as PDF files (see LITH in “Supplementary material”). Standard sedimentological observations of lithology, sedimentary structures, color, bioturbation, and accessories were entered into the form, as well as specific comments, when necessary, on these features. Information in the Accessories column includes documentation of macroscopic biogenic remains, such as shells, worm tubes, and isolated gravel-sized clasts such as lonestones. When possible, clasts were described as sedimentary, igneous (plutonic or volcanic), or metamorphic (foliated or nonfoliated). Further, consideration of whole-round magnetic susceptibility and natural gamma ray data supported the identification and interpretation of distinct sedimentary features or intervals within the cores (e.g., reduced magnetic susceptibility and lower density often indicate the occurrence of diatom oozes).

Color

Sediment color was determined qualitatively for core intervals using Munsell Soil Color Charts (Munsell Color Company, Inc., 2009).

DESClogik data capture software

Visual core description forms were compiled and entered into the LIMS database using the DESClogik application. Direct entry of descriptive and interpretive information in the program was performed using Tabular Data Capture mode. Before core description began, a spreadsheet template was constructed in Tabular Data Capture mode. Two templates containing category columns for texture and relative abundance of biogenic/mineralogic components were configured specifically for recording smear slide and thin section data, respectively. Another template containing category columns for texture and relative abundance of tephric components was configured specifically for recording volcanic grains. Data entered in DESClogik were then uploaded into the LIMS database.

Standard graphical report

A one-page graphical representation of each section was generated using the LIMS2Excel application and a commercial program (Strater, Golden Software). VCDs are generated with a CSF-A depth scale, split-core high-resolution color images, graphic lithology, volcanic grain content, drilling disturbance, bioturbation intensity, sedimentary structures, lithologic accessories (e.g., clast abundance or macrofossils), age, magnetic susceptibility, GRA bulk density, color reflectance (b*), and shipboard sample collection (Fig. F2). Graphic lithologies, sedimentary structures, and other visual observations shown on the VCDs by graphic patterns and symbols are explained in Figure F3.

Smear slides

To aid in lithologic classification, smear slide microscope analysis was used to determine mineralogy, microfossil, and volcanic constituents and abundance. Toothpick samples were taken from representative sections of lithology and at a frequency of at least one sample per core (~9.5 m). Each slide was prepared by completely mixing the sediment with several drops of distilled water on a glass microscope slide and dried on a hot plate at 50°C. The dried sample was then mounted in Norland optical adhesive 61 and fixed in an ultraviolet light box. Type and relative abundance of biogenic, volcanic, and mineralogic components were estimated (Fig. F4) for each smear slide using a transmitted-light petrographic microscope equipped with a camera. Data were entered into the LIMS database using a custom tabular template in DESClogik. Images of some smear slides were taken and uploaded into DESClogik as well.

Lithologic classification scheme

The lithologic description for granular sediments was based on the classification schemes used during Ocean Drilling Program (ODP) Leg 178 (Shipboard Scientific Party, 1999), IODP Expedition 317 (Expedition 317 Scientists, 2011), and IODP Expedition 318 (Expedition 318 Scientists, 2011).

The principal sediment/sedimentary rock name was based on the relative abundance of components present, including siliciclastic, carbonate, biogenic, and/or volcanic grains. If well indurated or cemented, the modifier of “stone” was used. These sediments were classified using Figure F5, based on Expedition 317. The principal name of sediments with >50% siliciclastic grains was based on an estimate of the grain sizes present. The Wentworth (1922) scale was used to define size classes:

• If no gravel was present, the principal sediment/rock name was determined based on the relative abundances of sand, silt, and clay (after Shepard, 1954) (Fig. F5).
• If the sediment/rock contains siliciclastic gravel, then the principal name was determined from the relative abundance of gravel (>2 mm) and sand/mud ratio of the clastic matrix following the classification of Moncrieff (1989) and used during Expedition 318 (Fig. F6).

The primary name for sediment/rock with >50% biogenic grains was “ooze,” modified by the most abundant specific biogenic grain type that forms 50% or more of the sediment or rock (Fig. F7). For example, if diatoms exceed 50%, then the sediment is called “diatom ooze.” However, if the sediment is composed of 40% diatoms and 15% sponge spicules, then the sediment was termed “biosiliceous ooze.” This scheme was also used with carbonate biogenic grains.

Major and minor modifiers for biogenic sediment were also applied to the principal sediment/rock names following the scheme of Expedition 318 (Expedition 318 Scientists, 2011):

• Major modifiers are those components with abundances between 25% and 50% and are indicated by the suffix “rich” (e.g., “diatom-rich”).
• Minor modifiers are those components with abundances of 10%–25% and are indicated by the suffix “-bearing” (e.g., “diatom-bearing”).

The products of volcanic eruptions can be introduced into the sedimentary environment at the drill sites by both primary (air fall and sedimentation through the water column) and secondary (e.g., reworking by sediment-gravity flows, ice rafting, etc.) processes. The term “volcaniclastic-bearing” was used for sediment containing between 10% and 50% volcanic grains without consideration of the origin and environment (Brown, 2007; Fisher and Schmincke, 1984). “Volcaniclastic-rich” was used for sediment with 50%–90% volcanic grains. The principal name of a sediment/rock with >90% primary volcanic grains, such as glass shards and other pyroclastic components, was “ash” (Fisher and Schmincke, 1984) (Fig. F8). Using this definition, ash is defined as unconsolidated, fine (<2 mm) pyroclastic material, whereas the term “tuff” is used for the consolidated or lithified counterpart.

Bed/Lamination and thickness

Boundaries between different lithologies are classified as sharp or gradational. Bedding and lamination are defined following Mazzullo et al. (1988):

• Thinly laminated (1–3 mm thick),
• Laminated (3 mm to 1 cm),
• Very thin bedded (1–3 cm),
• Thin bedded (3–10 cm),
• Medium bedded (10–30 cm),
• Thick bedded (30–100 cm), and
• Very thick bedded (>100 cm).

For units in which two lithologies are closely interbedded (the individual beds are <15 cm thick and alternate between one lithology and another), three “interbedded” lithology names are used as primary units (Fig. F3) modified from Expedition 317 (Expedition 317 Scientists, 2011): interbedded sand and mud, interbedded silt and mud, and interbedded mud and diamict. When beds are distributed throughout a different lithology (e.g., beds of sand several centimeters to tens of centimeters thick within a mud bed), they are logged individually and the associated bed thickness and grain size ranges are described.

Bioturbation

Ichnofabric description included the extent of bioturbation and notation of distinctive biogenic structures. To assess the degree of bioturbation semiquantitatively, a modified version of the Droser and Bottjer (1986) ichnofabric index scheme was used (1 = no apparent bioturbation, 2 = slight bioturbation, 3 = moderate bioturbation, 4 = heavy bioturbation, and 5 = complete bioturbation) (Fig. F9). Massive mud may be deposited rapidly in a glacial environment and record no evidence of bioturbation; therefore, it was assigned a value of 1. However, mud may also completely lack sedimentary structures because of complete bioturbation (e.g., 5 on the scale), which may be accompanied by color mottling. This index is graphed using the numerical scale in the Bioturbation intensity column of the VCDs. Recognizable biogenic structures and trace fossils were noted and logged in the LIMS database through DESClogik.

Clast abundance

Clast abundance was determined by counting lonestones and diamict clasts that were visible on the archive-half sediment surface. The working half was examined when only holes or depressions caused by lonestones and diamict clasts were observed in the archive half. Size, lithology, and shape of lonestones and diamict clasts were determined whenever possible (i.e., without disrupting the sediment surface). Only lonestones and diamict clasts larger than 2 mm were counted. In the case of isolated lonestones contained in mud, the number of clasts per described subunit of a core section was entered into DESClogik. When the number of clasts per subunit of a core section became relatively common, the number of lonestones was not directly counted. The modifiers “with dispersed,” “with common,” or “with abundant” clasts and “clast-rich” and “clast-poor” then accounted for the clast abundance following the Moncrieff (1989) classification (Fig. F6). Details on size, lithology, shape, and the specific lithology of observed lonestones and diamict clasts are provided in the written core descriptions and/or the DESClogik General interval comments column.

Core disturbance

Core disturbance from the drilling process may alter the cores slightly (bent/bowed bedding contacts) or greatly (complete disruption of stratigraphic sequence). To document drilling disturbances, the following classification scheme is used. Drilling disturbance of relatively soft or firm sediment was classified into three categories:

1. Slightly disturbed: bedding contacts are slightly bent or bowed in a concave-downward appearance.
2. Extremely disturbed: bedding is completely deformed and may show diapiric or minor flow structures.
3. Soupy: sediment is water saturated and shows no traces of original bedding or structure.

Drilling disturbance of harder sediment (i.e., lithified by compaction or cementation) was classified into four categories:

1. Slightly fractured or biscuited: core pieces are in place and have very little drilling slurry or brecciation.
2. Moderately fractured or biscuited: core pieces are from the cored interval and are probably in the correct stratigraphic sequence (although the entire section may not be represented); intact core pieces are broken into rotated discs (or “biscuits”) as a result of the drilling process, and drilling mud has possibly flowed in.
3. Highly fractured or brecciated: pieces are from the cored interval and are probably in the correct stratigraphic sequence (although the entire section may not be represented), but the original orientation is totally lost and drilling mud has flowed in.
4. Highly fractured or drilling slurry: pieces are from the cored interval and are probably in the correct stratigraphic sequence (although the entire section may not be represented), but the original orientation is totally lost; loose pieces of core material are mixed with the drilling slurry.

In addition to these categories for soft and hard sediment, several other terms were used to characterize drilling disturbances. Although many other forms of drilling disturbance were observed (Fig. F3), the most common types are as follows:

• Suck-in: completely disturbed stratigraphic record due to soft (often sandy) sediment sucked into the core liner while pulling the drill string upward. Characteristic features are vertical stratification and flow-in structures in the middle of the section.
• Washed gravel: fine material was probably lost during drilling, with only washed coarse material remaining, commonly pebbles or cobbles. This may result from problems recovering coarse-grained diamict.
• Flow-in: soupy, displaced sediment was drawn into the core liner during APC coring.
• Fall-in: downhole contamination results from the falling of loose material from the drill hole walls into the top of the core. The uppermost 10–15 cm of each core was inspected during description for potential fall-in.

In additional to drilling-related artifacts, disturbance also occurred during core handling. The split core surface of relatively high porosity lithologies occasionally was disturbed because of excess pressure applied by the point magnetic susceptibility SHMSL lander. Pore water sampling by rhizons also disturbed the sedimentary fabric within several centimeters of the sampling hole.

Digital color imaging

The SHIL captures continuous high-resolution images of the archive-half surface for analysis and description. Images were collected shortly after core splitting and core description in an effort to avoid color changes resulting from sediment drying and oxidation of the surface. The shipboard system uses a commercial line-scan camera lens (AF Micro Nikon; 60 mm; 1:2.8 D), with illumination provided by a custom assembly of three pairs of light-emitting diode strip lights that provide constant illumination over a range of surface elevations. Each pair of lights has a color temperature of 6,500 K and emits 90,000 lux at 76 mm. The resolution of the line-scan camera was set at 10 pixels/mm. Available files include the original high-resolution TIFF image with grayscale and ruler, as well as reduced JPEG images cropped to show only the section-half surfaces.

X-ray diffraction analysis

Samples for XRD analyses were selected from the working half, generally at the same depth as sampling for solid-phase geochemistry and smear slides. Additional samples were taken when distinct lithologic changes occurred. In general, one 5 cm³ sample was taken of a representative lithology per core, typically in Section 1 or 2. Additional samples were occasionally taken and analyzed based on visual core observations (e.g., color variability and visual changes in lithology and texture) and smear slides. Samples taken once per core for XRD analysis also were generally analyzed for sedimentary inorganic (i.e., carbonate analysis) and organic (i.e., CHNS analysis) carbon in the Geochemistry Laboratory (see “Geochemistry”). Samples analyzed for bulk mineralogy were freeze-dried in the case of unlithified samples and ground by hand or in an agate ball mill as necessary. Prepared samples were top-mounted onto a sample holder and analyzed using a Bruker D-4 Endeavor diffractometer mounted with a Vantec-1 detector, using nickel-filtered CuKα radiation. The standard locked coupled scan was as follows:

• Voltage = 37 kV.
• Current = 40 mA.
• Goniometer scan = 4°–70°2θ.
• Step size = 0.0174°2θ.
• Scan speed = 1 s/step.
• Divergence slit = 0.3 mm.

Shipboard results yielded only qualitative results of the presence and relative abundances of the most common mineralogical components.

Diffractograms of bulk samples were evaluated with the aid of the EVA software package, which allowed for mineral identification and basic peak characterization (e.g., baseline removal and maximum peak intensity). Files were created that contained d-spacing values, diffraction angles, and peak intensities with and without the background removed. These files were scanned by the EVA software to find d-spacing values characteristic of a limited range of minerals, using aluminum oxide as an internal standard (Expedition 317 Scientists, 2011). Peak intensities were reported for each mineral to provide semiquantitative measures of mineral abundances downhole and among sites. Muscovite/illite and kaolinite/chlorite have similar diffraction patterns and were not distinguished shipboard. Digital files with the diffraction patterns are available from the LIMS database (iodp.tamu.edu/tasapps/). The presence of expandable clay minerals was analyzed by treating the samples with ethylene glycol following Moore and Reynolds (1997).

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