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doi:10.2204/iodp.proc.320321.102.2010 LithostratigraphyThe lithostratigraphy of sediments recovered during Expedition 320/321 was determined by a combination of core description, smear slide and thin section analyses, and digital color imaging. The methods employed were based on those used during ODP Leg 199 (Shipboard Scientific Party, 2002), which recovered sediments similar to those drilled during Expedition 320/321, and IODP Expedition 320T, the first expedition to use the new IODP core description and database systems employed here (Expedition 320T Scientists, 2009). New core description process and databaseA new core description process and database, first implemented and assessed during Expedition 320T, was used during Expedition 320/321 using DESClogik software. Prior to drilling operations, two spreadsheet templates were constructed in Tabular Data Capture and customized for Expedition 320 using the lithostratigraphic results of ODP Legs 199 and 138 as a guide (Shipboard Scientific Party, 2002; Mayer, Pisias, and Janecek, et al., 1992). The first of these spreadsheet templates was used to record core descriptions (Fig. F5). The second template was used to record descriptions of smear slides and thereby quantify the texture and relative abundance of biogenic and nonbiogenic components (Fig. F6). Visual core description and barrel sheetsVisual core descriptions of the archive half of the split cores provide a summary of the data obtained during shipboard analysis. Visual core description data were recorded digitally in real time using DESClogik. DESClogik includes a graphic display mode of core data display (e.g., aligned digital images of section halves and various measurement data) that can be used to aid core description. During Expedition 320/321, the STRATER software package was used to compile the visual core descriptions for each core. Site, hole, and core number are given at the top of the visual core description together with a summary core description (Fig. F7). Core depth below seafloor, core length, and section breaks 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 show data collected by the WRMSL and SHMSL (see "Physical properties"). These include GRA densitometer bulk density, corrected magnetic susceptibility, and lightness as determined by color reflectance (L*). Columns to the right of these data show stratigraphic age, shipboard samples taken, sedimentary structures (bioturbation intensity and sedimentary structure type), lithologic accessories, and sediment disturbance. Digital color imageThe SHIL imaged the flat face of split cores using a line scan camera. Sediment cores are imaged as soon as possible after splitting to minimize color changes that occur through oxidation and drying. The flat face of the archive-half section was scraped with the edge of a glass slide to provide a "clean" surface for imaging. 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 light emitting diode line lights with fully adjustable angles to the lens axis. Note that compression of line-scanned images into compiled stacks may result in artifacts (e.g., false appearance of lamination). Graphic lithologyEach recovered lithology is shown in the Graphic Lithology column on the visual core description (Figs. F7, F8). 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 compose 25% or more of the total sediment. The width of each pattern in the column approximates the relative abundance of that component. Sedimentary structuresSedimentary structures formed by natural processes (i.e., not a result of drilling disturbance) are represented on the barrel sheet with symbols in the Structures column. Structures formed by both biogenic and physical processes are included. An estimate of bioturbation intensity is indicated on the left side of the Structures column. Bioturbation intensity is classified as nonvisible, indicating either the complete absence of bioturbation (0% of the surface area) or a completely bioturbated sediment (100% of the surface area); minor (<30% of the surface area); moderate (30%–60% of the surface area); and intense (>60% of the surface area) following Droser and Bottjer (1986) and Kemp (1995). When identifiable, ichnofossils of Zoophycos, Skolithos, Chondrites, and Planolites burrows were reported in the lithologic description; however, during Expedition 320 these were included in the electronic database "comments." Sedimentary structure types are indicated on the right side of the Structures column (e.g., nodules and soft-sediment deformation structures). All contacts between lithologies are gradational unless otherwise specified. Drilling disturbanceSediment disturbance resulting from the coring process is illustrated in the Drilling Disturbance column on the visual core description (e.g., flow-in, biscuits, and drilling breccia) (Fig. F8). 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. Sample typesVisual core descriptions display 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 pore water and micropaleontology samples. Samples taken to aid core description including "toothpick" samples were analyzed for smear slides, thin section billets, and discrete samples for mineralogical analysis by XRD (Fig. F8). 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 pore 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. ColorIn addition to the digital color image captured by the SHIL, visual core descriptions show sediment color and the corresponding hue, value, and chroma data as determined qualitatively using Munsell Soil Color Charts (Munsell Color Company, 1994). These data were recorded immediately after cores were split to avoid color changes associated with drying and redox reactions. Core summaryThe written description for each core contains a brief overview of both major and minor lithologies, their Munsell color, and notable features such as sedimentary structures and major disturbances resulting from the coring process. Smear slide descriptionsSmear slide samples were taken by toothpick sampling of the split core to establish major and minor lithologies. Visual percentage estimates of biogenic, nonbiogenic, and textural features were made from each slide (Fig. F6). Biogenic components were divided into major microfossil groups (e.g., nannofossils, radiolarian, fish debris). Basic mineralogies were identified. The zeolite minerals phillipsite and clinoptilite were identified and tabulated individually in the smear slide template; however, they are depicted in the final visual core description as "zeolites." Sediment classificationLithologic names consist of a principal name based on composition, degree of lithification, and/or texture as determined from visual core description, smear slide, and 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. Minor components that represent between 10% and 25% of the sediment follow the principal name (after a "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. Sedimentary components that make up <25% of the total sediment are not designated in the Graphic Lithology column. Our naming conventions follow the ODP sediment classification scheme (Mazzullo et al., 1988). 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 (see below), and shipboard XRD analyses (see below) (Fig. F9). When encountered, fragments of pumice are noted in each visual core description. Size divisions for grains are those of Wentworth (1922) (Fig. F9). Terms that describe lithification vary depending upon the dominant composition, as described below:
Core curation and shipboard sampling of igneous rocksWe examined core sections containing igneous rocks prior to cutting the core with a diamond-impregnated saw to describe important mineralogic and structural features. Each piece was numbered sequentially from the top of the core section and labeled on the outside surface. Pieces that could be fit together were assigned the same number and lettered consecutively (e.g., 1A, 1B, 1C, etc.). Plastic spacers were placed between pieces with different numbers. The presence of a spacer may represent a substantial interval of no recovery. If it was evident that an individual piece had not rotated about a horizontal axis during drilling, an arrow was added pointing to the top of the section. Visual core descriptions and barrel sheets for igneous rocksThe few fragments of basalt recovered during Expedition 320/321 were described in the database templates and annotated visual core descriptions of the core were used for the sediments. X-ray diffraction analysesBecause of technical difficulties, only a handful of XRD analyses were conducted during Expedition 320/321. Bulk sample analyses were performed using a Bruker D-4 Endeavor X-ray diffractometer with a Vantec detector using Ni-filtered CuKα radiation. Instrument conditions were as follows: 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, and count time = 0.5 s. |
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