75%), common (50%-<75%), moderate (10%-<50%), rare (<10%), and barren (none); these levels were illustrated with graphic symbols in the "Bioturbation" column (Fig. F2).
Core description forms, or "barrel sheets," provide a summary of the data obtained during shipboard analysis of each sediment core. Detailed observations of each section were recorded initially by hand on standard ODP Visual Core Description (VCD) forms. Copies of original VCD forms are available from ODP upon request. This information was subsequently entered into AppleCORE (version 8.1m) software, which generates a simplified, annotated graphical description (barrel sheet) for each core. These barrel sheets appear alongside corresponding core photographs (see the "Core Descriptions" contents list). Site, hole, and depth in mbsf are given at the top of the barrel sheet, with mbsf positions of core sections indicated along the left margin. Columns on the barrel sheets include graphic lithology, bioturbation, sedimentary structures, accessory lithologies, fossils, sediment disturbance, sample types, and remarks. These columns are discussed below, followed by an outline of the lithostratigraphic classification used during Leg 198.
Lithologies of the core intervals recovered are represented on barrel sheets by graphic patterns in the column titled "Graphic Lithology" (Fig. F1). For intervals containing homogeneous mixtures of multiple lithologies, 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 greater of the total sediment. The width of each pattern in the column approximates the relative abundance of that component. Relative abundances reported in this volume are useful for general characterization of the sediment, but they are not precise, quantitative data.
Sedimentary structures formed by natural processes and not as a result of drilling disturbance are represented on the barrel sheet under the "Sedimentary Structures" column (Fig. F2). Structures formed by both biogenic and physical processes are included. These include varying degrees of bioturbation, types of trace fossils, parallel laminations, and soft sediment deformation structures.
Using a scheme similar to that proposed by Droser and Bottjer (1986), five levels of bioturbation were recognized. Bioturbation intensity was classified as abundant (75%), common (50%-<75%), moderate (10%-<50%), rare (<10%), and barren (none); these levels were illustrated with graphic symbols in the "Bioturbation" column (Fig. F2).
Symbols are used to denote accessory lithologies, authigenic minerals, concretions, fossils, and sediment disturbance induced by the coring process (Fig. F2). Symbols are positioned at the location in the section where that feature is observed. If the feature extends over an interval, the symbol appears centered on a vertical line to denote the stratigraphic extent of occurrence.
Colors were determined qualitatively using the Munsell rock color charts (Rock-Color Chart Committee, 1991) and, to avoid color changes associated with drying and redox reactions, were described immediately after the cores were split.
Sample material taken for shipboard sedimentologic and chemical analysis consisted of pore water whole-round samples, "toothpick" samples (smear slides), thin section billets, and discrete samples for XRD and coulometric analysis. Typically, two to three smear slides (or thin sections) were made per core, one pore water, and two coulometer samples taken per core. XRD samples were taken only where needed to assess the lithologic components. Additional samples were selected to better characterize lithologic variability within a given interval. Tables summarizing data, such as grain size and relative abundance of sedimentary components from smear slides, were generated using a spreadsheet program (Sliders).
The written description for each core contains a brief overview of major and minor lithologies that are present, as well as notable features (e.g., sedimentary structures).
Lithologic names consist of a principal name based on composition, degree of lithification, and/or texture as determined from visual description and smear slide 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. Thus, 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 ranging from 10% to 25% were reflected in the sediment name in the description column as "WITH," but these components were not designated in the graphic lithology column. These naming conventions follow the ODP sediment classification scheme (Mazzullo et al., 1988), with the exception that during Leg 198 a separate "mixed sediment" category was not distinguished. During Leg 198, we did not encounter neritic sediments or chemical sediments except as accessory minerals and do not address these categories below.
Sediment was classified on the basis of composition estimated by visual examination of the core, smear slides, and thin sections, and by shipboard measurements of carbonate content (see "Carbonate Analysis" in "Organic Geochemistry") and shipboard XRD analyses (see "X-Ray Diffraction"). In volcaniclastic sediments, the term "ash" (or "tuff" if lithified) is used in place of "sand," whereas "lapilli" is used for granule and cobble size categories. Larger volcanic clasts (breccia) were not encountered, but discrete pumice lapilli are noted as "pumice clasts." Size divisions for grains are those of Wentworth (1922) (Fig. F3). Size-textural qualifiers were not used for pelagic sediment names (e.g., nannofossil clay implies that the dominant component is detrital clay rather than clay-sized nannofossils).
Terms that describe lithification vary depending upon the dominant composition:
In addition to visual estimates of the color, reflectance of visible light from soft sediment cores was routinely measured using a Minolta spectrophotometer (model CM-2002) mounted on the AMST. The AMST measures the archive half of each core section and provides a high-resolution stratigraphic record of color variations for visible wavelengths (400-700 nm). Freshly split cores were covered with clear plastic wrap and placed on the AMST. Measurements were taken at 2.5 cm spacing. The AMST skips empty intervals and intervals where the core surface is well below the level of the core liner, but the AMST cannot recognize relatively small cracks or disturbed areas of core. Thus, AMST data may contain spurious measurements that should, to the extent possible, be edited out of the data set before use. Each measurement recorded consists of 31 separate determinations of reflectance in 10-nm-wide spectral bands from 400 to 700 nm. Additional detailed information about measurement and interpretation of spectral data with the Minolta spectrophotometer can be found in Balsam et al. (1997, 1998) and Balsam and Damuth (2000).
To describe important mineralogic and structural features in both the archive and working halves, we examined core sections containing igneous rocks prior to cutting with a diamond-impregnated saw. 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 were 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 pointing to the top of the section was added.
Nondestructive physical properties measurements, such as natural gamma ray emission, were made on the core before it was split (see "Physical Properties"). After the core was split, lithologic descriptions were made of the archive half and the working half was sampled for shipboard physical properties measurements (see "Physical Properties"), thin sections, and XRD. The archive half was described on the VCD form and was imaged and then photographed.
We used VCD forms to document each section of the igneous rock cores. The left column on the form represents the archive half. A horizontal line across the entire width of the column denotes a plastic spacer. Oriented pieces are indicated on the form by an upward-pointing arrow to the right of the piece. Locations of samples selected for shipboard studies are indicated in the column headed "Shipboard Studies," with the following notation: XRD = X-ray diffraction analysis and TSB = thin section billet. Core summaries of VCD descriptions for each core were produced in AppleCORE. Copies of VCDs are available from ODP upon request.
We subdivided the core into consecutively numbered lithologic units (denoted in the "Lithologic Unit" column on the VCD) on the basis of changes in color, structure, brecciation, grain size, vesicle abundance, mineral occurrence and abundance, and the presence of sedimentary interbeds. Intercalated sediment horizons were designated as "A" and the underlying volcanic rock as "B" within the same unit.
Written descriptions accompany the schematic representation of the core sections and include the following:
We examined thin sections from the core intervals noted on the VCD forms to complement and refine the hand specimen observations. In general, the same terminology was used for thin section descriptions as for the visual core descriptions. The percentages of individual phenocryst, groundmass, and alteration phases were estimated visually, and textural descriptions are reported in table format. The textural terms used are defined by MacKenzie et al. (1982). Thin section examination resulted in modification of some rock names. At least one thin section per subunit was described.
Selected samples were taken for qualitative mineral analysis with an XRD Philips model PW1729 X-ray diffractometer using Ni-filtered CuK
radiation. Instrument conditions were as follows: 40 kV, 35mA; goniometer scan from 2° to 70°2
(air-dried samples) and from 2° to 12°2 (glycolated samples); step size of 0.01°2; scan speed at 1.2°2/min; and count time of 0.5 s for each step.
Some samples were decalcified using 10% acetic acid then washed repeatedly with demineralized water in a centrifuge. The carbonate-free fraction was deflocculated with a 1% Calgon (sodium hexametaphosphate) solution and homogenized in a sonic dismembrator for 1 min. The clay fraction (<2 µm) was then separated by centrifugation, and the clay residue was deposited onto glass slides. MacDiff software (version 4.1.1 PPC by Rainer Petschick) was used to display diffractograms, and identifications are based on multiple peak matches, using the mineral database provided with MacDiff. Diffractograms were peak corrected to match the calcite peak at 3.035 Å. In the absence of calcite, no peak correction was applied.
A digital core imaging system (DIS) was installed during the port call prior to Leg 198. For the first time, systematic, high-resolution digital scanning of the archive half of each core was included in the shipboard core flow (following core description). The images for each core section are available in the Janus database (see the "Related Leg Data" contents list).
