Biogenic Sediments

Unlike siliciclastic sediments, biogenic sediments, defined as containing >60% biogenic components, are not described based on texture. Rather, the principal name for all biogenic sediments is ooze. If the siliciclastic component represents 5%-40% of a sediment, the naming conventions using "-rich" and "-bearing" described above are followed. Thus, a sediment composed of 30% siliciclastic clay and 70% sand-sized foraminifers is called a clay-rich foraminifer ooze, not a clay-rich foraminifer sand.

Mixed Sediments

Subequal mixtures of biogenic and nonbiogenic material, where the biogenic content is 40%-60%, are termed "mixed sediments" in the ODP classification (Mazzullo and Graham, 1988). The name of a mixed sediment consists of a major modifier(s) consisting of the name(s) of the major fossil group(s), with the least common fossil listed first, followed by the principal name appropriate for the siliciclastic components (e.g., foraminifer clay). The same naming conventions for using "-bearing" and "-rich" apply to mixed sediments as described above. An unconsolidated sediment containing 5% foraminifers, 40% nannofossils, and 55% silt is, thus, called a foraminifer-bearing nannofossil silt. Sediment containing 5% diatoms, 40% clay, and 55% nannofossils is called a diatom-bearing nannofossil clay.

The definition of Gealy et al. (1971) was used to identify the firmness of recovered sediments. Siliceous sediments and rocks are divided into two classes of firmness:

1. Soft sediments, which are composed of gravels, sands, silts, and clays (sediment core can be split with a wire cutter), and
2. Hard sediments, which are composed of conglomerates, sandstones, siltstones, and claystones (sediment core must be cut by a band or diamond saw).

Calcareous sediments and rocks are divided into three classes of firmness:

1. Unlithified: soft sediments that readily deform under the pressure of a fingernail or spatula;
2. Partly lithified: firm but friable sediments that can be scratched with a fingernail or the edge of a spatula; and
3. Lithified: hard, nonfriable cemented rocks that are difficult or impossible to scratch with a fingernail or the edge of a spatula.

The definitions and nomenclatures of special rock types were adopted and modified from ODP Legs 112 and 138 (Suess, von Huene, et al., 1988; Mayer, Pisias, Janacek, et al., 1992) and adhere as closely as possible to conventional terminology. One special rock type, authigenic carbonate, was particularly important during Leg 204.

Authigenic carbonates (aragonite, calcite, and dolomite) are present in various morphologies, as nodules or irregular precipitates, and are designated as "C" in the "Diagenesis" column of the core description forms ("barrel sheets") (Figs. F4, F5). In cases where it was possible to clearly identify the carbonate mineralogy, symbols for the respective carbonate minerals were used (see Fig. F3). The degree of lithification is noted in the core description as "friable" (the rock showed only partial lithification) or "lithified" (the rock is fully cemented).

Detailed sedimentologic observations and descriptions were recorded manually for each core section on VCD sheets. A wide variety of features that characterize the sediments were recorded, including lithology, sedimentary structures, color, diagenetic precipitates, and core disturbance. Compositional data were obtained from smear slides. The color (hue and chroma) of the sediments was determined by both color spectrophotometry and, more often, by visual comparison with the Munsell soil color chart (Munsell Color Co., 1975). This information was synthesized for each core in the AppleCORE software (version 8.1m), which generates a simplified one-page graphical description of each core, or barrel sheet (Fig. F4). Barrel sheet symbols used during Leg 204 are described in Figure F5. For more detailed information on sedimentary features, VCD forms are available from ODP upon request.

Of particular interest during Leg 204 were the visual indications of disruption to the sediment caused by the dissociation of gas hydrate in the recovered sediment. Massive hydrate was removed on the catwalk prior to core description, but sampled intervals were noted in the barrel sheets. The two primary textures identified as resulting from the dissociation of gas hydrate are soupy and mousselike. Soupy sediments are watery, homogeneous, and fluidized (Fig. F6). These sediments are often associated with void spaces in the core because they are able to flow from their original position during core recovery and therefore retain no original sedimentary structures. Sediments containing mousselike textures can be divided into two descriptive types based on water content. Wet, watery mousselike sediment texture is soft and deforms plastically under slight pressure from one finger (Fig. F7). Mousselike texture can also occur in drier sediments that are stiffer and tend to form brittle flakes, which break off under the pressure of one finger (Fig. F8). These drier, stiffer sediments often appear foliated when split by the core cutter wire. Both types of mousselike texture contain gas vesicles and obscures primary sedimentary structures.

Mousselike and soupy textures related to the dissociation of hydrate not sampled prior to description were observed at all sites and noted on the barrel sheets. Remarks made on barrel sheets for each core describe any additional potential indications of gas hydrate near the sampled intervals, including the presence of dry, flaky sediment that may have been dewatered by the formation of hydrate nearby.

The lithology and grain size of the described sediments are represented graphically in a column of the barrel sheets using the symbols illustrated in Figure F5. A maximum of three different lithologies (for interbedded sediments) or three different components (for mixed sediments) can be represented within the same core interval. Intervals that are a few centimeters or greater in thickness can be portrayed accurately in the lithology column. Percentages are rounded to the nearest 10%, and lithologies that constitute <10% of the core are generally not shown but are listed in the "Description" column.

Visible bioturbation was classified into four intensity levels based on the degree of disturbance of the physical sedimentary structures:

1. Absent = no bioturbation; all physical sedimentary structures are preserved.
2. Rare = isolated trace fossils; up to 10% of physical sedimentary structures are disrupted.
3. Moderate = ~10%-40% disrupted physical sedimentary structures; burrows are generally isolated but may overlap locally.
4. Abundant = bedding completely disturbed; burrows are still intact in places.

These categories are based on the ichnofossil indices of Droser and Bottjer (1986) and are illustrated with graphic symbols in the "Bioturbation" column on the barrel sheets. Visual recognition of bioturbation was often limited in homogeneous sediments, particularly in hemipelagic clay zones without sulfide material.

Each type of sedimentary structure and its exact location are displayed in the "Structure" column of the barrel sheet. Symbols are used to note the wide variety of sedimentary structures encountered throughout Leg 204, and these are listed in the legend for the barrel sheets. Some of the more common structures observed were parallel bedding, fining-upward sequences, and the mottled appearance of sulfide-rich layers.

The presence of macroscopic fossils (including aggregates of sponge spicules, large diatoms [~1 mm], shell fragments, preserved whole shells, and gastropods) is displayed in a separate column on the barrel sheets.

Drilling-related sediment disturbance that persists over intervals of ~20 cm or more is recorded in the "Disturbance" column. Separate terms are used to describe the degree of drilling disturbance in soft and firm sediments:

1. Slightly disturbed = bedding contacts are slightly deformed.
2. Moderately disturbed = bedding contacts have undergone extreme bowing.
3. Highly disturbed = bedding is completely deformed as flow-in, coring/drilling slough, and other soft sediment stretching and/or compressional shearing structures attributed to coring/drilling (e.g., gas expansion).
4. Soupy = intervals are water saturated and have lost all primary sedimentary structures.

The degree of fracturing in indurated sediments and igneous rocks is described using the following categories:

1. Slightly fractured = core pieces in place and broken.
2. Moderately fractured = core pieces are in place or partly displaced, but original orientation is preserved or recognizable.
3. Highly fractured = core pieces are probably in correct stratiggraphic sequence (although they may not represent the entire sequence), but original orientations are lost.
4. Drilling breccia = core pieces (small and angular pieces) have lost their original orientation and stratigraphic position and may be mixed with drilling slurry.
5. Drilling biscuits = drilling slurry surrounding an intact or slightly fractured drilling biscuit.

Cores recovered from gas and gas hydrate-bearing sediments are often disturbed by gas expansion and fracturing. In cases where it is possible to distinguish between disturbance of the core resulting from drilling and disturbance resulting from gas expansion, notes were made in the comments section of the barrel sheets listing the depths at which gas fracturing was observed.

The position of whole-round samples, as well as smear slides and samples taken to support and verify the observations of the smear slide and thin section analyses, are indicated in the "Sample" column on the barrel sheets. The abbreviations used are as follows: smear slide (SS), interstitial water (IW), microbiology (MBI), gas hydrate (HYD), personal sample (PS), and micropaleontology (PAL).

The relative position of features that are related to diagenesis are displayed in the "Diagenesis" column on the barrel sheets. These are mineral precipitates (e.g., pyrite, gypsum, and carbonate).

Smear slides were prepared from the archive halves of the cores. With a toothpick, a small amount of sediment was taken and put on a 1 in x 3 in glass slide, homogenized, and dispersed over the slide with a drop of deionized water. The sample was then dried on a hot plate at the lowest effective temperature. A drop of Norland optical adhesive and a 1 in x 1 in cover glass were added. The smear slide was fixed in an ultraviolet light box. With a transmitted light petrographic microscope, both the grain size and abundance of dominant components in a sample were determined. Abundance was estimated with the help of a comparison chart for visual percentage estimation (after Terry and Chilingarian, 1955). Note that smear slide analyses tend to underestimate the amount of sand-sized and larger grains because these grains are difficult to incorporate into the slide. Table T1 is an example of data obtained from smear slide analyses and was generated using the spreadsheet program SLIDERS. This table includes information about the location of samples, their grain-size distribution, and whether the sample represents the dominant (D) or the minor (M) lithology in the core. Additionally, it provides estimates of the major mineralogical and biological components from the examination of each smear slide. The presence of authigenic minerals, such as manganese oxides, iron sulfides, or carbonates, as well as the presence of rare trace minerals, was noted in the "Comments" column. The mineralogy of the major smear slide components was also validated by XRD analyses, and the relative proportion of carbonate and noncarbonate material was validated by chemical analysis of the sediments (see "Carbonate Carbon and Carbonate" in "Organic Geochemistry").

Thin sections were taken from several authigenic carbonate precipitates and from a few well-indurated sediments. Tables summarizing thin section data, such as grain size and relative abundance of sedimentary components, were also generated using SLIDERS. A Zeiss Axioplan microscope equipped with a digital camera was used to obtain images of the thin sections on board. Digital photomicrographs were obtained and stored as TIFF (.tif) files. Thin section results were used to complement the VCDs and as a substitute for smear slides when sediments were well indurated.

XRD analyses were used to support and verify the smear slide and thin section descriptions. Each sample was freeze-dried, ground, and mounted with a random orientation into an aluminum sample holder. For these measurements, a Philips PW-1729 X-ray diffractometer with a CuK source (40 kV and 35 mA) and a Ni filter was used. Peak intensities were converted to values appropriate for a fixed slit width. The goniometer scan was performed from 2° to 70° (2), at a scan rate of 1.2°/min (steps = 0.01° and count time = 0.5 s). Diffractograms were peak corrected to match the (100) quartz peak at 4.26 Å. Common minerals were identified based on their peak position and relative intensities in the diffractogram using the interactive software package MacDiff 4.1.1 (Petschick, 2000).

In addition to visual estimates of the color, reflectance of visible light from soft sediment cores was often 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 record of downcore color variations for the visible wavelengths (400-700 nm). Freshly split cores were covered with clear plastic wrap and placed on the AMST. Measurements were taken at 2.0 cm spacing. The AMST skips empty intervals and intervals where the core surface is well below the level of the core liner but does not 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 the measurement and interpretation of spectral data with the Minolta spectrophotometer can be found in Balsam et al. (1997, 1998) and Balsam and Damuth (2000). When sediment color became uniform, use of the Minolta system was discontinued.

All core sections were imaged using the Geotek X-Y digital imaging system (DIS) immediately after being split and scraped. We found it particularly useful to scrape the cores immediately prior to imaging in order to capture the ephemeral nature of some sedimentary features, particularly sulfide precipitates, which become oxidized within minutes of core splitting. It is worth noting that this fresh scraping did not occur prior to the regular photo imaging, which can take place between 30 and 60 min after scraping. Consequently, some of the sedimentary details may be lost using the more conventional archive-core table photographs. For this reason, cores were scraped before close-up photographs were taken. For digital imaging, core sections were routinely loaded into each imaging tray as soon as the previous sections were imaged so that the imaging system ran continuously during section loading and unloading. Some problems were encountered with occasional system crashes, however, which were probably a result of automated network uploading of files during image transfer from the camera hardware to the local computer.

In the core section dialogue box, we set the subbottom depth (SBD) of section at zero for Section 1 and let it automatically increment down the remaining sections for each core. All images were acquired at a crosscore and downcore resolution of 100 pixels/cm. At the beginning of Leg 204, the acquisition aperture was varied in an attempt to maximize the dynamic range captured. However, after Site 1244, we found it more expedient to fix the aperture at a value that would image most cores without the need for further adjustment. It was, therefore, normally set to f4.7, which maximized the dynamic range for most of the core sections, but it had to be decreased when bright ash-rich or carbonate layers were imaged to prevent image saturation. To prevent saturation of the bar-code label (placed at the end of each section) and of the polystyrene inserts (e.g., for voids and whole-round samples), we placed pieces of dark overhead-transparency film over them to reduce the brightness. This was a satisfactory but inelegant solution that would be improved with some high-quality gray translucent overlays. Care was taken to ensure that the system was correctly calibrated using the "white tile" procedure and that the camera position was correctly set up. If the image field is incorrectly set such that the ruler on the left side of the core is in the camera's field of view, then horizontal stripes, caused by pixel blooming, occur on the resulting image, which results in relatively dark cores. This is caused by the bright nature of the lettering on the ruler and could be prevented by having darker lettering (gray instead of white). Output from the DIS includes an uncompressed TIFF file (available upon request) and a compressed Mr.Sid (.sid) file (available in the Janus database) for each scanned section. Red-green-blue (RGB) profiles for all images were also automatically saved (available upon request) but were generally not used on board. Additional postprocessing of the color imagery was done to achieve a "medium"-resolution JPEG (.jpg) image of each section and a composite PDF (.pdf) image of each core. The PDF full-core color images were useful for quick reference at sea, particularly when writing the site chapters.