Core material recovered on Leg 181 was described according to ODP procedures and conventions. Shipboard procedures included visual core descriptions, smear-slide analysis, X-ray diffraction, and color spectrophotometry, all of which are outlined in general terms in the following section.
Sediments collected on Leg 181 are classified using the scheme presented by Mazzullo et al. (1988). This classification method is descriptive as opposed to genetic and is based predominantly on texture and composition. However, as with other ODP legs (e.g., Leg 113; Shipboard Scientific Party, 1988), minor modifications have been made to the scheme (Table T1, also in ASCII format). According to Mazzullo et al.'s (1988) plan, cores from Leg 181 are primarily granular and pelagic sediments that incorporate varying proportions of siliciclastic and volcaniclastic components (Fig. F3):
The main lithologic classes are defined on the basis of the relative proportions of the aforementioned components. Therefore, pelagic sediments are those containing >50% biogenic planktonic grains, and siliciclastic sediments have >50% terrigenous grains. This contrasts with Mazzullo et al. (1988) who use a 60% cut-off value for the main classes, with components in the 40%-60% range being ascribed to a "mixed" class. A 50% cut-off eliminates the somewhat nonspecific "mixed" category.
Granular sediment has (1) a principal name that defines its class and (2) major and minor modifiers that capture the texture, composition, fabric, and other features of the granular components (Table T1). Pelagic sediments have principal names relating to the composition and degree of compaction. Thus, (1) ooze = unconsolidated calcareous and/or siliceous deposits; (2) chalk = firm pelagic sediment mainly composed of calcareous biogenic components; (3) limestone = hard pelagic sediment composed of calcareous biogenic components; (4) radiolarite, diatomite = firm pelagic sediment with a dominance of radiolarians or diatoms.
Siliciclastic sediments have principal names based on texture and are derived as follows:
Lithified sediments have the suffix "-stone" added to the textural group/subgroup term. Again, where silt and clay concentrations are unspecified, "mudstone" is used and if fissile, "shale."
Volcaniclastic sediments are described as "tephra" where deposits are unconsolidated and grains are <2 mm diameter. Consolidated deposits are termed "tuff."
Major (>25%) and minor (~10%-25%) modifiers describe the composition and textures of the sediment in greater detail. Components with concentrations of <10% are not named unless they are of special significance to the interpretation. Major modifiers can also be used to describe color, grain fabric, and shape. The principal name is preceded by the major modifier and then the minor modifier, which is accompanied by the term "-bearing" (e.g., tephra-bearing).
Examples:
Cores are designated using leg, site, hole, core number, and core type (See "Introduction") and the cored interval (mbsf and meters composite depth [mcd]; see "Composite Depths").
The visual description of each core was summarized using core description sheets ("barrel sheets"; Fig. F5). Barrel sheets were generated using AppleCORE (v. 0.7.5g) and are presented with whole-core photographs (see the "Core Descriptions" contents list). The lithology of the recovered material is represented on barrel sheets by a column titled "Graphic Lithology." In addition, a wide variety of features that characterize the sediment is indicated in the columns to the right of the graphic log. These features are briefly described below. The legend of symbols used in the barrel sheets is shown in Figure F6. Symbols are placed as close as possible to their proper stratigraphic position although they remain schematic.
The lithologic text description consists of three parts: (1) a heading that lists all the major sediment lithologies observed in the core, (2) a general description of major and minor lithologies, and (3) a general description of the core including the location and type of any significant features in the core. The terminology for the thickness of sedimentary beds and laminae follows McKee and Weir (1953), who use the following categories: very thick bedded (>100 cm), thick bedded (30-100 cm), medium bedded (10-30 cm), thin bedded (3-10 cm), thickly laminated (0.3-3 cm), and thinly laminated (<0.3 cm). Included in this text is any information regarding sediment disturbance produced by natural processes or coring/drilling procedures.
Any deformation and disturbance of the recovered sediment interpreted as resulting from the coring process is listed in the "Drilling Disturbance" column. The absence of drilling disturbance is indicated by a blank column. The degree of drilling "disturbance" is described for soft and firm sediments using the following categories:
For indurated sediments the degree of "fracturing" was described using the following categories:
Standard ODP nomenclature was used (Mazzullo et al., 1988).
Ichnologic analysis included both an evaluation of the extent of bioturbation and, where possible, the identification of ichnofossil types.
The degree of bioturbation was semiquantitatively assessed using a slightly modified version of the Droser and Bottjer (1991) ichnofabric index (e.g., barren or no bioturbation, rare, moderate, common, and abundant; see Fig. F6). These indices are illustrated using grayscale shading in the "Relative Bioturbation" column of the barrel sheets.
Identification of ichnofossil types was restricted to intervals where biogenic structures were discrete (e.g., where burrows exhibited sharp walls or had fills that contrasted well with the surrounding sediments in terms of texture, composition, or color). Discrete biogenic structures (burrows, burrow systems, borings, and so on) were identified on the basis of their morphology as observed on the core surface. Identifiable biogenic structures are illustrated in the "Ichnofossils" column of the barrel sheets, using symbols depicted in Figure F6.
Visual description of the sediment was complemented by data from smear slides. Tables summarizing these data may be found in the "Core Descriptions" contents list. These tables include information about the sample location together with the estimated abundances of the sand, silt, and clay fractions and the major biogenic components. An indication is also provided as to whether the sample represents a dominant or a minor lithology in the core.
Identification of finer grained mineral particles is difficult using only a binocular microscope, and the quantity of sand-sized particles tends to be underestimated in smear slides because they cannot be incorporated evenly into the smear. Therefore, it is stressed that smear-slide data provide only estimates of the relative abundances of detrital constituents. The mineralogy of some smear-slide components was validated by X-ray diffraction.
The positions of samples taken from each core for analysis are indicated by letters in the "Sample" column of the barrel sheets as follows: SS (smear slide), PAL (micropaleontology), IW (interstitial water), and XRD (X-ray diffraction).
Graphic summaries of the lithology from each Leg 181 site are presented for each site chapter and are derived from information given on the barrel sheets. Components with concentrations of <10% are omitted from the Summary Columns as are beds and layers of <20 cm thick. Thus, the lithology columns only show the major lithologic units and subunits. Additional columns outlining 550-nm light reflectance, natural gamma ray, magnetic susceptibility, and calcium carbonate are also included where available. Reflectance and susceptibility, in particular, provide a high-resolution record of downcore change. Stratigraphic control is provided by biostratigraphic zones derived from foraminifers, nannofossils, radiolarians, and diatoms with additional control from magnetic polarity data.
Included in the core description is a qualitative impression of sediment color estimated by the sedimentologists using the standard Munsell color chart. In addition, the reflectance of visible light from each core was determined using a Minolta Spectrophotometer CM-2002, mounted on the split-core analysis track (SCAT). This instrument determines the reflectance at 31 separate 10-nm-wide spectral bands from 400 to 700 nm, covering the visible spectrum. Measurements were taken as soon as possible after the cores were split. Strips of very thin, transparent plastic film were used to cover the cores to prevent fouling the spectrophotometer lens, which was lowered to almost touch the core for measurement (except at Site 1119). Routine measurements were made at 2-cm intervals down each core section, but spacings were sometimes modified according to void space and sedimentary layering.
Before obtaining measurements from each core, the spectrophotometer was calibrated by attaching a white calibration cap covered with film (Balsam et al., 1997). Spectrophotometric measurements were then recorded using the program Spectrolog (v. 3.0). Additional detailed information about measurement and interpretation of spectral data with the Minolta spectrophotometer can be found in Schneider et al. (1995) and Balsam et al. (1997).
In selected core samples, the relative abundances of the main silicate and carbonate minerals were determined using a Philips model PW-1729 X-ray diffractometer with Cu-K
radiation (Ni filter) and automatic divergence slit. Bulk-sediment samples were freeze-dried, ground, and mounted with a random orientation in an aluminum sample holder. The instrument conditions were 40 kV, 35 mA, goniometer scan from 2° to 70° (2
) for bulk samples, step size 0.01% (2), scan speed at 1.2° /min, count time 0.5 s. Peak intensities were converted to values appropriate for a fixed slit width.
Graphic evaluation of the diffractograms was performed using the interactive MacDiff (v. 3.3 PPC) software. The main minerals were identified according to peak position and the relative abundances established on the basis of both peak intensities and integrated peak areas. The ratios and relative abundances reported in this volume should be considered only as providing a general characterization of the sediments and not as a detailed quantitative measure.
