radiation at 40 kV and 35 mA with a focusing graphite monochromator and the
following slit settings:The description of sedimentary units recovered during Leg 191 included estimates of sediment composition based on smear slides, thin sections, carbonate and XRD measurements, and documentation of sedimentary and deformational structures, drilling disturbance, presence and type of fossils, bioturbation intensity, induration, diagenetic alteration, and color. These data were recorded manually for each core section on standard Visual Core Description (VCD) paper forms that are archived by ODP. This section is chiefly a reproduction of the "Sediment Description" section in the "Explanatory Notes" chapter of the Leg 185 Initial Reports volume (Shipboard Scientific Party, 2000), with modifications reflecting the pelagic lithologies encountered during Leg 191.
Information on the VCDs was summarized and entered into AppleCORE (version 8.1f) software, which generates a one-page graphical log of each core (barrel sheet). Barrel sheets are presented with core photographs in the "Core Descriptions" contents list. A wide variety of features, such as sediment lithology, bed thickness, primary sedimentary structures, bioturbation parameters, soft-sediment deformation, and structural and diagenetic features are indicated by patterns and symbols in the graphic logs. A key to the full set of patterns and symbols used on the barrel sheets is shown in Figure F2. The symbols are schematic but are placed as close as possible to their proper stratigraphic position or arrows indicate the interval to which the symbol applies. For exact positions of sedimentary features, copies of the detailed section-by-section VCD forms can be obtained from ODP. Barrel sheets consist of the following columns, each of which is discussed in detail below: lithology, texture and structure, sedimentary structures, bioturbation, drilling disturbance, samples, color, and description.
Sediment lithologies are represented by patterns in the Lithology column (see the "Core Descriptions" contents list for examples). This column may consist of up to three vertical strips depending on the number of the major end-member constituents (see "Sediment Classification"), thus reflecting intermixing of different components. Sediments with only one major component group (i.e., all other component groups being below 10% each) are represented by one strip. Because of the limitations of the AppleCORE software, thin intervals of interbedded lithologies cannot be adequately displayed at the scale used for the barrel sheets but they are described in the Description column of the barrel sheets, where appropriate.
With exceptions as noted below, the terms and parameters for texture and sedimentary structure that are used for this leg are compatible to those mentioned in the handbooks of Mazzullo and Graham (1988) or Folk (1980). Sediments were examined in terms of texture, including grain size, shape, sorting, and fabric, with the aid of hand lenses, smear slides, and thin sections. See, however, the comments below on the near-worthlessness of mandated grain-size estimates taken from smear slides, especially in reference to pelagic oozes. Primary and secondary sedimentary structures were described from observations of the split surface of the archive cores.
Symbols for bioturbation, structures, accessories, and drilling disturbances are shown in Figure F2.
The bulk of the pelagic sediments recovered during Leg 191 were relatively homogeneous. Stratification, bioturbation, or other sedimentary structures were usually only discernible where textural or compositional differences were present (e.g., close to ash layers). In the homogeneous background sediment, however, it was difficult to distinguish the destruction of primary structures by bioturbation from the actual absence of primary structures. Given the light gray and reddish colors of the siliceous and clayey sediments, the absence of lamination, and the very low organic matter contents, it is reasonable to assume that the remaining sediment has been pervasively bioturbated as well because benthic burrowing activity was not limited by oxygen deficiency. To convey the maximum amount of information without confusing interpretation with observation, we used the Bioturbation column to display only visible bioturbation or sediment mottling. The Bioturbation column of the barrel sheets shows four levels of intensity as follows:
Stratification thickness was characterized using a combination of the terms given by McKee and Weir (1953) and Ingram (1954):
Natural structures (physical or biological) can be difficult to distinguish from disturbance created by the coring process. Deformation and disturbance of sediment that resulted from the coring process are illustrated in the Drilling Disturbance column, using symbols shown in Figure F2. Blank regions indicate the absence of drilling disturbance. The degree of drilling disturbance for soft sediments was described using the following categories:
There are standard ODP categories for describing fragmentation in indurated sediments and rock, but the only one appropriate for Leg 191 was the following, for the cherty interval:
The stratigraphic position of samples taken for shipboard analysis is indicated in the Samples column of the barrel sheet according to the following codes:
Sediment color was determined visually by comparison with standard color charts (Munsell Color Company, Inc., 1975; Rock Color Chart Committee, 1991) and is reported in the VCD Color column and the Description column of the barrel sheets. In addition to determining color visually, all cores were scanned at 2- to 5-cm intervals using a Minolta CM-2002 spectrophotometer mounted on the AMST. The spectrophotometer measures reflectance in thirty-one 10-nm-wide bands of the visible spectrum (400-700 nm) on the archive half of each core section. Spectrophotometer readings were taken after cleaning the surface of each core section and covering it with clear plastic film (Glad Cling Wrap). Calibration of the color scanner did not include a correction for the plastic film because we found the effect to be very minor even with very brightly colored lithologies. The measurements were taken automatically and recorded by the AMST at evenly spaced intervals along each section. There was no way to program the AMST software to avoid taking measurements in intervals with a depressed core surface or in disturbed areas of core with drilling slurry or biscuits. The data are part of the Janus database and can be obtained from ODP (see the "Related Leg Data" contents list). Additional detailed information about measurement and interpretation of Minolta spectrophotometer spectral data can be found in Balsam et al. (1997, 1998, 2000).
A summary of the lithologic observations is given in the Description column of the barrel sheets. It consists of three parts: (1) a heading in capital letters that lists only the dominant sediment lithologies observed in the core; (2) a general description of the core, including color, composition, sedimentary structures, bed thicknesses, drilling disturbance, as well as any other general features in the core; and (3) descriptions and locations of thin, interbedded, or minor lithologies.
Sediments were analyzed petrographically using smear slides and thin sections. Tables summarizing these data (see the "Core Descriptions" contents list) include information about the sample location, whether the sample represents a dominant (D) or a minor (M) lithology in the core, and an estimate of the percentages of sand-, silt-, and clay-sized fractions, together with all identified components. We emphasize here that smear-slide and thin-section analyses provide only estimates of the relative abundances of the constituents. The comparison charts of Baccelle and Bossellini (1965, in Flügel, 1982) were used to refine abundance estimates in thin sections. However, these charts cannot be used for smear slides because they are designed to simulate a field of view that is completely and evenly covered with particles. Quantification of data from smear slides is further complicated by the difficulty of identifying fine-grained particles using only a microscope and by the tendency to underestimate sand-sized grains because they cannot be incorporated evenly into the smear. Biogenic opal and its diagenetic modifications are particularly difficult to determine from smear slides (van Andel, 1983). Previous experience has shown that the largest variations in smear-slide determination correlate with the change from one observer to another or with shift changes. The accuracy problem is indicated in the "Explanatory Notes" chapters of several recent ODP Initial Reports volumes in which sedimentologic numerical data in general, and those of smear-slide determination in particular, are consistently deemphasized and replaced by semiquantitative categories (e.g., Shipboard Scientific Party, 1998a, 1998b, 1998c, 2000). A limitation to semiquantitative categories, such as the ones proposed during previous legs, would have seemed all the more appropriate during Leg 191. Current ODP policies, however, require the input of numerical data, and so the reader is warned that the tabulated smear-slide results (see the "Core Descriptions" contents list) largely reflect the need to comply with these regulations rather than actual accuracy. Smear-slide and thin-section data were reviewed for internal consistency and correct sedimentologic nomenclature, and the qualitative composition was confirmed by XRD. Accuracy of the carbonate content estimated from smear slides and thin sections was confirmed by chemical analyses.
Selected samples were
taken for qualitative mineral analysis by XRD using a Philips diffractometer
with CuK
radiation at 40 kV and 35 mA with a focusing graphite monochromator and the
following slit settings:
.
/min.
Bulk samples were
freeze-dried, ground using an agate mortar and pestle, and packed in sample
holders, which, together with the ship's movement, probably imparted some
orientation to the mineral powder. These samples were scanned from 2°-70°2.
MacDiff software (v. 4.0.4 PPC, by Rainer Petschick) was used to display
diffractograms and to identify the minerals. Most diffractograms were corrected
to match the main peaks of quartz, calcite, or clinoptilolite at 3.343, 3.035,
and 8.95 Å, respectively. Identifications are based on multiple peak matches
using the mineral database provided with MacDiff. Relative abundances reported
in this volume are useful for general characterization of the sediments, but
they are not quantitative concentration data.
We evaluated the methods and the classification used for sediment description and found that there is a need for a less ambiguous and more flexible classification than the proposed ODP standard classification by Mazzullo et al. (1988). Our classification is neither comprehensive nor entirely descriptive, but it is simple to use for the purpose of Leg 191 and it circumvents some of the disadvantages of the Mazzullo classification. Notably, it avoids the impression of a level of accuracy that is not achievable under the conditions of most ODP cruises. Also, we tried to use common and relatively simple names, which led us to abandon a number of terms. For the deep pelagic Site 1179, we used the three end-members, diatoms, radiolarians, and clay (Fig. F3), that represent virtually all of the section (except for some ashy and zeolitic intervals) that lies above the cherts.
Sediment names at Site 1179 consist of one of four principal names relating to the dominant composition of the sediment (e.g., clay, diatom ooze, and radiolarian ooze, where any one of those three is more abundant than 50%) and siliceous ooze (if neither diatoms nor radiolarians is more than 50% but together they are) and of modifiers that precede the principal name. A component with 10%-30% abundance is termed component bearing, and one with 30%-50% is termed component rich (e.g., ash-bearing clay and radiolarian-rich diatom ooze).
Sedimentary rock is named according to composition and induration. Except for some thin zeolitic layers firm enough to be called mudstone or claystone, sedimentary rock recovered is termed porcellanite if it is firm to hard and chert if it is hard enough to resist scratching by a stainless steel probe. Porcellanite is dull in appearance and is commonly less dense than vitreous to waxy dense chert. In addition to this field classification, the terms porcellanite and chert bear a strong compositional notion. Thus, porcellanite is typically composed of opal-CT (cristobalite-trydimite) but may also contain diagenetic quartz, carbonate, and silicates (mostly clay minerals). Chert is usually dominated by quartz and tends to be purer silica but may also contain clay minerals and carbonate (Isaacs, 1982; Isaacs et al., 1983). The hard lithologies we recovered were almost entirely pieces of chert.
No lithology had sufficient detrital sand or silt to earn it a sediment name. Detrital components, however, include fine sand- through silt-sized grains of quartz and feldspar of eolian origin. A few grains of hornblende and pyroxene were found. Sand grain size is between 2 mm and 63 µm, silt is between 63 and 2 µm, and clay is material finer than 2 µm. Where they do not compose ooze, mixtures of sand, silt, and clay texture are named according to the classification of Folk (1980) and for a few thin layers the suffix -stone is added if the sediment is indurated. Note that the silt-to-clay boundary has been placed at 2 µm, as suggested by Doeglas (1968). The principal name for sediments dominated by volcaniclastic components in the silt- and fine sand-size range (250-2 µm) is volcanic ash. No ash was sufficiently indurated to earn the name tuff, and no hyaloclastite was observed, except that interlayered with basalt crust.
Clay that is present in almost all cores probably is partly eolian, partly far traveled in suspension, and partly authigenic. The grain size of zeolite that is present in many cores seems to depend largely on the vigor of the smear-slide preparation, and so in the classification, clay and zeolite are commonly lumped as zeolitic clay.
