The description of sedimentary units recovered during ODP Leg 205 included estimates of sediment composition based on smear slides, thin sections, carbonate measurements, ICP-AES, and X-ray diffraction, documentation of sedimentary and deformational structures, drilling disturbance, presence and type of fossils, bioturbation intensity, induration, diagenetic alteration, and color.
Information from the cores was entered into AppleCORE (version 8.1m) software, which generated a one-page graphical log of each core ("barrel sheet"). A wide variety of features, such as sediment lithology, primary sedimentary structures, bioturbation, soft-sediment deformation, and core disturbance is 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 they are placed as close as possible to their proper stratigraphic position, or arrows indicate the interval for which the symbol applies. The columns on the barrel sheets are as follows.
Sediment lithologies are represented by patterns in the "Graphic Lithology" column. This column may consist of up to three vertical strips, depending on the number of the major end-member constituents present in the sediment mixture. Sediments with only one major component group (i.e., all other component groups are <10% each) are represented by one strip. Sediment nomenclature follows the scheme shown in Figure F3 in the case of sediment comprising a mixture of carbonate, clastic, and volcaniclastic components. For clastic and volcaniclastic sediments, the sediments are further classified on the basis of grain size and the relative proportion of different grains sizes in the total. A ternary diagram (Fig. F4) shows the naming scheme for sediment composed of a mixture of clay, silt, and sand. Because of the limitations of the AppleCORE software, intervals <20 cm thick cannot be adequately displayed at the scale used for the barrel sheets, but they are described in the "Description" columns of the barrel sheets where appropriate.
Depositional structures were noted with regard to large-scale patterns seen in the sediment and the rocks. They may include laminations, massive rip-up clasts, cross-bedding, cross-laminations, and erosional structures. Most clay-rich sediments recovered during ODP Leg 205 were either parallel laminated or heavily bioturbated. Primary structures were often impossible to identify where bioturbation was common.
Natural structures (physical or biological) can be difficult to distinguish from disturbances created by the coring process. Deformation and disturbance of sediment that resulted from the coring process are illustrated in the "Drilling Disturbance" column with the 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:
Moderately disturbed = bedding contacts bowed.
Soupy = water-saturated intervals that have lost all original structure.
Fragmentation in indurated sediments and rock was described using the following categories:
The stratigraphic position of samples taken for shipboard analysis and the location of close-up photographs is indicated in the "Samples" column of the barrel sheet according to the following codes:
Any disruptions or faults in evidence during examination of the core were recorded in this section, as were brecciation and textures associated with deformation. Veins and their infillings were noted with as much detail as possible.
The written description for each core, located under the "Description" column on the barrel sheets, contains a brief overview of both major and minor lithologies present, color gradation, grain-size gradation, and fossils. Sediment color was determined using a Minolta CM-2002 spectrophotometer mounted on the archive-half multisensor track (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 covering the surface of each core section with clear plastic film. Calibration for the color scanner did not include a correction for the plastic film because the effect is minor even with very brightly colored lithologies. The area measured is a circle 8 mm in diameter, and the spectrophotometer integrates the sensed color over this area. The AMST was not programmed to avoid taking measurements in intervals with a depressed core surface or in disturbed areas of core with drilling slurry or biscuits. The color data are a part of the ODP Janus database. Additional 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 the relationship between two units that are in contact with one another within a core section, sharp and gradational (denoted with S or G, respectively) contacts were noted.
The size of grains was assessed based on Wentworth's (1922) classification. Size grading refers to the change in grain size within an individual unit. Normal grading refers to the grains becoming finer upward. Reverse grading refers to the grains becoming coarser upward. The word "Normal" or "Reverse" was included in the description to denote the type of grading observed, if any.
Grain size and composition of sediments were determined using smear slides. These were prepared according to the procedures described in the handbook for shipboard sedimentologists (Mazzullo et al., 1988). Identification in terms of general components was undertaken in accordance with Rothwell (1989). For semiquantitative visual estimation of sediment textures and determination of major components, we used the Comparison Chart for Visual Percentage Estimation determined by Terry and Chilingar (1955). In the case of smear slides, however, the percentage had to be corrected to compensate for the degree of dispersion of the grains in the smear slide. Quantitative estimate of grain size and composition are made in the tables located in the site summary. We did not follow the system used by the ODP Leg 170 Scientific Party at Sites 1039 and 1040 in which they distinguished whether components were abundant, common, or present in trace amounts. Because Leg 170 scientists did not specify percentages for their categories, a direct comparison is difficult, although >30% is considered to be abundant, 10%-30% to be common, and <10% to be trace.
Smear slides provide only rough estimates of the relative abundances of detrital constituents. This is the result of some fundamental limits:
We examined thin sections from the core intervals noted on the barrel sheets to complement and refine the hand-specimen observations. The same terminology was used for thin section descriptions as for the visual core descriptions (VCDs). In sediments, the proportions of lithic, crystal, and vitric components as well as the finer-grained matrix were estimated. The textural terms that we used are defined in MacKenzie et al. (1982). Tables summarizing data from thin sections and smear slides are included (see the "Core Descriptions" contents list). These tables include information about the locations of samples in the core and an estimate of the abundance of different grain sizes and different grain types where appropriate.
We subdivided the core into consecutively numbered lithologic units, under the "Lithologic Unit" column, on the basis of changes in grain size, mineral presence, and abundance. Where appropriate, the stratigraphic units defined by earlier drilling during ODP Leg 170 were employed to assist in correlation.
We employed the ODP standard sediment classification scheme of Mazzullo et al. (1988). Sediment names consist of a principal name relating to the dominant composition of the sediment (e.g., claystone, marl, or chert) and one or two modifiers that precede the principal name (e.g., ash-bearing siliceous clay). Principal names may be preceded by a major modifier (e.g., diatomaceous) that relates to a component group that is common or abundant but not dominant. As an example, diatomaceous claystone describes a hard sediment that contains >50% siliciclastic clay and >30% diatoms. Minor modifiers were used to specify a common component group (i.e., <30%, but >10% of the sediment). Minor modifiers are used with the suffix "-bearing" and precede the major modifiers. Thus, a soft sediment with dominant radiolarians, abundant calcareous nannofossils, and common volcanic ash would be called a nannofossil-rich radiolarian ooze with volcanic ash. Besides composition, principal names vary according to the grain size and the induration of the sediment. Principal sediment names are as follows:
Routine samples for shipboard X-ray diffraction (XRD) analysis were taken primarily from the squeeze cake residues of interstitial water whole rounds. Additional samples were collected periodically from minor lithologies such as carbonate-cemented claystone and volcanic ash. Samples were freeze-dried, crushed either by hand with agate mortar and pestle or with a ball mill, and mounted as random bulk powders. The XRD laboratory aboard the JOIDES Resolution is equipped with a Philips PW-1825 programmable X-ray generator and a Philips PW-1710/00 diffraction control unit with a PW-1770 automatic sample changer. Machine settings for calibration standards and all samples for all sites were as follows:
).
.
/min.
to 70°2.
The software used for XRD data reduction is MacDiff (version 4.1.1). This shareware application for Macintosh supports digital data processing and measurement of peak geometry. Peak intensity (counts per step) and peak area (total counts) were recorded after creating a baseline (200 iterations for all °2 values) and smoothing the counts (17-term filter of standard weighted means).
Normalization factors for shipboard conversion of integrated peak areas to semiquantitative relative abundances of dominant minerals were used previously during ODP Leg 190, when matrix singular value decomposition was used to solve for reliable normalization factors using the method of Fisher and Underwood (1995). Peak areas were recorded for the (101) quartz, (104) calcite, (002) plagioclase, (100) quartz, (101) cristobalite, and composite clay peak at d = 4.5 Å. Calibration depends upon the analysis of known weight percent mixtures of mineral standards that are fair matches for the natural sediments encountered along the Middle America Trench. All XRD-determined abundances were normalized to 100%. It is imperative that the normalization factors presented in Table T1 be used only to reduce data gathered on the Philips XRD instrument currently in use on the JOIDES Resolution, and only when operating parameters match those presented above (Table T1). The inherent problems of analyzing mixtures of highly crystalline minerals (quartz and plagioclase) and randomly oriented phyllosilicates (clay minerals) prevent use of the bulk-powder data in characterizing abundance of individual clay minerals. Although it would be premature at this time to assign relative abundance values to the minerals tracked by shipboard XRD, postcruise biogenic silica assessment will make it possible to evaluate calculated relative abundance of dominant minerals with depth. The normalization factors presented here will be crucial in that next step.
Error associated with calculating relative mineral abundance is expressed in Table T2 for each standard. The difference between the known weight percentage of a given mineral and the calculated, normalized weight percentage of that mineral is provided. The maximum difference encountered (for plagioclase) was 6.07 wt%. Relative percent error, calculated as [(normalized weight percentage - measured weight percentage)/measured weight percentage]x100 average <3%, although errors are significantly higher than the average where mineral abundances are low.
Beyond routine semiquantitative assessment of relative mineral abundance, we also used XRD to characterize representative samples of volcanic ash. Because of the variability of crystal content, amorphous glass content, and alteration products in such samples, we simply recorded the intensity and area of representative peaks generated by common minerals (e.g., clays, zeolites, quartz, plagioclase, cristobalite, calcite, amphibole, pyroxene, pyrite, and halite). A final parameter to measure is the ratio of peak areas for (100) quartz (d = 4.257 Å) to (101) cristobalite (d = 4.0397 Å). The accuracy of this ratio suffers from interference between the highest-intensity cristobalite peak and a secondary plagioclase peak at ~22°2.
Whereas samples for bulk XRD were taken at intervals at every site, clay fraction XRD analyses were carried out only when the clay fraction was of specific geological interest. Approximately 5 cm3 of freeze-dried sediment was set into a 600-mL glass beaker and broken into small (gravel-sized) pieces if the sample was considerably indurated. Enough 3% H2O2 was poured over the sample to cover it completely in order to remove organic material, and the beaker was covered with a watch glass until the reaction was complete, ~24 hr in most cases. At this stage for carbonate samples, 10% acetic acid was added to remove the carbonate and the mixture was well stirred and left for 12 hr then spun down completely using the 21K Marathon centrifuge. Acetic acid was decanted, and the sample was twice washed with deionized water (DI). All washes were accomplished by spinning the centrifuge tubes until all solids had settled to the bottom of the centrifuge tube. Approximately 175 mL of dispersant (Calgon: 4 g/1000 mL DI) was added to the sample, and the mixture was well stirred and left overnight. Throughout these solution baths, occasional stirring maintained the sample suspension and an ultrasonic bath was used to help disaggregate stubborn samples. Following disaggregation, we washed samples in 50-mL centrifuge tubes three times to generate a washed bulk sample.
The sonic dismembrator disaggregated the sample thoroughly in preparation for making the clay split; the dismembrator was not used for >3 min duration to avoid clay grain damage. In centrifuge tubes, the washed bulk sample was run at 1000 rotations per minute for 3.2 min to settle the >2-µm fraction. A 10-mL aliquot of the suspension was drawn into a large syringe and emptied slowly into the funnel of a Millipore suction filtration apparatus using 0.45-µm Millipore filters (4.5 cm in diameter). After the water passed through and before the clay caught on the filter dried, the filter was removed from the apparatus and laid carefully on a slide, transferring the clay layer to the slide surface. Each slide was analyzed unaltered and again after 24 hr in a room-temperature glycol chamber. The XRD instrument operating parameters were the same as for bulk XRD except step size was 0.010°2, and the scan was from 2°2 to 35°2.
