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doi:10.2204/iodp.proc.333.102.2012

Lithology

Visual core descriptions

For the visual description of sediment core sections, we followed the conventional Ocean Drilling Program (ODP) and IODP procedures for recording sedimentologic information (Mazzullo and Graham, 1988). VCD forms were compiled on a section-by-section basis. Hand-written core descriptions were transferred to section-scale templates using J-CORES software and then converted to core-scale depictions using Strater software. Sediment texture (defined by the relative proportions of sand, silt, and clay) follows the classification of Shepard (1954). The classification scheme for siliciclastic lithologies follows Mazzullo et al. (1988). The Graphic lithology column on each VCD plots to scale all beds that are at least 2 cm thick. Graphic patterns do not show persistent interbedded layers <2 cm thick. These thinner layers are identified as laminae or color banding in the Sedimentary structures column.

In practice, it was difficult to discriminate between the dominant lithologies of silty clay, clayey silt, and clay without undertaking quantitative grain size analyses on board. Nevertheless, we tried to make a qualitative visual distinction between silty clay, clay, and silt on the basis that clay contains <25% silt (Shepard, 1954). The dividing line between more heavily indurated examples of the same lithologies (e.g., silty clay versus silty claystone) is arbitrary. For the inputs sites, Expedition 322 Scientists (2010), therefore, used terms for lithified rock because they only obtained RCB cores. However, since Expedition 333 also recovered HPCS and ESCS cores from the upper nonlithified stratigraphic sections at IODP Sites C0018, C0011, and C0012, this approach was not applicable and we used qualitative observational criteria to distinguish induration degree, while also consulting information from logging while drilling (at Site C0011) and physical properties data. Figure F3 displays graphic patterns for all sedimentary lithologies encountered during Expedition 333. Also shown are symbols for internal sedimentary structures, soft-sediment deformation structures, and severity of core disturbance, for the description of which, we also consulted X-ray CT images (see “X-ray computed tomography”).

Where calcareous sediments occur, we used the term “calcareous mudstone” for mud deposits that contain 10%–50% calcite (following the classification Mizutani, Saito, and Kanmera, 1987) and “lime mudstone” for mud-supported fabric with >90% carbonate component (Dunham, 1962; expanded by Embry and Klovan, 1972).

To emphasize the differences in the composition and grain size of volcaniclastic material, we modified the classification scheme of Fisher and Schmincke (1984). In general, coarser grained sediments (63 µm to 2 mm average grain size) are named siliciclastic “sand” where volcaniclastic components are <25% of the total clasts. Volcaniclastic grains can be (1) reworked and commonly altered heterogeneous fragments of preexisting volcanic rock, tuff, or tephra or (2) fresh, or less altered, compositionally homogeneous pyroclasts. Pyroclasts are produced directly by many types of processes associated with volcanic eruptions, without reference to the causes of the eruption or origin of the particles. Pyroclasts can include crystals, glass shards, and rock fragments. If the sediment contains >25% but <75% volcaniclasts with a predominant grain size approaching that of sand, it is designated a “volcaniclastic sand/sandstone.” If the total clast composition is >75% pyroclasts, then the sediment is classified as “ash.” Depending on the grain size and degree of compaction, the nomenclature is adjusted accordingly (e.g., tuff versus ash and coarse sand-size ash versus fine silt-size ash). Because of the qualitative nature of the descriptions, compositions close to the dividing lines of the classification scheme are challenging to the interpreters. In addition, with the exception of fresh glass shards in the population of pyroclasts, it is difficult to use smear slides (see below) to discriminate unequivocally between primary eruptive products (crystals, glass shards, or rock fragments) versus particles of identical composition created by subaerial or subaqueous erosion of fresh volcanic materials. On stratigraphic columns, we therefore use the term ash as a general descriptive term for all volcaniclastic sediments and depict coarse ash beds with longer lines than fine ash beds.

Bioturbation intensity in deposits was estimated using the following semiquantitative ichnofabric index:

• Slight bioturbation = <33% of original bedding disturbed.

• Moderate bioturbation = 33%–66% of original bedding disturbed.

• Intense bioturbation = >66% of original bedding disturbed.

Separating slightly and moderately bioturbated mud may, in some cases, be biased by the smoothness of the section surface. The ichnofabric index in cores was identified with the help of visual comparative charts (Heard et al., 2008).

Smear slide description

Smear slides are useful for identifying and reporting basic sediment attributes (texture and composition). Results are only semiquantitative. Errors can be large, especially for textural estimates of the fine silt and clay-size fractions. Smear slide analysis also tends to underestimate the amount of sand-size grains because they are difficult to incorporate evenly onto the slide. As during Expeditions 315, 316, and 322, we suspect that the counting errors can be considerable (e.g., “Lithology” in Expedition 315 Scientists, 2009) and easily as high as ~20%.

We estimated the texture of the sediments with the help of a visual comparison chart (Rothwell, 1989). The various sediment components were divided into several categories (e.g., feldspar, pyroxene, metamorphic lithic fragments, sedimentary lithic fragments, etc.) and grouped into descriptive categories according to the following classification (Figs. F4, F5):

• D = dominant (>50% of total grains).

• A = abundant (>20%–50% of total grains).

• C = common (>5%–20% of total grains).

• P = present (>1%–5% of total grains).

• R = rare (0.1%–1% of total grains).

• T = trace (<0.1% of total grains).

This classification was used to optimize the reproducibility of results among different scientists. For fine-grained sediments (silt/siltstone and silty clay/claystone), rough estimations were made regarding the matrix, using the visual comparison chart after Rothwell (1989). Detailed results are summarized in the smear slides (see “Core descriptions”).

The sample location for each smear slide was entered into J-CORES with a sample code of “SS,” using the Samples application. The position of each specimen is shown on the VCD slide editor column of the VCD application. In addition, the relative abundance of some of the major components (e.g., quartz) was also validated by bulk powder XRD (see “X-ray diffraction”), and the absolute weight percent of carbonate was verified by chemical analysis (see “Organic geochemistry”).

X-ray diffraction

We completed routine XRD analyses of bulk powders using a PANalytical CubiX PRO (PW3800) diffractometer. Our principal goal was to estimate relative weight percentages of total clay minerals, quartz, feldspar, and calcite using peak areas. Most of the samples were selected from intervals adjacent to whole-round samples, and most are part of sampling clusters colocated with physical properties, carbonate, and XRF. A few additional samples were collected periodically from such unusual lithologies as carbonate-cemented claystone and volcanic ash. Samples were freeze-dried, crushed with a ball mill (along with powder for XRF and carbonate), and mounted as random bulk powders.

The instrument settings were as follows:

• Generator = 45 kV and 40 mA.

• Tube anode = Cu.

• Wavelength = 1.54060 Å (Kα₁) and 1.54443 Å (Kα₂).

• Step spacing = 0.005°2θ.

• Scan step time = 0.648 s.

• Divergent slit = automatic.

• Irradiated length = 10 mm.

• Scanning range = 2°2θ to 60°2θ.

• Spinning = yes.

To match ODP Leg 190, NanTroSEIZE Stage 1 expeditions, and Expedition 322 as closely as possible, MacDiff 4.2.5 software was used for data processing. The software can be acquired at: (www.ccp14.ac.uk/​ccp/​web-mirrors/​krumm/​macsoftware/​macdiff/​MacDiff.html). Each peak’s upper and lower limits were adjusted following the guidelines shown in Table T2. Calculations of relative mineral abundance utilized a matrix of normalization factors derived from integrated peak areas and singular value decomposition (SVD) (Table T3). These SVD normalization factors for NanTroSEIZE were recalculated during Expedition 315 after the diffractometer’s high-voltage power supply was replaced. To help illustrate, a computation for the “absolute” percentage of quartz in a given specimen would be (see also Table T3)

As thoroughly described by Fisher and Underwood (1995), calibration of SVD factors depends on the analysis of known weight-percent mixtures of mineral standards that are appropriate matches for natural sediments. Bulk powder standards for the NanTroSEIZE project are the same as those reported by Underwood et al. (2003): quartz (Saint Peter sandstone), feldspar (Ca-rich albite), calcite (Cyprus chalk), smectite (Ca montmorillonite), illite (Clay Mineral Society IMt-2, 2M1 polytype), and chlorite (Clay Mineral Society CCa-2). The Expedition 322 Scientists (2010) published examples of diffractograms for all of the standard mixtures.

If a natural mineral mixture generates peak areas close to those of the standards, then the sum of the four “absolute” percentages will be close to 100%. In practice, all of the values must be normalized such that total clay minerals + quartz + feldspar + calcite = 100%. If trace quantities of a mineral or minerals generate peaks with low intensity, use of negative SVD normalization factors may result in computed values of absolute weight percent that are negative. This is a common problem with calcite. In such cases, we inserted the numerical value of 0.1% as a proxy for “trace.”

Average errors (SVD-derived estimates versus true weight percent) needed to be recalculated during Expedition 333, so the 14 standard mineral mixtures were run three times each. This test provides a measure of operator bias for choosing the lower and upper limits of each peak (Table T2). The average precision (standard deviation) based on three analyses of each standard mixture is: total clay minerals = 1.6%, quartz = 1.2%, feldspar = 0.8%, and calcite = 1.0%. The average accuracy (true wt%–calculated wt%) is: total clay minerals = 4.3%, quartz = 2.4%, feldspar = 1.8%, and calcite = 2.2%.

In spite of its precision with standard mixtures, the SVD method is only semiquantitative, and results for natural specimens should be interpreted with some caution. One of the fundamental problems with any bulk powder XRD method is the difference in peak response between poorly crystalline clay minerals at low diffraction angles and highly crystalline minerals at higher diffraction angles (e.g., quartz and plagioclase). Contents of clay minerals are best characterized by measuring the peak area, whereas peak intensity may be easier and more accurate to quantify quartz, feldspar, and calcite. Analyzing oriented aggregates enhances basal reflections of the clay minerals, but this step is time consuming and requires isolation of the clay-size fraction to be effective. Errors also propagate as more minerals and peaks are added to the procedure. Counts for “plagioclase” may also include K-feldspar, so we refer to that value in natural specimens as “feldspar.” For clay mineral assemblages, the two options are to individually measure one peak for each mineral and add the estimates together (thereby propagating the error) or to measure a single composite peak that extends from 19.4° to 20.4°2θ. Chlorite does not contribute to this composite peak, so errors grow if natural mixtures contain abundant chlorite. Another source of error is contamination of mineral standards by impurities such as quartz (e.g., the illite standard contains ~20% quartz).

In the final assessment, calculated values of a mineral’s weight percent should only be regarded as relative percentages within a four-component system, where total clay minerals + quartz + feldspar + calcite = 100%. How close those estimates are to their absolute percentages within the mass of total solids will depend on the abundance of amorphous solids (e.g., biogenic opal and volcanic glass), as well as the total of all other minerals that might occur in minor or trace quantities. For most natural samples, the absolute errors are probably between 5% and 10%. Thus, the primary value of bulk powder XRD data should be to identify spatial and temporal trends in sediment composition.

X-ray fluorescence

Whole-rock quantitative XRF spectrometry analysis was undertaken on core samples for the major elements. Analyses were performed on samples of 10 cm³ taken from every interstitial water cluster sample. Additional samples were collected from the working half of the core, usually next to samples for XRD and carbonate analysis, from unusual lithologies such as ash.

Collected samples were first vacuum-dried at least 24 h and crushed to fine powder with a tungsten carbide-ball mill. Loss on ignition was measured using weight changes on heating at 105°C for 2 h and then at 1000°C for 3 h. Glass beads were created for XRF analyses by fusing 0.9 g of sample with 4.5 g of SpectromeltA12 flux and 3 drops of LiI 20 wt% in a Pt crucible for 7 min at 1150°C. Analyses were performed on the wavelength dispersive XRF spectrometer Supermini (Rigaku) equipped with a 200 W Pd anode X-ray tube at 50 kV and 4 mA.

Major elements were measured using the fused glass bead method and are presented as weight percent oxide proportions (Na₂O, MgO, Al₂O₃, SiO₂, P₂O₅, K₂O, CaO, TiO₂, MnO, and Fe₂O₃). Rock standards of the National Institute of Advanced Industrial Science and Technology (Geological Survey of Japan [GSJ]) were used as the reference materials for quantitative analysis. Processed data were uploaded into J-CORES as a comma-delimited data file. Data are reported as total counts on the peak and also as semi-quantitative oxide weight percents.

X-ray fluorescence scanning

Semi-quantitative XRF scanning was undertaken for major elements on split-core sections using the onboard JEOL TATSCAN-F2 energy dispersive spectrometry split-core scanner, equipped with a cryogenic Si semiconductor and a 76 mm beryllium window. Target for X-ray generation is rhodium (Rh) and the diameter of the collimator that directs the incident X-ray beam is 7 mm. Prior to scanning, split-core surfaces were carefully cleaned with a brush. Core sections were scanned with a 2 cm sampling step at 30 kV (0.17 mA), with a 40 s sampling time.

Semi-quantitative oxide weight percents for major elements (Na₂O, MgO, Al₂O₃, SiO₂, P₂O₅, K₂O, CaO, TiO₂, MnO, and Fe₂O₃) were calculated from background-corrected integrated peak intensities using software provided by the vendor (JEOL) for the TATSCAN. Processed data were uploaded into J-CORES as a comma-delimited data file. Data are reported as semi-quantitative oxide weight percents with a precision of the order of several weight percents. Therefore, results should be carefully interpreted for quantitative discussion. Data presented in “Lithology” in the “Site C0011” chapter and “Lithology” in the “Site C0012” chapter (Expedition 333 Scientists, 2012a, 2012b) was further calibrated using JB-1b reference material values measured the day of the analysis (Table T4).

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