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doi:10.2204/iodp.proc.317.102.2011 LithostratigraphyThe lithostratigraphic procedures used during Expedition 317, including sediment classification, visual core description, smear slide and thin section preparation and description, and XRD, are outlined below. In addition, please see "Geochemistry and microbiology" for an explanation of carbonate, major element, and trace element analyses carried out on sediments and sedimentary rocks. Core preparationAfter imaging, but prior to description, the quality of the split surface of the archive half of each core was assessed, and when necessary (e.g., the surface was smeared or uneven) the split surface of the archive half was scraped lightly with a glass slide or spatula. Cleaned sections were occasionally reimaged if the visibility of sedimentary structures and fabric improved. Visual core descriptionSediment components and percentages in the core were determined using a hand lens, binocular microscope, smear slide examination, or thin section. Information from macroscopic and microscopic examinations of each core section was recorded by hand on a primary description form (Fig. F1). All handwritten forms were digitally preserved as PDF files (see LITH in "Supplementary material"). The schemes used for sediment description are detailed below. ColorSediment color was determined qualitatively for core intervals using Munsell soil color charts (Munsell Color Company, Inc., 2000). Lithologic classificationThe lithologic classification scheme used during Expedition 317, modified from Shipboard Scientific Party (2004), is based on three end-member grain components: biogenic silica, carbonate, and terrigenous or volcanic grains, along with alternative modifiers determined from smear slides (Fig. F2). This scheme is further divided according to the grain size of the terrigenous component (i.e., the relative proportions of gravel, sand, silt, and clay) (Wentworth, 1922). The term "mud" is used for a mixture of silt and clay (Fig. F2). Adjectives such as "silty" and "clayey" are used to differentiate mixtures of mud. Sand is grouped into very fine to fine sand and medium to very coarse sand fractions for description purposes. Mixtures of mud and sand are divided based on whether sand content is greater than or less than 50% (sandy mud or muddy sand, respectively). Mixtures of terrigenous, siliceous, and calcareous sediments are named according to the relative proportion of the three components (Fig. F2). "Marl" describes a mixture of sand- to mud-sized carbonate and terrigenous material, whereas sand-sized mixtures with little matrix are referred to as "bioclastic sand" and similarly "bioclastic gravel" for gravel-sized sediment with little matrix. The term "ooze" is used to describe <2 mm calcareous sediments containing >90% carbonate, whereas "shell hash" denotes >2 mm components. "Ash/tuff" is defined as sediment containing >50% silt- and sand-sized volcanic grains (Mazzullo et al., 1988; Shipboard Scientific Party, 2003b). Sediment names also indicate the degree of sediment induration (e.g., sand versus sandstone, silt versus siltstone, mud versus mudstone, ooze versus chalk, or limestone). Terrigenous sediments with >2 mm modal grain size are termed "gravel," whereas "conglomerate" and "breccia" are the principal names for consolidated gravels with well-rounded and angular clasts, respectively. Sedimentary structures, accessories, and beddingSedimentary structures, accessories, and other primary and secondary (diagenetic) features are noted in the core descriptions. Bed thickness was defined according to McKee and Weir (1953):
Laminae are described as <1 cm thick. For units in which two lithologies are closely interbedded (i.e., the individual beds are <15 cm thick and alternate between one lithology and another), three "interbedded" lithology names are used (Fig. F3): interbedded sand and mud, interbedded silt and mud, and interbedded clay and mud. When beds are scattered throughout a different lithology (e.g., beds of clay several centimeters to tens of centimeters thick within a mud bed), they are logged individually and entered as sedimentary structures, along with their associated thicknesses and textures, into the database using the DESClogik application. Lithologic accessories noted include the type and proportion of shells and organic material, the presence of glauconite or other minerals, concretions and nodules, veins, and so on (Fig. F3). BioturbationIchnofabric description analysis included evaluation of the extent of bioturbation and notation of distinctive biogenic structures. To assess the degree of bioturbation semiquantitatively, a modified version of the Droser and Bottjer (1986) ichnofabric index (1–5) scheme was employed (1 = no apparent bioturbation, 2 = slight bioturbation, 3 = moderate bioturbation, 4 = heavy bioturbation, 5 = complete bioturbation [no depositional structure remaining]) (Fig. F4). These indexes are illustrated using the numerical scale in the ichnofabric column of the standard graphic reports (barrel sheets). Sediments without recognizable depositional structures were recorded as Level 1 on this scale. Recognizable biogenic structures and trace fossils were noted and logged in the database. Drilling disturbanceThe type and/or degree of drilling disturbance is indicated using the terminology and intensities defined in Figure F3. Smear slide analysisToothpick samples were taken at select intervals in the core and used to create smear slides according to the method outlined in Mazzullo et al. (1988). One or more smear slide samples were collected from the main or dominant (D) lithology from the archive half of each core. Additional samples were collected from minor (M) lithologies and/or other areas of interest (e.g., laminations, mottles, etc.). Smear slides were viewed with a transmitted-light, petrographic microscope, and the percentages of different mineralogic, biogenic, and authigenic components were estimated along with the proportions of sand, silt, and clay (terrigenous only). These estimations were recorded on smear slide sample sheets, which were digitally preserved as PDF files (see LITH in "Supplementary material"), and entered into the database. This technique was limited in that sand grains were underemphasized, as were large calcareous components (shells and shell fragments), and the determination of percentages was subjective and varied slightly among different practitioners. Lithology descriptions from smear slides were calibrated by comparison with XRD analysis and calcium carbonate analyses by coulometry. Thin sectionsThin sections were created on board when representative lithified sediments were encountered (including concretions and nodules). Thin sections generally provide less biased samples of whole rock than do smear slides, and they allow for more accurate identification of the minerals present. However, the limitation of selecting samples from the concretions and nodules present in soft sediments means that there was some bias in the types of lithology sampled, including (1) sampling concretions in one lithology over unlithified sediments from another and (2) recovering concreted horizons at the expense of other sediments. Lithologies were defined according to the main lithologic classification scheme described above (Fig. F2). Additionally, a smear slide sample worksheet was completed before the results were added to the database. Representative locations for thin sections were selected when the sediments were generally too lithified for smear slide analysis. DESClogik data capture softwarePrimary description forms for core sections were compiled and entered into the database using the DESClogik application. Direct entry of descriptive and interpretive information into the DESClogik program was performed using the Tabular Data Capture (TDC) mode. Before core description, a spreadsheet template was constructed in TDC. Tabs and columns were customized to include description from information categories (e.g., lithology, drilling disturbance, and bioturbation). A second template containing category columns for texture and relative abundance of biogenic/mineralogic components was configured specifically for recording smear slide data. DESClogik includes a graphic display mode for core data (e.g., digital images of section halves and measurement data) that can be used to augment core description. The data entered in DESClogik were then uploaded into the Laboratory Information Management System (LIMS) database. Standard graphic report (barrel sheet)The LIMS2Excel application was used to extract data in a format that could be used to plot descriptive as well as instrumental data in core graphic summaries using a commercial program (Strater, Golden Software). The Strater program was then used to produce a simplified, annotated, publication-quality standard graphic report (also known as a barrel sheet) of each core. Beginning with the leftmost column, each barrel sheet displays depth scale (m CSF-A), core length, and section information. A fourth column displays the concatenated section-half images adjacent to a graphic lithology column in which core lithologies are represented by graphic patterns illustrated in Figure F3. Subsequent columns provide information on drilling disturbance, sedimentary structures, lithologic accessories, ichnofabric, and shipboard samples (see legend in Fig. F3). Additional columns present age data (see "Biostratigraphy") and plots of core logging data such as magnetic susceptibility, natural gamma radiation (NGR), and color measurements (see "Physical properties"). X-ray diffraction analysisSamples for XRD analyses were selected from working halves based on visual core observations (e.g., color variability, visual changes in lithology, and texture) and smear slides. XRD analyses were performed on one sample per core from the dominant lithology in the same interval from which samples were taken for smear slide, carbonate, and carbon content analyses (coulometry, CHNS; see "Geochemistry and microbiology"). One 5–10 cm3 sample per core was frozen, freeze-dried in the case of unlithified samples, and ground by hand or in an agate ball mill as necessary. Prepared samples were top-mounted onto a sample holder and analyzed using a Bruker D-4 Endeavor diffractometer mounted with a Vantec-1 detector using nickel-filtered CuKα radiation. The standard locked coupled scan was as follows:
Diffractograms of single samples were evaluated with the Bruker DiffracPlus software package, which allowed only for mineral identification and basic peak characterization (e.g., width and maximum peak intensity) and could not deconvolve overlapping peaks, which were commonly observed in samples from all sites. The locations of peaks used for mineral recognition are shown in Table T2. Secondary diffraction peaks were used for certain minerals (e.g., quartz and K-feldspar) because there was interference at the primary diffraction peak position. Relative abundances of various minerals were established on the basis of maximum peak intensity. Quantification of mineral contents was not possible because the samples were not spiked with a defined amount of a mineral standard for calibration and because of a possible preferred orientation caused by top-mounting the samples. Therefore, shipboard results yielded only qualitative results on the relative occurrences and abundances of the most common mineralogical components. Further identification of clay mineralogy from these bulk powder analyses was not attempted on board the ship. Based on the comparison of calcite peak intensity with total carbonate content from coulometry, the minimum detectable peak intensity was set at 60 counts. |
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