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doi:10.2204/iodp.proc.303306.110.2006 LithostratigraphyThis section outlines the procedures used to document the basic sedimentology of the deposits recovered during Expedition 306, including visual core description, smear slide description, digital color imaging, color spectrophotometry, and XRD studies. For consistency of Expedition 303 and 306 sediment classifications, the procedures described in the “Site U1302–U1308 methods” chapter were mainly followed. Only general procedures are outlined, except where they depart significantly from Ocean Drilling Program (ODP) and IODP conventions. Sediment classificationThe sediments recovered during Expedition 306 were composed of biogenic and siliciclastic components. They were described using the classification scheme of Mazzullo et al. (1988). The biogenic component is composed of the skeletal debris of open-marine calcareous and siliceous microfauna (e.g., foraminifers and radiolarians, respectively) and microflora (e.g., calcareous nannofossils and diatoms, respectively) and associated organisms. The siliciclastic component is composed of mineral and rock fragments derived from igneous, sedimentary, and metamorphic rocks. The relative proportions of these two components are used to define the major classes of “granular” sediments in the scheme of Mazzullo et al. (1988). The lithologic names assigned to these sediments consist of a principal name based on composition, degree of lithification, and/or texture as determined from visual description of the cores and from smear slide observations. The total calcium carbonate content of the sediments, determined on board (see “Geochemistry”), was also used to aid in classification. For a sediment that is a mixture of components, the principal name is preceded by major modifiers (in order of increasing abundance) that refer to components making up ≥25% of the sediment. Minor components that represent between 10% and 25% of the sediment follow the principal name (after a “with”) in order of increasing abundance. Minor sedimentary components <10% in abundance are not reflected in the lithologic name. For example, an unconsolidated sediment containing 30% nannofossils, 50% clay minerals, 15% foraminifers, and 5% quartz would be described as a nannofossil clay with foraminifers. For biogenic sediments (i.e., >50% biogenic grains; see below), major or minor modifiers of siliciclastic components were summed, resulting in a more inclusive/general term for the name of the sediment. For example, an unconsolidated sediment containing 55% nannofossils, 15% foraminifers, 10% clay minerals, 10% quartz, and 10% detrital calcite would be described as a silty clay nannofossil ooze with foraminifers. Minor sedimentary components are not designated in the Graphic Lithology column of the barrel sheets. These naming conventions follow the ODP sediment classification scheme (Mazzullo et al., 1988), with the exception that during Expedition 306 a separate “mixed sediment” category was not distinguished (Fig. F1). As a result, biogenic sediments are those that contain >50% biogenic grains and <50% siliciclastic grains, whereas siliciclastic sediments are those that contain >50% siliciclastic grains and <50% biogenic grains. For the classification of biogenic carbonate oozes, three categories were used:
Some sediments contained >50% biogenic components, but no single biogenic component was >50%. In these instances, the siliceous and calcareous components were summed, resulting in a lithologic name that reflects the relative contribution of each biogenic component by listing these components in order of increasing abundance. For example, an unconsolidated sediment containing 45% nannofossils, 15% diatoms, 10% sponge spicules, 10% clay minerals, 10% quartz, and 10% detrital calcite would be described as a silty clay biosiliceous-nannofossil ooze. Size divisions for siliciclastic grains are those of Wentworth (1922) (Fig. F2), with 10 major textural categories defined on the basis of the relative proportions of sand, silt, and clay (Fig. F3); however, distinguishing between some of these categories is difficult (e.g., silty clay versus clayey silt) without accurate measurements of grain size abundances. For siliciclastic sediments, the term “clay” is only used to describe particle size and is applied to both clay minerals and other siliciclastic/detrital mineral components <4 µm in size. Size-textural qualifiers were not used for biogenic sediment names (e.g., nannofossil clay implies that the dominant component is detrital clay rather than clay-sized nannofossils). During Expedition 306, neritic and chemical sediments were not encountered except as accessory components; therefore, these categories are not addressed below. Sediments containing >50% silt- and sand-sized volcanic grains were classified as ash layers. Terms that describe lithification vary depending upon the dominant composition as described below:
Visual core descriptionPreparation for core descriptionThe standard method of splitting a core by pulling a wire lengthwise through its center tends to smear the cut surface and obscure fine details of lithology and sedimentary structure. When necessary during Expedition 306, the archive halves of cores were gently scraped across, rather than along, the core section using a stainless steel or glass scraper to prepare the surface for unobscured sedimentologic examination and digital imaging. Scraping parallel to bedding with a freshly cleaned tool prevented cross-stratigraphic contamination. Sediment barrel sheetsCore description forms, or “barrel sheets,” provide a summary of the data obtained during shipboard analysis of each sediment core. Detailed observations of each section were recorded initially by hand on standard IODP visual core description (VCD) forms. Copies of original VCD forms are available from IODP upon request. This information was subsequently entered into AppleCORE (version 9.4), which generates a simplified annotated graphical description (barrel sheet) for each core (Fig. F4). Site, hole, and depth (in mbsf or mcd if available) are given at the top of the barrel sheet, with the corresponding depths of core sections along the left margin. Columns on the barrel sheets include Graphic Lithology, Bioturbation, Structures, Accessories, Sediment Disturbance, Sample Types, Color, and Description. These columns are discussed in more detail below. Graphic lithologyLithologies of the core intervals recovered are represented on barrel sheets by graphic patterns in the Graphic Lithology column using the symbols illustrated in Figure F5. A maximum of two different lithologies (for interbedded sediments) or three different components (for a uniform sediment) can be represented within the same core interval. Minor lithologies, present as thin interbeds within the major lithology, are shown by a dashed vertical line dividing the lithologies. Lithologic abundances are rounded to the nearest 10%. Lithologies that constitute <25% of the core are generally not shown but are listed in the Lithologic Description section. However, some distinctive minor lithologies, such as ash layers, are included graphically in the lithology column. Contact types (e.g., sharp, scoured, gradational, and bioturbated) are also shown in the Graphic Lithology column. Relative abundances of lithologies reported in this way are useful for general characterization of the sediment but do not constitute precise, quantitative observations. BioturbationFor bioturbation classification, five levels of bioturbation were used during Expedition 306. These levels are illustrated with graphic symbols in the Bioturbation column (Fig. F6), and are presented in a scheme based on Droser and Bottjer (1986) (Fig. F7). Bioturbation intensity is classified as:
Most of the Expedition 306 sediments fall into one of these four categories. As a fifth category, the classification “homogeneous” occasionally was used to represent a totally bioturbated or homogenized sediment. Note, however, that in a homogeneous sediment section with no color changes at all it is often not clearly possible to estimate the degree of bioturbation, which may range from absent to totally bioturbated or homogenized. Sedimentary structuresThe locations and types of sedimentary structures visible on the prepared surfaces of the split cores are shown in the Structure column of the core description form using the symbols represented in Figure F6. Symbols in this column indicate the locations of individual bedding features and any other sedimentary features, such as scours, ash layers, ripple laminations, and fining-upward or coarsening-upward intervals. AccessoriesLithologic, diagenetic, and paleontologic accessories, such as nodules, sulfides, and shells, are indicated in the Accessories column on the barrel sheets. The symbols used to designate these features are shown in Figure F6. Sediment disturbanceDrilling-related sediment disturbance that persists over intervals of ~20 cm or more is recorded in the Disturbance column using the symbols shown in Figure F6. The degree of drilling disturbance is described for soft and firm sediments using the following categories:
Sample typesSample material taken for shipboard sedimentologic and chemical analysis consisted of pore water whole rounds, micropaleontology samples, smear slides, and discrete samples for XRD. Typically, one to five smear slides were made per core, one pore water sample was taken at a designated interval, and a micropaleontology sample was obtained from the core catcher of most cores. XRD samples were taken only where needed to assess the lithologic components. Additional samples were selected to better characterize lithologic variability within a given interval. Tables summarizing relative abundance of sedimentary components from the smear slides were generated using a spreadsheet exported from a standard program (Sliders) used during the cruise. ColorColor is determined qualitatively using Munsell Soil Color Charts (Munsell Color Company, 1994) and described immediately after cores are split to avoid color changes associated with drying and redox reactions. When portions of the split core surface required cleaning with a stainless steel or glass scraper, this was done prior to determining the color. Munsell color names are provided in the Color column on the barrel sheet, and the corresponding hue, value, and chroma data are provided in the Description column. DescriptionThe written description for each core contains a brief overview of both major and minor lithologies present and notable features such as sedimentary structures and disturbances resulting from the coring process. Smear slidesSmear slide samples were taken from the archive halves during core description. For each sample, a small amount of sediment was removed with a wooden toothpick, dispersed evenly in deionized water on a 1 inch × 3 inch glass slide, and dried on a hot plate at a low setting. A drop of mounting medium and a 1 inch × 1 inch cover glass was added, and the slide was placed in an ultraviolet light box for ~30 min. Once fixed, each slide was scanned at 100×–200× with a transmitted light petrographic microscope using an eyepiece micrometer to assess grain-size distributions in clay (<4 µm), silt (4–63 µm), and sand (>63 µm) fractions. The eyepiece micrometer was calibrated once for each magnification and combination of ocular and objective using an inscribed stage micrometer. Relative proportions of each grain size and type were estimated by microscopic examination. Note that smear slide analyses tend to underestimate the abundance of sand-sized and larger grains (e.g., foraminifers, radiolarians, and siliciclastic sand) because these are difficult to incorporate into the smear. Clay-sized biosilica, being transparent and isotropic, is also very difficult to quantify. Clay minerals, micrite, and nannofossils can also be difficult to distinguish at the very finest (less than ~4 µm) size range. After scanning for grain-size distribution, several fields were examined at 200×–500× for mineralogical and microfossil identification. Standard petrographic techniques were employed to identify the commonly occurring minerals and biogenic groups as well as important accessory minerals and microfossils. Smear slide analysis data tables are included in “Core descriptions.” These tables include information about the sample location, whether the sample represents a dominant (D) or a minor (M) lithology in the core, and the estimated percentages of sand, silt, and clay together with all identified components. Relative abundances of different grain types below 5% were assigned on a semiquantitative basis using the following abbreviations: tr = trace (0%–1%), and R = rare (1%–5%). Abundance of gravel-sized grainsOne of the primary objectives of Expeditions 303 and 306 was to collect cores that record the paleoclimatic history of the North Atlantic at millennial and longer timescales. Because of the geographic location and the geologic age of these sediments, ice-rafted debris (IRD) was expected to be an important part of that paleoclimatic record. The most easily identified IRD consists of outsized clasts in a fine-grained pelagic matrix and is often defined operationally as gravel-sized grains. During Expedition 306, the abundance of IRD was estimated by counting the number of gravel-sized grains (i.e., granule size and larger) in each 1 cm increment of the core. Wherever possible, composition, size, and angularity of each clast were also determined. All of this information is recorded in a table within each site chapter. Spectrophotometry (color reflectance)Reflectance of visible light from the surface of split sediment cores was routinely measured using a Minolta spectrophotometer (model CM-2002) mounted on the AMST. The AMST measures the archive half of each core section and provides a high-resolution stratigraphic record of color variations for visible wavelengths (400–700 nm). Freshly split soft cores were covered with clear plastic wrap and placed on the AMST. Measurements were taken at 2.0 cm spacing. The AMST skips empty intervals and intervals where the core surface is well below the level of the core liner but does not skip relatively small cracks, disturbed areas of core, or large clasts. Thus, AMST data may contain spurious measurements, which should be edited out of the data set. Each measurement recorded consists of 31 separate determinations of color reflectance in 10 nm wide spectral bands from 400 to 700 nm. Additional detailed 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). Digital color imagingSystematic high-resolution line scan digital core images of the archive half of each core were obtained using the Geotek X-Y DIS (Geoscan II). This DIS collects digital images with three line-scan charge-coupled device arrays (1024 pixels each) behind an interference filter to create three channels (red, green, and blue). The image resolution is dependent on the width of the camera and core. The standard configuration for the Geoscan II produces 300 dpi on an 8 cm wide core with a zoom capability up to 1200 dpi on a 2 cm wide core. Synchronization and track control are better than 0.02 mm. The dynamic range is 8 bits for all three channels. The FrameStore card has 48 MB of onboard random-access memory for the acquisition of images with an Industry Standard Architecture interface card for personal computers. After cores were visually described, they were placed in the DIS and scanned. A spacer holding a neutral gray color chip and a label identifying the section was placed at the base of each section and scanned along with each section. Output from the DIS includes a Windows bitmap (.BMP) file and a JPEG (.JPEG, .JPG) file for each section scanned. The bitmap file contains the original data. Additional postprocessing of data was done to achieve a medium-resolution JPEG image of each section and a composite JPEG image of each core, which is comparable to the traditional photographic image of each core. The JPEG image of each section was produced by an Adobe Photoshop batch job that opened the bitmap file, resampled the file to a width of 0.6 inch (~15 mm) at a resolution of 300 pixels/inch, and saved the result as a maximum-resolution JPEG. The DIS was calibrated for black and white approximately every 12 h. No significant change in this calibration was observed during Expedition 306. A constant aperture setting of f/11 was used. X-ray diffraction analysisSelected samples were taken for qualitative mineral analysis by using an XRD Philips model PW1729 X-ray diffractometer using Ni-filtered CuKα radiation. Instrument conditions were as follows: 40 kV, 35 mA; goniometer scan from 2° to 70°2θ (air-dried samples) and from 2° to 12°2θ (glycolated samples); step size = 0.01°2θ; scan speed = 1.2°2θ/min; and count time of 0.5 s. Some samples were decalcified using 10% acetic acid then washed repeatedly with demineralized water in a centrifuge. The carbonate-free fraction was deflocculated with a 1% Calgon (sodium hexametaphosphate) solution and homogenized in a sonic dismembrator for 1 min. MacDiff software (version 4.1.1 PPC by Rainer Petschick) was used to display diffractograms, and identifications are based on multiple peak matches using the mineral database provided with MacDiff. Diffractograms were peak corrected to match the calcite peak at 3.035 Å. In the absence of calcite, no peak correction was applied. |
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