The quantity and nature of smear slides to be prepared from a given core section depends upon the sediment character and the research questions being asked. Analyzing many slides made at a regular, arbitrary interval (e.g., every 10 cm) can lead to fatigue with the process, and is not recommended. In general we suggest initially taking one or two slides of each routine "background" sediment type or lithology per core section, as well as a slide of any anomalous sediment type. During core description, toothpicks can be lightly inserted into the core to mark locations where smear slides should be taken, and the toothpicks then used to make the slides. The researcher should view the smear slides while the core is also available for viewing whenever possible (i.e., ideally when the core has just been split and is being described), in order to mentally relate the sediment composition (as determined from the smear slide) with its expression in core face, and regularly return to the core face to address questions raised by the smear slide analysis. By this method the researcher can learn to infer composition from the core face alone, and can prepare fewer slides as she continues analysis of the core. Slides of previously-analyzed lithologies should be periodically prepared in order to check the identification, and replicate slides should be analyzed if semiquantitative analysis is a goal. Core description conducted iteratively using smear slides, core face, and (if available) analytical data such as multisensor core logs provides the optimal setting for the researcher to understand the sedimentary matrix and its variations, recognize important sedimentary features, and form hypotheses about processes in the paleoenvironment to guide the core sampling and analysis strategy.
The casual analysis of smear slides provides insight into the components that make up lacustrine or marine sediments, but to fulfill their potential as a semiquantitative analytical tool, smear slide observations must be made systematically. Adherence to a standard form(at) prompts the analyst to make the same observations on each slide, allowing relative changes in lithology to be recognized. Several suggested formats are provided on the TMI website.
Analysis is made using a petrographic microscope; an additional fiber-optic light source is useful for providing the reflected light required to distinguish between certain components, e.g., some opaques. The distribution of material on the slide may be uneven, so each analysis should begin with a scan of the entire slide area at low magnification, e.g., 100x. Although the variety of components viewed in smear slide may appear overwhelming, the novice analyst should remember that virtually all can be grouped into one of four broad categories: clastics, organics, diatoms, and carbonates (the latter of which may or may not be present, depending on water chemistry). In addition to these dominant parts of the sediment, there may be components with minor or trace abundance but which have paleoenvironmental significance, such as diagenetic minerals and mineraloids (e.g., pyrite, vivianite, hematite), volcanic glass, and anthropogenic debris such as fly ash. Evaporite minerals (e.g., gypsum, halite) are common in some environments and can also occur as secondary precipitates from pore water as the core desiccates over time. Observations of the grain size distribution, sorting, and rounding of clastic components (i.e., sediment texture) should be made using alternating plane- and cross-polarized light, and rotation of the stage, to illuminate all mineral grains, including the fine silt- to clay-sized fraction that may show very low relief. Percentage estimates of each sedimentary component should be attempted, and the eye can be "calibrated" by comparing visual estimates with numerical compositional data from the same sediments as analyzed at a later date. Partial dissolution of components such as diatoms and carbonate minerals should be noted, as their condition provides information about saturation with respect to these phases in the water column and pore waters. Exceptional preservation, e.g., of algal pigments and unstable minerals, can occur in environments with rapid deposition, anoxic bottom waters, or both.
One of the primary goals in conducting smear slide analysis is to generate compositionally-based names for sedimentary lithologies and facies, to be used in core description and archived with the core data and metadata. The lacustrine sediment classification scheme outlined in Schnurrenberger et al. (2003) provides a flexible terminology for continental sediments, and sediment names can be readily determined based on smear slide descriptions.
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