Abstract
Abstract. Smear slide petrography has been a standard technique during scientific ocean drilling expeditions to characterize sediment composition and classify sediment types, but presentation of these percent estimates to track downcore trends in sediment composition has become less frequent over the past 2 decades. We compare semi-quantitative smear slide composition estimates to physical property (natural gamma radiation, NGR) and solid-phase geochemical (calcium carbonate, CaCO3 %) measurements from a range of marine depositional environments in the northern Indian Ocean (Bay of Bengal, Andaman Sea, Ninetyeast Ridge) collected during International Ocean Discovery Program (IODP) Expedition 353. We show that presenting smear slide estimates as percentages, rather than abundance categories, reveals similar downcore variation in composition to the more quantitative core analyses. Overall downcore trends in total calcareous components from smear slides (foraminifers + nannofossils + shell fragments + authigenic carbonate) follow similar downcore trends to samples measured by CaCO3 coulometry. Total lithogenic components (clay + mica + quartz + feldspars + lithic grains + vitric grains + glauconite + heavy minerals + iron oxides) and clay from smear slides track reasonably well with NGR measurements. Comparison of site averages of absolute percentages of total calcium carbonate from coulometry and total calcareous components from smear slide observations reveals an overestimation in carbonate percentages in smear slides (likely due in part to underestimation of the clay fraction), especially in sediments rich in smectite clays. Differences in sediment color between sites and settling of clay particles during slide preparation may contribute to this discrepancy. Although smear slide estimates range in accuracy depending on the training of the operator, we suggest that sedimentologists describing cores obtained during scientific drilling can use the percent estimates of sedimentary components in smear slides to identify trends and cyclicity in marine sediment records.
Highlights
Visual estimation has long been used as a standard method for describing sediments and sedimentary rocks (e.g., Folk, 1951; Terry and Chilingar, 1955; Reid, 1985)
Smear slide petrography has become a standard procedure for identification of microscopic components and lithology classification for sediments recovered during scientific drilling, including Deep Sea Drilling Project (DSDP), Ocean Drilling Program (ODP), Integrated Ocean Drilling Program/International Ocean Discovery Program (IODP), and International Continental Scientific Drilling (ICDP) expeditions (Musich, 1984; Mazzullo and Graham, 1988; Rothwell, 1989; Myrbo et al, 2011; Marsaglia et al, 2013, 2015)
We observe downcore trends in smear slide clay fraction estimates that follow the downcore trends in natural gamma radiation (NGR), which is primarily controlled by the lithogenic content, primarily clay minerals, in the cores
Summary
Visual estimation has long been used as a standard method for describing sediments and sedimentary rocks (e.g., Folk, 1951; Terry and Chilingar, 1955; Reid, 1985). Smear slide petrography has become a standard procedure for identification of microscopic components and lithology classification for (predominantly unconsolidated) sediments recovered during scientific drilling, including Deep Sea Drilling Project (DSDP), Ocean Drilling Program (ODP), Integrated Ocean Drilling Program/International Ocean Discovery Program (IODP), and International Continental Scientific Drilling (ICDP) expeditions (Musich, 1984; Mazzullo and Graham, 1988; Rothwell, 1989; Myrbo et al, 2011; Marsaglia et al, 2013, 2015). These smear slide descriptions are used in tandem with macro-scale visual core descriptions. Littler: Utility of smear slide percent estimates – IODP Expedition 353 for the classification of recovered sediments using a variety of classification and nomenclature schemes (e.g., Folk, 1954; Shepard, 1954; Davies et al, 1977; Dean et al, 1985; Mazzullo et al, 1988; Carozzi, 1988, 1993; Flügel, 2010; Milliken, 2014)
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