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. 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 percentestimates to track downcore trends in sediment composition has become lessfrequent 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 thenorthern 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, ratherthan abundance categories, reveals similar downcore variation in compositionto 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 smearslides track reasonably well with NGR measurements. Comparison of siteaverages of absolute percentages of total calcium carbonate from coulometryand total calcareous components from smear slide observations reveals an overestimation in carbonate percentages in smear slides (likely due in partto underestimation of the clay fraction), especially in sediments rich in smectite clays. Differences in sediment color between sites and settling ofclay particles during slide preparation may contribute to this discrepancy.Although smear slide estimates range in accuracy depending on the trainingof the operator, we suggest that sedimentologists describing cores obtainedduring scientific drilling can use the percent estimates of sedimentarycomponents in smear slides to identify trends and cyclicity in marinesediment records.

scientific drilling

Comparison of sediment composition by smear slides to quantitative shipboard data: a case study on the utility of smear slide percent estimates, IODP Expedition 353, northern Indian Ocean