Smear slide grain counts from IODP Site 323-U1342

Here we present new grain count data from from Integrated Ocean Drilling Program (IODP) Expedition 323 Site U1342 Holes A, C, and D. New records of sediment composition from laminated and non-laminated were determined from smear slide (quantitative grain counts) and particle size analyses. A subset of samples from three laminated intervals (672-697 ka, 826-836 ka, and 847-851 ka) and four adjacent massive intervals (668-671 ka, 822-825 ka, 803-845 ka, and 853-856 ka) were selected for analyses. Sediment smear slides were examined with a transmitted light petrographic microscope equipped with a standard eyepiece micrometer. Biogenic (pennate and centric diatoms, silicoflagellates, sponge spicules, foraminifers, and coccolithophores), mineral (clay minerals, silt- to sand-size siliciclastics), and volcaniclastic components were identified and their percentage abundances were visually determined under a petrographic microscope using a 40X objective and 10X eyepiece. For each sample, 3 counts were done on different parts of a smear slide using a random walk, and the average value of the 3 counts was used. Particle size analyses on the same samples were carried out with a Beckman-Coulter LS 13 320 laser particle size analyzer (LPSA) attached to an aqueous module equipped with a pump and a built-in ultrasound unit. Size distributions were analyzed from 0.04 µm to 2 mm. Measurements of such a wide particle size range are possible because the particle sizer is composed of two units: a laser beam for conventional (Fraunhofer) diffraction (from 0.4 µm to 2 mm) and a polarized intensity differential scatter (PIDS) unit, which measures particles based on the Mie theory of light scattering (0.04 µm). The samples for the analyses were subsampled and dispersed in the deionized water of the aqueous module of the particle sizer until obscuration values of 10%–15% and PIDS obscuration values of 48%–52% were obtained. The optical model chosen for the grain size determination is the default Fraunhofer model, based on the Fraunhofer theory of light scattering. Data interpolation and statistical analyses were calculated with the laser particle sizer proprietary software (56). Because all samples analyzed tend to log-normal grain size distributions in the 0.04 µm to 2 mm spectrum, geometric rather than arithmetic statistics were applied to the values obtained by the logarithmically spaced size channels of the particle sizer. Smear slide analyses show that laminated sediments contain significantly more diatomaceous biogenic material relative to siliciclastic grains. Laminated samples contain a higher percentage of total diatoms compared to non-laminated samples (72 versus 30%) and a higher percentage of well-preserved (i.e. whole, non-fragmented) diatoms (29 versus 9%), indicative of a high siliceous flux rate. Laminated sediments also display higher proportions of other biogenic particles (including silicoflagellates, sponge spicules, foraminifera, and coccolithophores).