The samples for this study were taken from ODP Core 178-1098A-6H (Barker, Camerlenghi, Acton, et al., 1999) and are composed of hemipelagic sediments consisting of an alternation of laminated to thinly bedded orange-brown diatom ooze with blue-gray diatom-bearing terrigenous silt and clays (Fig. F2). These sediments overlie a dense, blue-gray diamict with many prominent dropstones. Overlapping sediment slabs, 15 cm long and ~1 cm wide, were removed from the core using a sediment slab cutter (Pike and Kemp, 1996; Schimmelmann et al., 1990). These slabs were subsampled for thin section preparation and also for examination by SEM secondary electron imagery (SEI).
Sediment for polished thin section preparation was prepared following the fluid-displacive, low-viscosity Spurr resin embedding technique of Pike and Kemp (1996), as adapted by Pearce et al. (1998). Thin sections were analyzed using light microscopy and SEM backscattered electron imagery (BSEI) to document lamina composition and thickness variations and sediment fabric. BSEI analysis was carried out following the standard techniques developed for paleoceanographic analysis of laminated sediments (Grimm, 1992; Kemp, 1990; Pike, 2000). In essence, the BSEI photograph records the differences in the average atomic number and density of the target; therefore, dense terrigenous grains (hence, terrigenous-rich laminations) that have relatively high average atomic numbers and densities produce bright images. Diatom ooze laminae contain diatom frustules that are filled with low-atomic-number, carbon-based resin and therefore produce dark images. In Palmer Deep sediments, the BSEI photomosaic can be considered as a porosity map with dark layers representing highly porous diatom ooze and bright layers representing dense, diatom-bearing terrigenous silt and clay laminae.
In addition to BSEI, thin sections were examined using light microscopy. In plane-polarized light, the diatom ooze layers produce bright images (optically transparent biogenic silica and resin) and the diatom-bearing silt and clay layers produce darker images. This is the opposite sense to the BSEI images, so care should be taken when examining the figures in this data report.
Blocks of sediment with dimensions of <0.5 cm were cut from the slabs and carefully fractured open to reveal surfaces parallel and perpendicular to the laminated sediment fabric. These blocks were correlated with corresponding laminae in the BSEI photomosaics. Care was taken not to touch the fractured surfaces with either scalpel or tweezers. Blocks were mounted onto standard SEM stubs using epoxy glue, left to dry at room temperature for 48 hr, and then gold-coated for topographic SEI analysis. Surfaces were analyzed for composition, fabric, and relationships between components.