Diatoms were found to be an abundant part of the sedimentary composition of the well-defined bulk-density lows at Site 1063. The porous valve morphology of diatoms provides an open framework for the sediment, lowering the sediment density. Abundant terrigenous material will also impart a low density signal, which is what accounts for the low observed at the top of Hole 1063D and at 1.4 meters below seafloor (mbsf) (see Fig. F1). This uppermost depth interval (0-1.7 mbsf) scarcely contains any diatoms or diatom fragments, so this density low results entirely from the high terrigenous component in the sediment. A nearly complete lack of diatoms is also noted in depth intervals 2.4-3.3 mbsf (Section 172-1063D-2H-1) and 3.6-8.9 mbsf (bottom of Section 172-1063D-2H-1 to the top of Section 172-1063D-2H-5). Because of the low diatom content in the upper 9 m of sediment, the following 8 m was not analyzed in order to focus on the lower depths of this study, which exhibit the largest changes in density. This study seeks to characterize the diatom assemblages, not only within the bulk-density lows but through entire density cycles.
Biogenic sedimentary material recovered from Site 1063 can either reflect in situ production and deposition or transport and redeposition of such material (or some of both). The presence of species restricted ecologically to coastal or littoral waters (Delphineis surirella, Raphoneis amphiceros, Cocconies disculoides, and Cymatosira lorenziana) indicates that some amount of transport is occurring. Currently, there is little diatom accumulation in this area of the Atlantic, whereas other regions with very different oceanographic regimes enjoy abundant diatom production and preservation (Jousé and Kazarina, 1974; Pokras and Molfino, 1986; Abrantes, 1991).