IODP Proceedings    Volume contents     Search


Materials and methods

The sediment at Site U1371 consists of ~104 m of diatom ooze and ~20 m of pelagic clay (Fig. F2). The strata of this site are divided into two lithologic units based on their markedly different modal composition. Unit I is ooze with average diatom and clay content of 56% and 17%, respectively. Unit II is a mixture of clay, zeolite, and red-brown to yellow-brown semiopaque iron manganese oxides. Unit II contains an average modal abundance of up to 26% diatoms but only in the upper 5 m of the unit where the lithology transitions from ooze to clay. Other minor constituents of the sediment include quartz, pyrite, manganese oxide/hydroxide, and biogenic particles including radiolarians, spicules, and silicoflagellates.

Eighty-two microslides prepared for diatom analyses contained abundant and well-preserved fossil diatom, resting spore, radiolarian, and silicoflagellate assemblages. To prepare the microslides, silty to clayey sediments containing diatoms were selected between Samples 329-U1371D-1H-1W, 66–67 cm (top depth = 0.66 m uncompressed core depth below seafloor [CSF-A]), and 14H-4W, 35–36 cm (126.25 m CSF-A).

For standard diatom analysis, ~0.5 g of each wet sample was processed; the methods of sample preparation, counting, and other procedures that we followed were basically the same as those of Koizumi (1968) and Akiba (1982b, 1986) with a minor modification. Counting methods for vegetative cell valves of “normal” diatoms and Chaetoceros resting spores were followed after Akiba (1986) and Suto (2006b), respectively (Table T1). A single vegetative cell valve of Centrales species was counted as one when more than a half of a valve was observed. Broken specimens of Pennales species were counted as one valve when two apices were observed. Other criteria for identification of specific genera were used following after several papers (see the floral reference list for diatom taxonomy in the “Appendix”).

Diatom abundance is expressed as an approximate number of diatom valves per slide calculated using the length of scanning lines. To determine the fluctuation of diatom assemblages, 100 vegetative valves of normal diatoms were counted at species level for each sample. After counting, the slides were scanned to record the presence of species missed in the original tally as indicated by a “+” in Table T1. The resting spore abundance is defined here as the number of spore valves encountered during a count of 100 vegetative cell valves of other diatom species.

In diatom biostratigraphic assignments, we applied the diatom zonal scheme of Harwood and Maruyama (1992) for examined samples. Diatom event (first appearance datum [FAD] and last appearance datum [LAD]) ages used in this study were after the midpoint of average range model (ARM) of Cody et al. (2008), which recalibrated the ages to the Cande and Kent (1995) timescale (Figs. F2, F3), although the usefulness of their ages of bioevents has been up for discussion. Magnetostratigraphic ages are presumed with the diatom biostratigraphic results according to the shipboard results in Expedition 329 Scientists (2011), although the polarity chrons were not described in this study. All subchron boundaries and diatom bioevent ages were converted to the Gradstein et al. (2004) timescale.