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doi:10.2204/iodp.proc.317.202.2013

Materials and methods

During Expedition 317, we drilled three shelf sites (U1351, U1353, and U1354) and one slope site (U1352) along a transect perpendicular to the margin of the Canterbury Basin (Fig. F1). For age control, all core catcher samples were analyzed aboard ship for calcareous nannofossil, planktonic and benthic foraminifer, and diatom biostratigraphy in Holocene to Eocene sediments (see the “Expedition 317 summary” chapter [Expedition 317 Scientists, 2011a]). Continental slope Site U1352 represents a complete section of modern slope terrigenous sediment, mainly in Holes U1352A and U1352B, to hard Eocene limestone sediments at the bottom of Hole U1352C, with all the associated lithological, biostratigraphical, physical, geochemical, and microbiological transitions.

Hole U1352B was the deepest water site drilled during the expedition. Shipboard study revealed few diatoms, but onshore diatom investigation recognized moderately continuous occurrences of fossil diatom assemblages in upper Pliocene to Pleistocene sediments with better preservation than that seen in samples from the shelf sites. Our postcruise diatom analysis, therefore, focused on 437 samples from Hole U1352B. Silt to clay samples were selected from every core section (yielding a sample interval of approximately every 1.5 m).

Counting and other procedures followed those of Akiba (1986), with a minor modification:

1. About 1.0 g of each sample was oven-dried for 2 h at 100°C, mixed with ~30 mL of hydrogen peroxide (H₂O₂, 10%–15%), and boiled in a 200 mL beaker to decompose sediment aggregates and oxidize organic matter.

2. After cooling, ~30 mL of hydrochloric acid (HCl, 30%) was added and boiled to dissolve calcareous material (including calcareous nannofossils and foraminifers) to concentrate the siliceous fossil material.

3. After boiling for ~5 min, the beaker was filled with distilled water and allowed to stand for 8 h.

4. After standing, the residue was separated by decanting the acid water, and the beaker was refilled with distilled water. This process was repeated five times.

5. In order to remove clay fractions, a flocculating solution (Na₄P₂O₇, 0.01 N) was added to the beaker and allowed to stand for 8 h. The residue was separated by decanting the water. The beaker was filled again with sodium phosphate solution, and this process was repeated until the supernatant liquid became clear. The remaining residue contained diatoms, other siliceous fossils (e.g., silicoflagellates and radiolarians), and coarse unbroken mudstone fragments. The residues were preserved in 5 mL bottles with distilled water.

Diatom abundance is expressed as an approximate number of diatom valves per slide (18 mm × 18 mm coverslip) calculated using the length of scan lines needed to count at least 100 diatom valves. In the case where there were fewer than 100 valves, counts were stopped after covering ~320 mm² (i.e., whole coverslip observation). Resting spores of the shallow-marine diatom genus Chaetoceros, which have generally been neglected in diatom analyses although they indicate coastal upwelling and a high-nutrient environment, were counted separately. Diatom and spore occurrences and the presence (indicated by a “+” in Table T1) of silicoflagellates, radiolarians, and pollen in Hole U1352B recognized until at least 100 diatom valves were enumerated in separate scan line(s) or until ~320 mm² of the coverslip was scanned. The results are given in Table T1.

For diatom biostratigraphic assignments, ages of bioevents that define or characterize certain horizons are based mainly on the diatom biochronology for the Antarctic Neogene of Cody et al. (2008).

Light microscope observations and the photomicroscopy of diatoms and other microfossils (i.e., silicoflagellates, radiolarians, and pollen) were carried out using an Olympus BX40 light microscope with a differential interference contrast condenser at magnifications of 200× for scanning and 400× and 600× for identification and photography using a digital camera system (Olympus DP20). Scanning electron microscopy was performed using a Hitachi High-Technology SEM SU6600 at several magnifications in the Laboratory of Geobiology at Nagoya University (Japan).

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