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Samples were processed in different ways depending on organic and mineral content. For samples that were organic rich (e.g., coals), 2 g of sediment was crushed using a mortar and pestle and then placed directly into concentrated HNO3 (~15 mL) for 20 min. The residue was then sieved through a 10 µm mesh sieve with at least 1 L of water. The residue was subsequently oxidized using 2 mL of sodium hypochlorite solution for 10 s in an ultrasonic bath and further sieved to remove all chemicals and to clean the residue of excess fine organic matter. For mineral-rich sediments such as clays and siltstones, treatment commenced on ~5 g of sediment using 35% HCl to dissolve carbonates. The next stage was to remove silicate minerals by heating the sediment in 49% hydrofluoric acid (HF) (15 mL) on a hot plate heated to 50°C for 1 h. The residue was sieved at 20 µm (or 10 µm for more organic-rich samples with potential for yields of pollen and spores) with at least 1 L of water and treated for 1 min in concentrated HCl before sieving to flush out remaining chemicals. Oxidation using 70% HNO3 was necessary to remove amorphous organic matter, and clastic samples were washed in 2 mL of concentrated HNO3 for 2 min. In all cases, samples were stained using safranin and then mounted onto two coverslips (24 mm × 24 mm), dried on a hot plate set to low temperature (50°C), and then mounted using a photocuring adhesive. Both coverslips were studied for palynomorphs, except where a count size of 300 grains or cysts was achieved from one coverslip. The remaining coverslip was scanned for the presence of important index taxa. Slides were analyzed for dinoflagellate cysts, spores, and pollen grains. Samples were analyzed on a Zeiss Axioplan 2 microscope in the paleontology laboratory on the Chikyu using mainly a 400× differential interference contrast and phase contrast lens.

Dinocyst nomenclature follows that of Williams et al. (1998) and studies from the northwestern Pacific by Matsuoka (1983), Bujak (1984), Kurita and Matsuoka (1994), Kurita and Obuse (2003), and Kurita (2004). The biostratigraphic schemes of Bujak (1984), Bujak and Matsuoka (1986), Matsuoka et al. (1987), Kurita and Obuse (2003), and Kurita (2004) were consulted to identify biostratigraphic index taxa and acme events. Nomenclature for fossil pollen and spores is not standardized, so the terms used here are based on regional standard names (Yamanoi, 1992; Sato, 1994; Wang et al., 2001; Wang, 2006), North American pollen and spore terminology, and similarly aged material in Europe and China (Kedves, 1969; Jingrong et al., 2000; Ruiqi et al., 2000; Wang, 2006).

Terms used in this report include first downhole occurrence and last downhole occurrence. In addition to first and last occurrence data, the abundance patterns of taxa were also noted because acmes have been indicated previously (Kurita and Obuse, 2003) and may have regional significance for biostratigraphy. Acmes are also important for assessing stratigraphic and environmental patterns in pollen and spore data (Yamanoi, 1992; Sato, 1994; Wang, 2006). Notes on palynomorph abundance within a sample or stratigraphic distribution were recorded as follows:

  • B = barren.

  • P = poor (1–75 specimens counted within a sample).

  • M = moderate (76–125 specimens counted within a sample).

  • G = good (>126 specimens counted within a sample).

  • X = reworked presence within a sample from an older stratigraphic interval.


Samples from RCB cuttings and cores were collected at 10 m intervals from Hole C0020A. Sample material of ~0.5 g was washed and soaked/softened in Milli-Q water then sieved using a 45 µm sieve. Strewn slides were prepared from the sieved sediment by first stirring/shaking the sediment into suspension, immediately removing part of the upper suspension with a disposable pipette, and injecting it into a droplet of Milli-Q water to cover a 24 mm × 36 mm coverslip. The strewn sample was dried on a hot plate (50°–60°C) then mounted to a glass slide using Norland Optical Adhesive 61 and cured in UV light. Samples were examined with a Zeiss Axioskop 40 polarizing light microscope at 400× magnification with identifications checked at 1000×.

Biostratigraphy is the primary objective of this study; therefore, only the occurrences of stratigraphically diagnostic diatoms were tabulated. Diatoms were counted when at least half of a valve was present and identifiable at the species level. Estimation of diatom abundance was qualitative and based on the following:

  • A = abundant (≥6 specimens per field of view [FOV] at 400× magnification).

  • C = common (1–5 specimens/FOV at 400×).

  • F = few (0.2–0.8 specimens/FOV at 400×).

  • R = rare (1–10 specimens/horizontal traverse at 400×).

  • B = barren.

Diatom preservation is recorded based on the degree of breakage and dissolution of diatom valves as described by Akiba (1986):

  • G = good.

  • M = moderate.

  • P = poor.

Strewn slides were also examined for calcareous nannofossils; however, restricted diversity and rare occurrence eliminate them as a useful biostratigraphic tool. Therefore, Neogene sediments were zoned using diatoms according to the Neogene North Pacific diatom zone code system of Yanagisawa and Akiba (1998).