Early Palaeocene through early Eocene silicoflagellate assemblages were examined from five southern subtropical

through subpolar deep-sea sites: DSDP Holes 208 and 524, and ODP Holes 700B, 752A, and 1121B. For each

site, the taxonomic composition of the silicoflagellate assemblage is documented in detail; Pseudonaviculopsis

gen. nov., Dictyocha castellum sp. nov. and Stephanocha? fulbrightii sp. nov. are proposed, along with several

new combinations. More importantly, however, these observations enable a considerable refinement to the existing

Palaeocene–Eocene silicoflagellate biostratigraphic zonation that for the first time uses datums calibrated to

the Geomagnetic Polarity Timescale. The Corbisema aspera Interval Zone occurs immediately above the K/Pg

boundary and is here described from Seymour Island. The Corbisema hastata Partial Range Zone extends from

near the K/Pg boundary to late early Palaeocene and has been observed in Hole 208. The Pseudonaviculopsis disymmetrica

Acme Zone occurs in Holes 208 and 700B. The Dictyocha precarentis Partial Range Zone, observed

in Holes 208, 700B, 752A and 1121B, is subdivided into D. precarentis, Naviculopsis primativa, N. cruciata

and Pseudonaviculopsis constricta subzones. The Naviculopsis constricta Partial Range Zone occurs in Holes

524, 700B, 752A and 1121B. This study is also the first to consider syn- and/or diachroneity in Palaeogene

silicoflagellate biostratigraphy.

Upper Miocene (~6 Ma) sediments from a north-south transect of Ocean Drilling Program Holes 848B, 849B, 850B, 851B across the equator in the Eastern Pacific Ocean record an episode of unusual skeletal variability associated with the silicoflagellate Bachmannocena diodon subsp. nodosa (Bukry) Bukry, 1987. Skeletons in this plexus persisted for between 55,000 and 75,000 years and include an arch across a 2-sided basal ring, a highly unusual morphology for the genus. Groups of skeletal morphotypes were common within the plexus episode, and occur in a systematic sequence that is consistent between sites. As a result of this study, silicoflagellates previously placed in the genus Neonaviculopsis Locker and Martini, 1986 are now considered part of the Bachmannocena diodon subsp. nodosa plexus.

The present contribution provides a study of calcareous nannofossils and siliceous microfossils from the Sawai Bay Formation on Car Nicobar Island, northern Indian Ocean. Two stratigraphically short sediment intervals near Sawai Bay have been examined. Qualitative and quantitative microfossil analyses show the Sawai Bay ‘A’ Section to be devoid of siliceous microfossils, while 24 well-preserved calcareous nannofossil taxa are identified. The Sawai Bay ‘B’ Section yields 18 calcareous nannofossil, 33 radiolarian and 25 diatom taxa. The calcareous nannofossil index taxa ( Ceratolithus armatus Müller, 1974a and C. cristatus Kamptner, 1950) indicate both sections to be from zones NN12 (CN10b) and NN13 (CN10c) of early Pliocene (Zanclean) age. The radiolarian taxa, i.e., Didymocyrtis avita Riedel, 1953, Euchitonia spp., Siphocampe lineata (Ehrenberg) Nigrini, 1977, Stichocorys peregrina Riedel, 1953, Semantis spp. and Stylochlamydium sp. are common in the Sawai Bay ‘B’ Section, which is assigned to Zone RN9. Most of the diatom taxa are represented by representatives of the genera Actinocyclus Ehrenberg, 1837, Azpeitia Peragallo in Tèmpere and Peragallo, 1912, Coscinodiscus Ehrenberg, 1839a, Grammatophora Ehrenberg, 1841 and Triceratium Ehrenberg, 1839b, with the benthic diatom species Triceratium favus Ehrenberg, 1839b being predominant (~35% of the total diatom count). Siliceous microfossils are also represented by silicoflagellates dominated by Dictyocha spp. and sponge spicules dominated by astrophorids.

Diatomaceous ooze sampled from near the Mariana Trench sediment surface by gravity corer (Core JL7KGC05) revealed a high sedimentary abundance of Ethmodiscus rex (Rattray, 1890) Wiseman and Hendey, 1953 fragments and tropical open ocean planktonic diatom taxa including Azpeitia nodulifera (Schmidt, 1878) Fryxell and Watkins in Fryxell, Sims and Watkins, 1986 and Alveus marinus (Grunow, 1880) Kaczmarska and Fryxell, 1996. Subsurficial sediments from the ooze are assigned a Marine Isotope Stage 2 age, approximately at the Last Glacial Maximum. The occurrence of Ethmodiscus ooze suggests massive late Pleistocene blooms in the Northwestern Pacific Ocean and provides a plausible link to paleoceanographic and paleoclimatic changes related to Antarctic Intermediate Water mass, which carried a high dissolved silica content as silicon leakage that reduced dissolution rate of diatom frustules. Northward flow of Antarctic Intermediate Water was probably related to surface current migration and southward shift of the Northwest Pacific Gyre to form oligotrophic conditions that triggered Ethmodiscus rex blooms under unusual nutrient recycling conditions within the ocean system. This bloom hypothesis may help explain differential silica dissolution during the last glacial stage.