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Abstract

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.
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Abstract

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.
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