Taxonomy and stratigraphic occurrence of Thalassiosira tetraoestrupii sp. nov. and related species in upper Miocene and lower Pliocene sediments from the Norwegian Sea, North Atlantic and North West Pacific

Three new Thalassiosira species are described. Thalassiosira baldaufii and T. labimarginata have previously never been reported whereas specimens of T. tetraoestrupii have probably often been assigned to T. oestrupii (Ostenfeld) Hasle (Fryxell and Hasle) in previous studies. The description of Thalassiosira praeoestrupii Dumont Baldauf and Barren is emended. Quantitative analysis of these Thalassiosira species has been completed for upper Miocene and lower Pliocene sediments from three different regions in the northern hemisphere. The material used is from DSDP Hole 552A in the North Atlantic, Site 578 in the NW Pacific and ODP Site 642 in the Norwegian Sea. The abundance patterns (number of valves per gram dry sediment) obtained allow the assessment of the biostratigraphic significance and precision of specific diatom events. Levels of reliability are estimated based on the degree of abundance change at the critical stratigraphic interval, level of abundance, consistency of occurrence and duration of consistency. The chronology of the events is obtained through direct correlation to the established magnetic polarity history. The following four first-order reliability (R1) diatom datums are proposed: in the North Atlantic the first abundant occurrence (FAO) of T. praeoestrupii (5.34 Ma), in the Norwegian Sea, first consistent occurrence (FCO) of T. tetraoestrupii (5.00 Ma), FCO of T. praeoestrupii (5.00 Ma) and last consistent occurrence (LCO) of T. baldaufii (4.30 Ma). Two R2 diatom datums are proposed: in the Norwegian Sea, FCO of T. baldaufii (5.13 Ma), LCO of T. praeoestrupii (4.87 Ma). Three R3 diatom datums are proposed: in the NW Pacific, FCO of T. tetraoestrupii (5.02 Ma) and FCO of T. praeoestrupii (5.54 Ma): in the Norwegian Sea, the first occurrence (FO) of T. labimarginata (5.83 Ma).