This thesis consists of three separate articles and are mainly concerned with the taxonomy of species from the genus Thalassiosira and quantitative diatom biostratigraphy in three oceanic regions of the northern hemisphere. The study material consists of Ocean Drilling Program (ODP) Holes 642B, 642C and 644A at about 67°N in the Norwegian Sea, Deep Sea Drilling Project (DSDP) Hole 552A at 56 N in the North Atlantic, and DSDP Site 578 at 34°N in the northwest Pacific. The stratigraphic sequences examined range in age from early Miocene to late Pliocene (18.0 to 1.8 Ma) in the Norwegian Sea, late Miocene to early Pliocene (5.7 to 4.4 Ma) in the North Atlantic, and late Miocene to early Pliocene (6.4 to 4.0 Ma) in the North Pacific.

The first article presents an empirically derived evaluation of the accuracy and reproducibility in Moore’s (1973) random settling method. This method is here adopted to quantitative diatom analysis. The results of the evaluation of Moore’s method provide estimates of errors to be expected at different abundance levels. These results made it possible to rank the reliability of diatom data produced throughout the thesis.

The second and third articles investigate a total of 53 diatom events. These articles also contain evaluations of the stratigraphic precision by which these 53 events can be determined on the basis of the observed absolute abundance patterns. Some of the 53 events discussed represent previously established datums. The quantitative data presented in this thesis allow establishment of several new biostratigraphic datums, from all three investigated regions. Of the 36 events investigated for the Norwegian Sea, 22 have been given chronological estimates through direct correlation to the established paleomagnetic polarity history. All 5 events for the North Atlantic and all 12 events for the North Pacific are also given age estimates through correlation to existing magnetostratigraphies. Five new Thalassiosira species are described, T. baldaufii, T. labimarginata, T. lindstroemii, T. tetraoestrupii and T. voeringensis. These species are all considered as stratigraphically useful.

Two Neogene hiatuses are identified in the outer Vdring Plateau sequence (ODP Site 642). The older falls in the late Miocene having a duration of approximately 3.5 m.y., representing the time interval between approximately 10 and 6.5 Ma. The younger hiatus represents the time interval between approximately 3.1 and 2.5 Ma in the late Pliocene.