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The North Pacific early Oligocene through early Miocene diatom zonation has been recently proposed based mainly on the materials from nearly continuous deep-sea cores. A series of eight zones was proposed based on the documented successive first occurrences of planktonic marker-species. The zones were an important contribution to the North Pacific diatom zonation to extend its application from the middle Miocene up to the basal Oligocene.
Biostratigraphic researches in the region of the last decade demonstrate a real efficiency of the proposed zonation. The application of oceanic zonation to studies of sequences on land and in marginal seas within the vast North Pacific region should be especially emphasized. The available data allow a new correlation scheme to be proposed for the Oligocene and lower Miocene. This zonal scheme is based on the comparative characteristics of the diatom assemblages from more than thirty localities in the region: from the Bering Sea in the north to Japan in the south. These results have great importance because calcareous plankton is essentially lacking above the Eocene in the North Pacific. That is why, until recently, the reliable dating and subdivision of the Oligocene and lower Miocene have proved difficult.
Materials on temporal and spatial distribution of the North Pacific Oligocene assemblages were a basis to reveal the features of diatom flora development and its productivity. The application and interpretation of these data are important in terms of historical geology. The development of diatom assemblages, as a main source of biosiliceous sedimentation, occurred against the background of various events affected oceanic circulation and climate. The Oligocene was characterized by accumulation of thick siliceous sedimentary successions in the North Pacific region. Since the Oligocene, siliceous sedimentation became especially characteristic of marginal parts of the Northwestern Pacific indicating an enhanced upwelling. This process associated diatom higher productivity, reflecting both paleoclimatic and paleoceanographic changes, and regional changes of paleogeographic conditions. Among the major events there were changes of the transitional period from the Eocene to Oligocene affected diatom assemblages. The North Pacific earliest Oligocene assemblages are similar to those from Norwegian-Greenland basin and southern high latitudes but not typical of low latitudes. It indicates an increase of provincialism between floras of high and low-latitude regions. These changes could be the result of enhanced latitudinal thermal gradients and narrowing of subtropical zone owing to cooling. However, an onset of progressive increased diatom flora spatial distribution and its productivity in the North Pacific taken place in the middle early Oligocene didn't coincide with cooling. Most likely, it might be determined by changes in oceanic deep-water circulation owing to paleoceanographic reorganizations.
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