The explosive diversification of the diatom genus Chaetoceros across the Eocene/Oligocene and Oligocene/Miocene boundaries in the Norwegian Sea

Eocene to middle Miocene stratigraphic changes in species richness, abundance and valve size of Chaetoceros resting spores in the Norwegian Sea (DSDP Site 338) were investigated in order to understand past productivity and paleoenvironmental changes in upwelling regions. As a result, drastic resting spore events were recognized in a ∼6 myr interval across the Eocene/Oligocene boundary (EO Event), the Oligocene/Miocene boundary (OM Event) and in the early middle Miocene (emM Event). The EO Event was characterized by explosive diversification at both the morpho-generic and specific levels, an increase in abundance, and a decrease in valve size from the upper Eocene through the lowest Oligocene. The OM Event was defined by a two-fold increase in species richness. During the emM Event spore abundance decreased rapidly, and species richness and valve size decreased gradually. These changes may indicate changes in the nutrient supply, especially in upwelling regions. The increased species richness suggests a change from a stable water column with a constant nutrient supply in the Eocene to an unstable one with a sporadic nutrient supply by increased vertical mixing in the Oligocene, based on evaluation of the ecologic differences between dinoflagellate cysts and Chaetoceros resting spores. The role of main primary producer might have switched from dinoflagellates and/or nannoplankton in the Eocene to diatoms, especially Chaetoceros, in the Oligocene in the Norwegian Sea. Increased resting spore species richness during the OM Event may show that environmental changes such as global cooling and nutrient mixing led to a diversification of the spore producing genus Chaetoceros. The emM Event might have been affected by changes in paleoceanographic conditions, perhaps a decrease in nutrient supply. This study presents the first paleoceanographic analysis using not only the total resting spore abundance but also the abundances of individual species, and establishes the value of spore taxonomy and diatom analysis including spores.