Paleoenvironmental changes affected on the diversity explosion and extinction events of the fossil diatom resting spore assemblage across the E/O boundary

The marine diatom genus Chaetoceros is known as a major contributor to primary production in near-shore upwelling regions and coastal areas, where it accounts for 20-25% of the total marine primary production. They produce heavily silicified resting spores which are easily preserved as fossils under nutrient-poor conditions. The diatom resting spores are therefore preserved as significant constituents in fossil marine diatom assemblages providing useful information for reconstructing paleoproductivity and paleoenvironmental changes. However, due to the importance of Chaetoceros in marine primary production, it is crucial to investigate fossil resting spores in upwelling regions. As the result of revising the taxonomy of fossil diatom Chaetoceros resting spores using DSDP 338, 436 and 438, and onland-samples (Newport Beach Section, California) from the late Eocene to the Recent, the Chaetoceros Explosion Event (when there was an increase in diversity and abundance, and a 50% reduction in valve size) across the Eocene/Oligocene (E/O) boundary was clarified. On the other hand, investigation of middle Eocene samples from the Integrated Ocean Drilling Program (IODP) Expedition 302, revealed an Extinction Event across the E/O boundary of diatom resting spores other than Chaetoceros. These two events indicate increased amounts of nutrient supply via upwelling and a change from a stable water column with a constant nutrient supply in the Eocene to an unstable one with sporadic nutrient supply due to increased vertical mixing in the Oligocene.