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8th International Nannoplankton Association Conference
ABSTRACTS
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Shiho Yuguchi, Teizou Murakami, Takato Saito, Tokiyuki Sato, Koji Kameo, Toshiaki Takayama:
We describe, in detail, the uppermost Cenozoic calcareous nannofossil assemblages of the Pacific Ocean for reconstruction of the palaeoceanography of the Pacific Ocean based on the analysis of 12 ODP holes located in the equatorial to high-latitude Pacific Ocean. Sato and Kameo have previously described the palaeoceanographic events related to the increase of the ice-sheet in the Arctic region, based on the analysis of the samples collected from the Arctic Ocean during ODP Leg 151. They found a remarkable floral change in the Late Pliocene assemblages of the Arctic Ocean, characterised by the floral change from abundantly occurring Reticulofenestra spp. (small) and Dictyococcites spp. (small), to barren, or to the cold-water nannofossil, Coccolithus pelagicus, assemblage. This bioevent was named as 'datum A' by Sato and Kameo, and is correlated to the palaeoceanographic event of drastic increasing of dropstones in the Arctic Sea. The event is calculated as 2.75Ma, based on the relationship between the bioevent and the magnetic polarity scale. As a result, they concluded that the drastic increase of the ice-sheets in the Northern Hemisphere occurred at 2.75Ma. On the other hand, both Kameo and Sato have described the timing of the final elevation of the Isthmus of Panama, based on analysis of the calcareous nannofossil assemblages. They analysed the nannofossils in the ODP samples collected from both the Caribbean Sea and the E equatorial Pacific Ocean to clarify the community of the nannofossils during the latest Cenozoic. Consequently, they described that the similarity of the assemblages between the Caribbean Sea and the E equatorial Pacific Ocean disappeared at 2.75Ma. This indicates that final closure of the C American Seaway occurred at 2.75Ma, which is the same time as the increase of the ice-sheet in the Northern Hemisphere. We analysed the Late Pliocene to Quaternary calcareous nannofossils in the equatorial N Pacific Ocean, focusing on nannofossil palaeobiogeography during the Late Cenozoic, in this study. The Pliocene marker-species of the Martini and Bukry biozonations, such as Discoaster brouweri, D. surculus, D. asymmetricus and D. tamalis, occur abundantly throughout the Upper Pliocene section in the equatorial Pacific, while they are not found in the high-latitude region of the Pacific Ocean. Instead of it, floral change recognised in the Arctic Ocean is clearly recognised in the Upper Pliocene section in both the N Pacific and the Japan Sea side of Japan regions. These results indicate that palaeobiogeographic provinces are divided into two provinces, characterised by warm-water assemblages and by cold-water assemblages. The boundary between these provinces is situated around 40°N. In addition, we also analysed the abundance of both warm- and cold-water species, focusing on the 2.75Ma event mentioned above. The warm-water species, discoasters, occur abundantly in the W equatorial Pacific Ocean, between 3.66 and 2.75Ma, and rapidly decrease after 2.75Ma. On the other hand, Coccolithus pelagicus, regarded as a typical cold-water species, shows different distribution patterns between the Atlantic and Pacific Oceans. In the Atlantic Ocean, C. pelagicus occurs abundantly before 2.75Ma in the Late Pliocene, but the abundance decreases after 2.75Ma. In contrast, C. pelagicus does not occur, or occurs rarely, between 3.66 and 2.75Ma in the Pacific Ocean, but rapidly increases after 2.75Ma. The distribution patterns of C. pelagicus recognised in the Atlantic Ocean contrast sharply with those seen in the Pacific Ocean. The differences represent the occurrences of different cold current systems during the Late Pliocene between the oceans. Between 3.66Ma and 2.75Ma, based on the absence of C. pelagicus in the eastern and western equatorial Pacific, it is inferred that the warm current originated off W Africa, flowed from the Atlantic to the Caribbean, and on into the equatorial Pacific. On the other hand, both the increase of C. pelagicus in the equatorial Pacific and the decrease in the NE Atlantic at 2.75Ma suggests that warm current communication between the equatorial Atlantic and the equatorial Pacific was cut off due to the closure of the C American Seaway, and a cold current from offshore California originated in the N Pacific, and flowed into the E equatorial Pacific at the time. Therefore, the contrast in the distribution patterns of the cold-water species, C. pelagicus, between the Atlantic and Pacific Oceans is related to the final elevation of the Isthmus of Panama, and indicates that a drastic change in the current system occurred at 2.75Ma.
[INA8] [Registration and Accomodation
Form]
[Division of Micropalaeontology] [Department of Geosciences] [Bremen University]
Copyright © 2000, most recent revision July 28, 2000 Tania Hildebrand-Habel (hiha@micropal.uni-bremen.de) |