8th International Nannoplankton Association Conference
Tokiyuki Sato, Takeshi Higuchi, Maki Nakajima, Koji Kameo, Toshiaki Takayama:
We describe, in detail, the cyclicity patterns of calcareous nannofossil assemblages recognised in the sequence of the Lower Pleistocene Nishiyama and Kitaura Formations, respectively distributed in the Niigata and Akita Prefectures on the Japan Sea side of Japan. Both the studied sections are characterised by the occurrence of Gephyrocapsa caribbeanica, G. oceanica, and Helicosphaera sellii, which indicate the Early Pleistocene. On the basis of the stratigraphic position of the Quaternary datum-planes in the sections (FAD G. oceanica and FAD large form Gephyrocapsa), the time-intervals of the sections are calculated as 25 000 years. Samples were collected at about 10cm stratigraphic intervals, except in the sandstone bed (turbidite sandstone), which is calculated as a 150-year interval, based on the relationship between the sediment accumulation rate and stratigraphic sampling interval.
A total of 200 samples was collected from the Niigata section, and well-preserved nannofossil specimens were found throughout the section. Nannofossil assemblages are characterised by the abundant occurrences of the cold-water nannofossil Coccolithus pelagicus and small Gephyrocapsa spp. On the basis of the comparison of the relative abundances of both C. pelagicus and Gephyrocapsa spp. (small form) with calcareous nannofossil productivity, a negative correlation is found in the relationship between the productivity and relative abundance of C. pelagicus. Furthermore, positive correlation is also clarified in the relation both between productivities of nannofossils and C. pelagicus, and between productivities of nannofossils and the relative abundance of Gephyrocapsa spp. (small form).
Based on the sequential change of the relative abundance of C. pelagicus and Gephyrocapsa spp. (small form) in both the Niigata and Akita areas, the cyclicity between them was analysed using FFT analysis. The productivity of calcareous nannofossils in the Niigata area shows high-frequency cycles, such as 2000, 1000 and 500 years. The relative abundance of the cold-water nannofossil, Coccolithus pelagicus, is also indicated by 2000 and 1000 year cyclicities in the Niigata section. The results of spectral analysis of Coccolithus pelagicus in the Akita area also shows 1000 and 500 year cyclicities. These results indicate that high-frequency cycles, related to cyclic environmental change, were present during the Early Pleistocene interval around Japan.
Furthermore, we analysed the relationships between the thickness of the turbidite sandstone and the relative abundance of the cold-water nannofossil, Coccolithus pelagicus, in the Akita area. The results may indicate a presence of a negative correlation between the sandstone thickness and the relative abundance of C. pelagicus in the low-frequency cycle. As the stratigraphic time-interval is very short (about 25 000 years), it needs further study to clarify the relationship between the turbidite thickness and the relative abundance of C. pelagicus, which is related to the timing of the occurrence of turbidity currents in the sequence stratigraphy of the monsoon district.
Copyright © 2000, most recent revision July 26, 2000Tania Hildebrand-Habel (firstname.lastname@example.org)