09:45 〜 10:00
[MIS04-04] 秋田県中新統女川層のサイクル層序の構築と日本海基礎生産の変動
キーワード：サイクル層序、生物源シリカ、地球軌道要素、XRF、XRD
Middle-to-Late Miocene is the time of the global climate transition from a relatively warm climate mode to a more glaciated mode. The global cooling in the earth history is highly related to changes in the Earth system dynamics such as tectonics, ocean circulation, and carbon cycling. During this epoch, diatomaceous sediments were widely deposited in the North Pacific margin and marginal basins including the Japan Sea, and deposition of these formations is supposed to have great influence on the global carbon cycling through the enhanced organic carbon burial that might have resulted in the global cooling. Despite years of efforts to study the role of the organic carbon burial associated with the formations of diatomaceous sediments along the North Pacific margins, a full consensus about their contributions on the Middle-to-Late Miocene cooling has not been reached.
The Onnagawa Formation, which is widely exposed in Akita Prefecture, northern Japan, is one of the representative expressions of the Miocene siliceous (diatomaceous) sediments deposited during Middle-to-Late Miocene, and known for its rhythmical sedimentary cycles in diagenetically recrystallized siliceous rocks. Since silica diagenesis prevents the construction of precise biostratigraphy, the periodicities of the sedimentary cycles have not been clarified.
We have obtained a numerical age of 11.18±0.37 Ma for the middle part of the Onnagawa Formation based on U-Pb dating of zircon grains obtained from volcanic ash layers (Yoshioka et al., submitted to Geochemical Journal). By integrating this age with the radiolarian biostratigraphy, we established the chronostratigraphy of the Onnagawa Formation. Then, we reconstructed the changes in the ratio of detritus to biogenic silica to apply cyclostratigraphic approach. The ratio of detritus to biogenic silica was estimated by using continuous and high-resolution records of mineralogical composition taken by an X-ray diffractometry (XRD) and major element composition taken by X-ray fluorescence (XRF) spectroscopy, and correlated with the short-eccentricity-tuned gamma ray attenuation (GRA) records of the deep-sea sediment cores at Integrated Ocean Drilling Program (IODP) Site U1430 in the Japan Sea. GRA is supposed to reflect the ratio of detritus to biogenic silica in the biosiliceous sediments.
Continuous and high-resolution XRD and XRF records enabled not only the application of cyclostratigraphy to the Onnagawa Formation but also the discussion of paleoceanography. The age-tuned XRF data indicate that the cyclicities of outstanding sedimentary cycles derived from the biogenic silica productivity are those of obliquity and precession cycles and that the biogenic productivity was highly related to the supply of nutrients such as silicon and phosphorus. The detailed age model by cyclostratigraphy will promote research on paleoceanography of the Onnagawa basin.
The Onnagawa Formation, which is widely exposed in Akita Prefecture, northern Japan, is one of the representative expressions of the Miocene siliceous (diatomaceous) sediments deposited during Middle-to-Late Miocene, and known for its rhythmical sedimentary cycles in diagenetically recrystallized siliceous rocks. Since silica diagenesis prevents the construction of precise biostratigraphy, the periodicities of the sedimentary cycles have not been clarified.
We have obtained a numerical age of 11.18±0.37 Ma for the middle part of the Onnagawa Formation based on U-Pb dating of zircon grains obtained from volcanic ash layers (Yoshioka et al., submitted to Geochemical Journal). By integrating this age with the radiolarian biostratigraphy, we established the chronostratigraphy of the Onnagawa Formation. Then, we reconstructed the changes in the ratio of detritus to biogenic silica to apply cyclostratigraphic approach. The ratio of detritus to biogenic silica was estimated by using continuous and high-resolution records of mineralogical composition taken by an X-ray diffractometry (XRD) and major element composition taken by X-ray fluorescence (XRF) spectroscopy, and correlated with the short-eccentricity-tuned gamma ray attenuation (GRA) records of the deep-sea sediment cores at Integrated Ocean Drilling Program (IODP) Site U1430 in the Japan Sea. GRA is supposed to reflect the ratio of detritus to biogenic silica in the biosiliceous sediments.
Continuous and high-resolution XRD and XRF records enabled not only the application of cyclostratigraphy to the Onnagawa Formation but also the discussion of paleoceanography. The age-tuned XRF data indicate that the cyclicities of outstanding sedimentary cycles derived from the biogenic silica productivity are those of obliquity and precession cycles and that the biogenic productivity was highly related to the supply of nutrients such as silicon and phosphorus. The detailed age model by cyclostratigraphy will promote research on paleoceanography of the Onnagawa basin.