11:15 AM - 11:30 AM
[MIS18-19] Orbital-Scale Fluctuations in the Japan-Sea Miocene Diatomaceous Sediment and Application to Cyclostratigraphy – An Example of the Nakayama Formation on Sado Island in Niigata Prefecture, Japan
Keywords:cyclostratigraphy, biogenic silica, orbital forcing, XRF, Nakayama Fm.
The Nakayama Formation, which is exposed on Sado Island in Niigata Prefecture, Japan, is one of the middle-to-late Miocene diatomaceous sediments deposited in the Japan Sea. Such sediments exist along the Japan Sea coast from the Hokkaido region to Shimane Prefecture, and in the deep basins in the Japan Sea. It means that enhanced production or blooming of diatom occurred in the Japan Sea during this era. Since most part of the Nakayama Formation avoids secondary alteration by silica diagenesis, it is expected that the Nakayama Formation is used for paleoceanographic reconstruction in the mid-latitude. In the southern part of the Osado area in Sado, diatom biostratigraphy of the Nakayama Formation has already been reported.
In this area, we measured major element compositions of diatomaceous mudstone (ca. 10.7–11.4 Ma based on the reported diatom biostratigraphy) from the Nakayama Formation by X-ray fluorescence (XRF) spectroscopy. As a result, it was revealed that the ratio of SiO2 to Al2O3 shows orbital-scale fluctuations. The ratio of SiO2 to Al2O3 is considered to reflect the ratio of biogenic silica (e.g., diatom frustules) to detritus. Then, we constructed cyclostratigraphy of the Nakayama Formation using the cyclic fluctuations of the ratio of biogenic silica to detritus. Since the ratio of biogenic silica to detritus shows a similar fluctuation pattern to the global benthic foraminiferal δ18O curve, we correlated the ratio of biogenic silica to detritus with the benthic δ18O curve, which is considered to record the global climate fluctuations through the change in the volume of polar ice sheets.
We also conducted a U-Pb measurement of zircon taken from an 8-cm-thick tuff layer intercalated with diatomaceous mudstone in the studied interval. In general, the zircon U-Pb age reflects when the zircon crystallized in magma. Therefore, a zircon U-Pb age gives only a lower limit of a depositional age of tuff if there is a time lag between zircon crystallization and volcanic ash sedimentation. Nevertheless, the obtained zircon U-Pb age was concordant with the diatom biostratigraphy within 2σ error.
In addition to reporting the results mentioned above, we will give a discussion about the controlling factors of the ratio of biogenic silica to detritus based on spectral analyses of the ratio of SiO2 to Al2O3.
In this area, we measured major element compositions of diatomaceous mudstone (ca. 10.7–11.4 Ma based on the reported diatom biostratigraphy) from the Nakayama Formation by X-ray fluorescence (XRF) spectroscopy. As a result, it was revealed that the ratio of SiO2 to Al2O3 shows orbital-scale fluctuations. The ratio of SiO2 to Al2O3 is considered to reflect the ratio of biogenic silica (e.g., diatom frustules) to detritus. Then, we constructed cyclostratigraphy of the Nakayama Formation using the cyclic fluctuations of the ratio of biogenic silica to detritus. Since the ratio of biogenic silica to detritus shows a similar fluctuation pattern to the global benthic foraminiferal δ18O curve, we correlated the ratio of biogenic silica to detritus with the benthic δ18O curve, which is considered to record the global climate fluctuations through the change in the volume of polar ice sheets.
We also conducted a U-Pb measurement of zircon taken from an 8-cm-thick tuff layer intercalated with diatomaceous mudstone in the studied interval. In general, the zircon U-Pb age reflects when the zircon crystallized in magma. Therefore, a zircon U-Pb age gives only a lower limit of a depositional age of tuff if there is a time lag between zircon crystallization and volcanic ash sedimentation. Nevertheless, the obtained zircon U-Pb age was concordant with the diatom biostratigraphy within 2σ error.
In addition to reporting the results mentioned above, we will give a discussion about the controlling factors of the ratio of biogenic silica to detritus based on spectral analyses of the ratio of SiO2 to Al2O3.