[S-GL22] Frontier research on chronostratigraphic unit boundaries

[SGL22-07] Construction of a high-resolution and correlatable age model of the Miocene diatomaceous sediment in the Japan Sea

*Jumpei Yoshioka^1,2, Junichiro Kuroda², Kenji Matsuzaki², Sota Niki³, Takafumi Hirata³, Takeru Kochi^1,2, Nanako O. Ogawa⁴, Naohiko Ohkouchi⁴ (1.Department of Earth and Planetary Science, the University of Tokyo, 2.Atmosphere and Ocean Research Institute, the University of Tokyo, 3.Geochemical Research Center, the University of Tokyo, 4.Japan Agency for Marine-Earth Science and Technology)

Keywords:Miocene diatomaceous sediment, diatom biostratigraphy, radiolarian biostratigraphy, cyclostratigraphy

During the middle to late Miocene, diatomaceous sediments were widely formed in the north Pacific region including the Japan Sea. Extensive organic matter burial with the diatomaceous sediments is thought to have had an influence on the Miocene global cooling through the carbon cycle. Therefore, the Miocene diatomaceous sediments with a high-resolution and correlatable age model are required to understand the Miocene global cooling mechanism. By the way, siliceous microfossil biostratigraphy is widely used to date most of the diatomaceous sediments deposited in the Japan Sea. Biostratigraphy used relative ages, which have the advantage in determining a rough age model allowing regional correlations. However, age error ranges of biostratigraphy are difficult to evaluate, and thus, biostratigraphy is not compatible with the high-resolution age determination.
In this study, we conducted (1) calculation of ages and error ranges of diatom and radiolarian biostratigraphic events and (2) construction of cyclostratigraphy of the Nakayama Formation, which is one of the Miocene diatomaceous sediments deposited in the Japan Sea and exposed on Sado Island in Niigata Prefecture.
First, about (1), we calculated the ages and the error ranges of biostratigraphic events by evaluating errors associated with the age model of the reference sedimentary sequence. The newly obtained ages and errors are more “numerical” than before and can be compared or used together with numerical ages such as U-Pb and K-Ar ages. Indeed, we used biostratigraphy and zircon U-Pb ages together to constrain the ages of the Nakayama Formation. The method proposed herein can be expanded to other geological ages. Providing numerical ages to biostratigraphy is expected to help the construction of more plausible age models for various sedimentary sequences.
Next, about (2), we reconstructed major element and mineral composition fluctuations of the Nakayama Formation by X-ray fluorescence (XRF) and X-ray diffraction (XRD) analyses. We identified outstanding orbital-scale cyclic fluctuations in the SiO₂/Al₂O₃ ratios and the opal-A/quartz ratios, both of which seem to represent the ratio of biogenic silica to terrigenous materials. In addition, we also recognized the cyclic appearance of diagenetic gypsum, which indicates the existence of calcium carbonate in the diatomaceous sediments. In this study, we correlated the fluctuations of the SiO₂/Al₂O₃ ratios with the global δ¹⁸O curves of benthic foraminifera and constructed a high-resolution age model. Such orbital-scale cyclic fluctuations of the ratio of biogenic silica to terrigenous materials as in the Nakayama Formation are reported in other regions in the Japan Sea, for example, the Onnagawa Formation in Akita Prefecture (Tada, 1991) and International Ocean Discovery Program (IODP) drilling cores (Kurokawa et al., 2019). In the future, we might use more various Miocene diatomaceous sediments in the Japan Sea with an integrated age model by using the cyclostratigraphic approach proposed in this study, which would have a lot of contribution to paleoceanography and paleoclimatology about the Miocene Epoch.