[S-GL22] Geochronology and Isotope Geology

[SGL22-P05] Osmium isotopic stratigraphy of the Upper Aptian-Lower Albian Stage in the Northwest Pacific and identification of OAEs.

*Kohei Hosogaya¹, Reishi Takashima², Junichiro Kuroda³, Hironao Matsumoto⁴, Toshiro Yamanaka⁵, Hiroshi Nishi⁶, Satoshi Oka¹ (1.Department of Earth Science, Graduate School of Science, Tohoku University, 2.Tohoku University Museum, Tohoku University, 3.Department of Ocean Floor Geoscience, Atmosphere and Ocean Research Institute, the University of Tokyo, 4.Institute of Life and Evironmental Sciecnces, University of Tsukuba, 5.Tokyo University of Marine Science and Technology, 6.Fukui Prefectural University)

Keywords:Yezo Group, Aptian, Albian, Cretaceous

The Cretaceous period is known as the hottest phase in Earth's history. In particular, the Aptian-Albian interval was the most active phase of tectonics. There is also evidence of drastic perturbations of environment and ecosystem during the period. It is important to know the mid-Cretaceous period for understanding Earth's responses to the carbon cycle and for assessing and predicting future impacts of global warming.
Several Ocean Anoxic Events (OAEs) have long been identified from the Upper Aptian to the Lower Albian interval. The Fallot Level is originally named for the interval intercalating six black shales in the Marnes Bleues Formation in the south-eastern France. OAE1b is known collectively as an interval intercalating frequent black shales (named Jacob, Kilian, Paquier and Leenhardt from the lower levels for the main black shales in the Vocontian Basin) in uppermost Aptian to lower Albian. Since few studies have been performed for the Fallot Event and OAE1b than the OAE1a and OAE2, further research is awaited to elucidate the scale and causal relationships of these events. These events have been studied in detail in Tethys where the lower Cretaceous strata is continuously exposed. On the other hand, in the strata accumulated in the Pacific Ocean, which was the largest ocean during Cretaceous, Fallot and OAE1b levels have not been identified because of scarcity of age-diagnostic fossils. Based on osmium and carbon isotopic stratigraphy of the Lower Cretaceous sequence in Hokkaido, Japan, here we have identified the first Fallot and OAE1b levels in the Northwest Pacific.
A continuous sequence of Aptian-Albian crops out in the Koyanosawa section, central Hokkaido. The sequence, belonging to the lower part of the Yezo Group, consists mainly of turbidite sandstone and hemipelagic mudstone accompanied with a huge oliststrome bed and frequent intercalations of felsic tuff, and is named the Shuparogawa Formation, Maruyama Formation and Hikagenosawa Formation in ascending order. The Shuparogawa Formation is subdivided into the Refureppu Sandstone Member, Kirigishiyama Olistostrome Member and Okusakainosawa Sandstone and Mudstone Member in ascending order. Planktic foraminiferal fossils indicate that the Refureppu Sandstone Member is Aptian while the other two members are lower Albian. In this study, osmium isotope was analyzed for 650 m stratigraphic interval which corresponds to whole Shuparogawa Formation at stratigraphic intervals of a few meters to several tens of meters. Our osmium isotope ratio (Osi) shows two pronounced decline about 0.1 in the lower and upper part of the Shuparogawa Formation, respectively. The lower decline, occurs at the lower part of the Refureppu Sandstone Member, is coincided with a 1‰ negative shift in δ¹³C curve. Given osmium, carbon, radiolaria and planktic foraminiferal biostratigraphies, the horizon is correlated with the Fallot Level in the Tethys Sea area. Another horizon showing decrease in osmium isotope ratios is observed at the basal part of the Okusakainosawa Sandstone and Mudstone Member. Radiolarian turnover has been reported at the corresponding horizon, and zircon U-Pb radiometric age of tuff, intercalated several-meters above the horizon, dates 110.0±0.60 Ma. These lines of evidence as well as planktic foraminiferal biostratigraphy exhibit that the horizon is correlative with the Leenhardt level of the OAE1b. Distinct decrease in osmium isotope ratio have been recognized at the Jacob and Killian levels in the OAE1b in the Italian section, while our result did not identify the corresponding decline in the studied sequence. Two possibilities can be given for this discrepancy. One is because the resolution of the osmium isotopic stratigraphy is insufficient to capture these short-duration (<40 000 years) event, and another is due to the lack of Jacob-Paquier interval on account of erosion and/or hiatus accompanied with deposition of olistostrome.