[B-CG06] Decoding the history of Earth: From Hadean to the present

[BCG06-15] Carbon isotopic perturbations and their international stratigraphic correlation of the Lower Cretaceous in NE Japan

*Taiga Tomaru¹, Reishi Takashima², Yuji Orihashi³, Toshiro Yamanaka⁴, Hisao Ando^5,6, Yoshihiro Asahara⁷, Azumi Kuroyanagi², Hiroshi Nishi⁶ (1.Department of Earth Science, Graduate School of Science, Tohoku University, 2.The Center for Academic Resources and Archives Tohoku University Museum , 3.Global Environment and Disaster Prevention Sciences, Graduate School of Science and Technology, Hirosaki University, 4. School of Marine Resource and Environment, Tokyo University of Marine Science and Technology, 5.Department of Earth Science, Graduate School of Science and Engineering, Ibaraki University, 6.Department of Dinosaur Paleontology and Geology, Faculty of Dinosaur Paleontology, Fukui Prefectural University, 7.Department of Earth and Environmental Sciences, Graduate School of Environmental Studies, Nagoya University)

Keywords:Lower Cretaceous, Somanakamura Group, Karakuwa Group, carbon isotope stratigraphy, U–Pb radiometric age

Stable carbon isotopic ratios on the surface of the Earth have fluctuated throughout the Phanerozoic, and global carbon isotopic perturbations were recorded for both organic and inorganic carbon (Kump & Arther, 1999; Cramer & Jarvis, 2020). Carbon isotopic perturbations during the Early Cretaceous Berriasian to Hauterivian (145–126 Ma) were affected by, for example, the emplacement of Large igneous provinces (LIPs) and the intensity changes of the monsoon cycle (Gomes & Vasconcelos, 2021; Martinez et al., 2023). Among δ¹³C fluctuations of this interval, the most significant one is called the Weissert Event (Weissert et al., 1998; Erba et al., 2004).
The Weissert Event (134.6–132.3 Ma; Martinez et al., 2023) occurred during the Late Valanginian, and this event is marked by a significant positive δ¹³C excursion (2.0–4.0‰). It is considered that this event has been induced by the emplacement of the Paraná–Etendeka LIPs, but its mechanism and implications for the environment are still under debate. In addition, carbon isotopic perturbations in the Panthalassa Ocean, including Japan, are barely known, and it is also unclear what environmental changes occurred in the Panthalassa during this interval. On the other hand, the continuous successions of the Lower Cretaceous are exposed in the Tohoku area and there is a possibility to reconstruct carbon isotopic fluctuations, including the Weissert Event, in the Panthalassa area.
Therefore, in order to elucidate carbon isotopic perturbations, their numerical ages, and paleoenvironmental changes in NW Panthalassa during Barreasian to Hauterivian, we conducted bulk organic carbon isotopic analysis, U–Pb radiometric dating of tuff, TOC, and DOP analysis for the Koyamada Formation, the Somanakamura Group, and the Kogoshio to Isokusa Formations, the Karakuwa Group.
As the results, a significant positive δ¹³C excursion (~ 2.5 ‰) was confirmed from the middle to the upper part of the Koyamada Formation, and zircon U–Pb ages of several interbedded felsic tuffs exhibited Valanginian. Whereas a significant positive δ¹³C excursion (about 1.5 – 2.0 ‰) was also confirmed in the middle part of the Isokusa Formation, zircon U–Pb ages from the lower part of this formation indicated Early Valanginian, and that from the upper part exhibited Hauterivian.
Thereafter, we demonstrated carbon isotopic correlation between the δ¹³C curves of this study and that of the representative Lower Cretaceous in Europe. The δ¹³C curves of this study align well with those of the Valanginian–Hauterivian GSSP section during the Weissert Event interval. In addition, there is a possibility the δ¹³C curves of the Isogusa Formation record δ¹³C fluctuations around the Berriasian/Valanginian boundary and the Valanginian/Hauterivian boundary. Furthermore, the U–Pb radiometric ages of this study were accorded well with the latest numerical age model (Martinez et al., 2023).
Moreover, chemical analyses of the Koyamada Formation indicated the oxic condition was dominant during the Weissert Event, but this condition fluctuated into dysoxic conditions during some intervals.

References
Erba et al., 2004, Geology, 32.2, 149–152.
Martinez et al, 2023, Earth–Science Reviews, 104356.
Percival et al., 2023, Geology, 51.8, 753–757.