[B-CG07] Decoding the history of Earth: From Hadean to the present

[BCG07-P08] Paleoenvironmental changes across the Triassic-Jurassic boundary based on marine Os isotopic records

*Ryota Murai¹, Junichiro Kuroda¹, Kazumasa Mukai², Yuki Okada³, Natsuko Yoshida³, Katsuhiko Suzuki⁴, Rie Hori, S.³ (1.the University of Tokyo, 2.Kumamoto University, 3.Ehime University, 4.Japan Agency for Marine-Earth Science and Technology)

Keywords:Triassic-Jurassic Boundary, Os isotopic record, bedded chert

The Triassic-Jurassic Boundary (~201.4 Ma) is marked by one of the “Big Five” mass extinctions in Phanerozoic. Before this event, the presence of stepwise extinction events in the Late Triassic was indicated (Rigo et al., 2020). The igneous activities of the Central Atlantic Magmatic Provinces, emplaced in the Pangea and related to its breakup, have been recognized as the main trigger of the End-Triassic Mass Extinction. However, their linkages are still poorly understood. To solve this problem, it is necessary to examine the paleoenvironmental changes through the end-Triassic time period.
The marine osmium (Os) isotopic compositions (¹⁸⁷Os/¹⁸⁸Os) vary in response to relative contributions from three sources: upper continental crust (¹⁸⁷Os/¹⁸⁸Os =~1.3), upper mantle (~0.13) and extraterrestrial materials (~0.13). Currently, temporal variation of marine Os isotopic compositions through the Late Triassic have not been well constrained.
In this study, we measured Os and rhenium (Re) concentrations and Os isotopic records of Late Triassic to earliest Jurassic sediments (bedded chert successions) from New Zealand (PakⅤ section, Pakihi Island) and Japan (AY and Miyanoura section, Akehama area, Ehime Prefecture), all of which were deposited in the Panthalassa Ocean, and calculated the initial Os isotopic compositions during deposition. The bedded cherts may have recorded the global changes of Os isotopic compositions because they are pelagic sediments, which are likely to be influenced by few local effects. Subsequently, we correlated the ¹⁸⁷Os/¹⁸⁸Os records obtained in this study with other sites previously reported (e.g., Kuroda et al., 2010; Sato et al., 2021) based on biostratigraphy records shown in previous researches. Based on the correlation, we discuss the fluctuation of global Os isotopic compositions and its paleoenvironmental significance, mainly its connection to extinction events.

References
Kuroda et al. (2010) Geology 38, 1095-1098
Rigo et al. (2020) Earth Sci. Rev. 204, 103180
Sato et al. (2021) Chem. Geol. 586, 120506