[M-IS28] Evolution of Pelagic Realm

[MIS28-04] Marine Os isotope record from the uppermost Barremian to the lowest Turonian

*Hironao Matsumoto¹, Rodolfo Coccioni², Fabrizio Frontalini², Kotaro Shirai¹, Junichiro Kuroda¹ (1.Atmosphere and Ocean Research Institute, the University of Tokyo, 2.University of Urbino)

Keywords:osmium, Cretaceous

Mid-Cretaceous is characterized by repeated episodic phases of organic-carbon burial in various oceanographic settings (oceanic anoxic events: OAEs) and following marine biotic crises. Since the radiometric ages of huge basaltic plateaus called large igneous provinces (LIPs) roughly correspond to these environmental perturbations, the massive volcanic eruptions associated with the emplacement of LIPs have been proposed as the trigger of the mid-Cretaceous environmental perturbations. Indeed, the volcanic gases released from these LIPs eruptions could have played an important role to sustain the warm, high-pCO₂ mid-Cretaceous climate. Recent studies on sedimentary Os isotopic records have provided evidence suggesting their linkages. Marine Os isotopic ratios (¹⁸⁷Os/¹⁸⁸Os) reflect the balance between radiogenic Os from the continental crust and unradiogenic Os sources (e.g., hydrothermal activity, weathering of the basaltic plateau, and meteorite impact). During Early Aptian OAE1a and end-Cenomanian OAE2, ¹⁸⁷Os/¹⁸⁸Os of the sedimentary rocks show rapid shifts to unradiogenic values (~0.2). Since radiometric ages of Ontong Java Plateau and Caribbean Plateau correspond to the sedimentary ages of OAE1a and OAE2, respectively, these Os isotopic declines have been interpreted as the massive input of mantle-derived unradiogenic Os through submarine volcanism forming these basaltic plateaus. However, the Os isotopic data during the mid-Cretaceous are limited to the short interval around major Cretaceous OAEs (e.g., OAE1a, OAE1b, and OAE2), which have hampered a long-term reconstruction of the hydrothermal activity. Here, we reconstruct the continuous marine Os isotopic record during the mid-Cretaceous using Tethyan sedimentary records in order to reveal the long-term variation of hydrothermal activity and the unknown massive volcanic activity associated with the formation of LIPs.