[M-IS17] New frontiers in geology

[MIS17-03] Geochemical and isotopic characteristics of the red shale layer deposited during the end-Cenomanian at the Exmouth Plateau, southern Indian Ocean

*Hironao Matsumoto¹, Hitoshi Hasegawa², Kotaro Shirai³, Takashi Hasegawa⁴, Erika Tanaka⁵, Kazutaka Yasukawa⁶, Junichiro Kuroda³ (1.Institute of Life and Evironmental Sciecnces, University of Tsukuba, 2.Department of Global Environmental and Disaster Prevention, Faculty of Science and Technology, Kochi University, 3.Atmosphere and Ocean Research Institute, The University of Tokyo, 4.Faculty of Geosciences and Civil Engineering, Institute of Science and Engineering, Kanazawa University, 5.Marine Core Research Institute, Kochi University, 6.Frontier Research Center for Energy and Resources, School of Engineering, The University of Tokyo)

Keywords:Oceanic Anoxic Event 2, southern high latitude, Plenus Cold Event

The end-Cenomanian Oceanic Anoxic Event 2 (94 Ma) is one of the largest OAEs of the Cretaceous, characterized by widespread deposition of organic-rich sediments under oxygen-depleted conditions. Such organic-rich sediments have been widely reported from the Tethys, Atlantic, and Pacific Oceans. However, while most previous OAE2 studies have focused on sedimentary archives from low latitudes, investigations of southern high-latitude sedimentary records remain limited.
The borehole core from Ocean Drilling Program (ODP) Site 762 (Exmouth Plateau) is known to contain a shale layer that may have been deposited during the OAE2 interval. While OAE2 sedimentary record from other locations predominantly consist of black shale layers deposited under the oxygen-depleted conditions, the corresponding interval at ODP Site 762C comprises red shale layers, suggesting that oceanic conditions at the southern high latitudes may have remained oxidizing even during OAE2. Therefore, understanding this sedimentary record is crucial for assessing the global extent of oxygen depletion during OAE2. However, geochemical and isotopic studies of these red shale layers have been extremely limited. In this study, we revealed the chemical composition of the red shale layer using an XRF core scanner and determined its precise depositional age based on Os and C isotopic stratigraphy. Using these data, we discuss oceanographic changes in the southern high latitudes during OAE2.
XRF-core scanner results show that the red shale layer is characterized by low sulfur concentration and enrichment in iron and manganese oxides, suggesting that this layer was deposited under the oxidizing condition. Additionally, Os and C isotopic records reveal that (1) lower part of the OAE2 is missing due to a hiatus, and (2) the onset of the red shale deposition corresponds to the end of “Plenus Cold Event”, a brief cooling episode during OAE2. From these pieces of evidence, we concluded that surface water downwelling was enhanced at the southern high-latitude during Plenus Cold Event, leading to the hiatus. As the surface water downwelling weakened at the end of the cooling event, red shale deposition started. Because similar hiatuses and red-colored deposits have been reported from multiple high-latitude southern sites, this region may have played a crucial role in surface water downwelling during OAE2.