[M-IS14] 古気候・古海洋変動

[MIS14-P23] 東インド洋深海底堆積物 900万年間の放散虫生層序と 800万年前のコズミックシャワーイベント

小島 陸¹、*岡崎 裕典¹、尾上 哲治¹ (1.九州大学大学院理学研究院地球惑星科学部門)

Biostratigraphy divides geologic strata into biozones defined based on the contained fossils, which enables stratigraphic correlation between geographically distant localities. Radiolarians are zooplankton with biogenic opal or strontium sulfate skeletons distributed in the world ocean from surface to deep water. Because of their high evolution rates, radiolarians are valid for constructing biostratigraphy. 3He flux increased temporarily at ~8.2 Ma in the Indian and Pacific oceans without any known crater. They suggest a cosmic shower event by extraterrestrial particles from asteroids breaking up and falling to the Earth. This study used sediment samples from Site 213 (10°12.71’S, 93°53.77’E, water depth 5609 m, penetration 172.5 m) in the eastern tropical Indian Ocean, drilled during Leg 22, Deep Sea Drilling Project (DSDP) in 1972. Core samples at Site 213 are composed of radiolarian ooze from late Miocene to Quaternary for the upper 75 m (Core 1R-8R) and pelagic clay in the middle Miocene for 75-135 m (Core 8R-15R). Site 213 is suitable for investigating extraterrestrial particles and radiolarian skeletons. Thus, we aim to establish radiolarian biostratigraphy since the late Miocene at Site 213 samples upper 0-95 m (Core1R-10R) and tried to detect cosmic spherules and spinels. We prepared 158 slides for light microscopic observations of radiolarian skeletons to establish radiolarian stratigraphy at Site 213. Each radiolarian bio-event was compared with magnetostratigraphy at Ocean Drilling Program (ODP) Hole 710A and calibrated with the ages in Geologic Time Scale 2020. We determined seven radiolarian fossil zone boundaries from the late Miocene to the Pleistocene and Cores 1R to 10R of Site 213 samples covering the last 9 Myrs. We also picked up magnetic grains under stereo microscope and performed SEM-EDX analysis to detect cosmic spherules and spinels. As a result, we found four cosmic spherules with Cr and Ni peaks and 197 spinels from Site 213 Core 9R3W-30.0-32.0 cm. A few spinels were also found from 9R3W60.0-62.0 cm and 9R3W120.0-122.0 cm. Based on radiolarian biostratigraphy, the age of Core 9R3W-30.0-32.0 cm is ~8.3 Ma. These are the first physical evidence for the pronounced extraterrestrial materials inflow event. Although we cannot determine the origin of the cosmic spherules and spinels, possible causes are asteroids' breakup and meteorite impact.