Japan Geoscience Union Meeting 2021

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[J] Oral

B (Biogeosciences ) » B-CG Complex & General

[B-CG04] Decoding the history of Earth: From Hadean to the present

Fri. Jun 4, 2021 9:00 AM - 10:30 AM Ch.26 (Zoom Room 26)

convener:Tsuyoshi Komiya(Department of Earth Science & Astronomy Graduate School of Arts and Sciences The University of Tokyo), Yasuhiro Kato(Department of Systems Innovation, Graduate School of Engineering, University of Tokyo), Katsuhiko Suzuki(Submarine Resources Research Center, Japan Agency for Marine-Earth Science and Technology), Kentaro Nakamura(Department of Systems Innovation, School of Engineering, University of Tokyo), Chairperson:Yoshida Satoshi(Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo), Tsuyoshi Komiya(Department of Earth Science & Astronomy Graduate School of Arts and Sciences The University of Tokyo)

9:15 AM - 9:30 AM

[BCG04-02] Discovery of the Miocene ocean impact event

*Tatsuo Nozaki1,2,3,4, Junichiro Ohta2,4, Takaaki Noguchi5, Honami Sato6, Akira Ishikawa7,1, Yutaro Takaya8,1,4, Jun-Ichi Kimura9, Qing Chang9, Kazuhiko Shimada10, Jun-ichiro Ishibashi10, Kazutaka Yasukawa2,4, Katsunori Kimoto11, Koichi Iijima1, Yasuhiro Kato2,4,1 (1.JAMSTEC/SRRC, 2.Univ. of Tokyo, 3.Kobe Univ., 4.Chiba Tech, 5.Kyushu Univ., 6.Univ. of Padova, 7.Tokyo Tech, 8.Waseda Univ., 9.JAMSTEC/VERC, 10.Kyushu Univ, 11.JAMSTEC/ESS)

Keywords:Ocean impact, Miocene, Ejecta layer, NW Pacific, Re-Os isotope, Platinum group element (PGE)

Meteorite impacts have caused catastrophic perturbations to the global environment and mass extinctions throughout the Earth’s history [1-3]. Here, we present petrographic and geochemical evidence of an impact ejecta layer, dating from about 11 Ma, in deep-sea clayey sediment in the Northwest Pacific [4]. This clay layer has high platinum group element (PGE) concentrations and features a conspicuous negative Os isotope anomaly (187Os/188Os as low as ~0.2), indicating an influx of extraterrestrial material. It also contains abundant spherules that include pseudomorphs suggestive of porphyritic olivine as well as spinel grains with euhedral, dendritic and spherical forms and NiO contents as great as 23.3 wt%, consistent with impact ejecta. Osmium isotope stratigraphy suggests the most plausible depositional age of ~11 Ma (Miocene) for this layer, as determined by fitting with the seawater evolution curve. No large impact crater of this age is known on land, even within the relatively large uncertainty range of the relative Os age. Thus, we suggest that an unrecognized ocean impact event in the middle or late Miocene produced the impact ejecta layer of the Northwest Pacific. This unrecognized ocean impact event is possibly related to the latest and enigmatic mass extinction during the middle Miocene (11.6 Ma) [5].



[1] Alvarez et al. (1980) Science, 208, 1095-1108. [2] Schulte et al. (2010) Science, 327, 1214-1218. [3] Lowery et al. (2018) Nature, 558, 288-291. [4] Nozaki et al. (2019) Sci. Rep., 9, 16111. [5] Raup and Sepkoski (1986) Science, 231, 833-836.