[B-CG04] 地球史解読：冥王代から現代まで

[BCG04-02] 中新世海洋隕石衝突イベントの発見

*野崎 達生^1,2,3,4、大田 隼一郎^2,4、野口 高明⁵、佐藤 峰南⁶、石川 晃^7,1、高谷 雄太郎^8,1,4、木村 純一⁹、常 青⁹、島田 和彦¹⁰、石橋 純一郎¹⁰、安川 和孝^2,4、木元 克典¹¹、飯島 耕一¹、加藤 泰浩^2,4,1 (1.JAMSTEC・資源、2.東大・工、3.神戸大・理、4.千葉工大・次世代資源、5.九大・基幹教育、6.パドバ大・理、7.東工大・理、8.早大・理工、9.JAMSTEC・火山、10.九大・理、11.JAMSTEC・地球表層)

キーワード：海洋隕石衝突、中新世、エジェクタ層、北西太平洋、Re-Os同位体、白金族元素

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.