[M-IS13] 遠洋域の進化

[MIS13-05] 美濃帯の遠洋性堆積物中から復元した三畳紀後期ノーリアン中期〜後期における海水中のオスミウム同位体記録

★招待講演

*佐藤 峰南¹、野崎 達生¹、尾上 哲治²、木村 純一¹、常 青¹、高谷 雄太郎³、加藤 泰浩⁴、鈴木 勝彦¹ (1.国立研究開発法人海洋研究開発機構、2.熊本大学、3.早稲田大学、4.東京大学)

キーワード：オスミウム同位体、チャート、三畳紀後期、美濃帯

Instead of a single mass extinction at the Triassic-Jurassic boundary, extinctions in major pelagic groups, such as radiolarians and conodonts, occurred stepwise during the last 15 Myr of the Triassic. Although a marked diversity decline on such pelagic fauna began in the end of Middle Norian, the cause of this extinction has been uncertain. In order to assess the Middle to the Late Norian environmental changes in the Panthalassa Ocean, we examined secular changes in the marine osmium isotope compositions (¹⁸⁷Os/¹⁸⁸Os) recorded in the Triassic bedded chert succession of the Mino Belt, central Japan. The osmium isotope compositions of seawater reflect contributions of the riverine (¹⁸⁷Os/¹⁸⁸Os ≈ 1.4), hydrothermal, and extraterrestrial (¹⁸⁷Os/¹⁸⁸Os ≈ 0.12–0.13) inputs of osmium to the global ocean. Given the distinctive ¹⁸⁷Os/¹⁸⁸Os compositions of these inputs and the relatively short residence time of Os in the ocean (several tens of thousand years), seawater ¹⁸⁷Os/¹⁸⁸Os compositions are highly sensitive to changes in these fluxes. The Os isotope compositions show a gradual increase in ¹⁸⁷Os/¹⁸⁸Os during the Middle Norian. Two negative Os isotope excursions are observed within a ~2 m stratigraphic interval located between the Middle and the Upper Norian. The onset of the first negative Os excursion is roughly located at the base of the Epigondolella bidentata conodont zone in the late Middle Norian. The timing of this Os isotope excursion coincides with both the abrupt increase in Os concentration and low Re/Os ratios, all of which suggest a significant input of extraterrestrial Os into the sediments. The Os isotope compositions exhibit the second negative Os isotope excursion from the initial Os isotope composition of ~0.7 to unradiogenic values of ~0.4 in the early Late Norian. However, it is difficult to specify the cause for the second negative excursion with the dataset acquired in this study. Additional research is needed to clarify the cause for this record in the upper Norian.