[M-IS22] [JJ] 地球掘削科学

[MIS22-14] 南太平洋ロードハウライズの白亜紀-古第三紀境界層の層序再検討

*黒田 潤一郎^1,2、斎藤 実篤²、臼井 洋一²、萩野 恭子³、熊 衎昕²、村山 雅史³、安藤 卓人⁴、大河内 直彦² (1.東京大学大気海洋研究所　海洋底科学部門、2.国立研究開発法人 海洋研究開発機構、3.高知大学 海洋コア総合研究センター、4.北海道大学 北極域研究センター)

キーワード：白亜紀-古第三紀(旧成紀)境界、オスミウム同位体、ロードハウライズ

The Lord Howe Rise is an elongate ribbon of submerged and extended continental crust that was separated from Australia during the Late Cretaceous. Deep Sea Drilling Project (DSDP) Leg 21 drilled on the Lord Howe Rise, and recovered Cenozoic and latest Cretaceous pelagic sediments at Sites 207 and 208. In this study we provide new geochemical, biostratigraphic and magnetostratigraphic data for the latest Cretaceous and the Paleocene sediments of the DSDP Site 208 (cores from 21-208-30R to 21-208-34R), to reassess the stratigraphy around the boundary between Cretaceous and Paleogene (K-Pg). The sediments are mainly composed of calcareous nannofossil chalk with an interval of siliceous mudstone and marlstone in the top 83 cm of the Core 21-208-33R, which was previously identified as the K-Pg transition zone. Both paleomagnetic data and calcareous nannofossil assemblages show that the sediment deposited nearly continuously from 550 to 590 meter below seafloor (mbsf), corresponding to ca. 62 through 68 Ma, respectively, with an average sedimentation rate of ~7 m/m.y. However, the sedimentation rate significantly dropped in the K-Pg transition zone, which was attributed to several hiatuses. Because radiogenic isotopic composition of osmium (¹⁸⁷Os/¹⁸⁸Os) of ocean water draws a unique and globally synchronous variation across the K-Pg boundary, it can be used as a stratigraphic correlation tool. Simply, the K-Pg boundary marks a very low ¹⁸⁷Os/¹⁸⁸Os value of <0.2 compared to the Maastrichtian (~0.5-0.6) and Danian (~0.4). Our ¹⁸⁷Os/¹⁸⁸Os data of sediment show a similar variation with the typical pattern of ocean water ¹⁸⁷Os/¹⁸⁸Os values from the Maastrichtian through Danian. More importantly, we found a siliceous mudstone sample with a low ¹⁸⁷Os/¹⁸⁸Os value of ~0.16. We propose that this sample represents the K-Pg boundary. However, paleomagnetic data suggest that this sample belongs to the chron C29N, substantially younger than the K-Pg boundary. In the presentation we will discuss the controversy, and further assess the stratigraphy around the K-Pg boundary.