[B-PT27] 原生代末／顕生代生物多様性変遷：絶滅と多様化

[BPT27-06] ペルム紀中期末における陸棚礁の崩壊：北上山地岩井崎石灰岩のSr同位体層序

*可児 智美¹、磯崎 行雄²、林 隆太郎² (1.熊本大学、2.東京大学)

The Iwaizaki Limestone in the South Kitakami Belt, NE Japan, represents shallow-marine shelf carbonates deposited along the Guadalupian (Middle Permian) South China margin. The main part of the limestone is composed of massive fossiliferous limestone (ca. 100 m) of reef complex, while the upper part (ca. 35 m thick) consists of interbedded limestone and mudstone. This facies transition corresponds to the onset of reef collapse, and the disappearance of tropical fauna. The main part and the lowermost upper part yielded Capitanian (Upper Guadalupian) fusulines but the rest of the upper limestone is barren of index fossils. As the overlying black mudstone yields Late Permian (Lopingian) ammonoids, the extinction-related G-L boundary exists somewhere within the uppermost limestone or in much higher mudstones. In order to constrain the age of the fossil-free uppermost limestone, we analyzed ⁸⁷Sr/⁸⁶Sr stratigraphy of the Iwaizaki Limestone, and correlated with the global secular trend. The present results confirmed that the topmost Iwaizaki limestone, as well as the main part, has the same low ⁸⁷Sr/⁸⁶Sr ratio of 0.7068-0.7069. The extremely low ⁸⁷Sr/⁸⁶Sr ratio (<0.7069; the lowest values of the Phanerozoic) continued to the topmost bedded limestone in the section. The coeval limestones in the world, including those at Akasaka and Kamura in Japan, recorded a unique interval of extremely low ⁸⁷Sr/⁸⁶Sr ratio (0.7068-0.7070) as called the “Capitanian minimum”. Therefore, the topmost Iwaizaki Limestone no doubt belongs to Capitanian, suggesting that the collapse of reef started clearly before the G-L boundary, probably by the onset of the putative Capitanian cooling.