[B-PT27_28PM1] 顕生代生物多様性の変遷：絶滅と多様化

[BPT27-08] ペルム紀中期末におきた顕著な海水準低下とグローバル寒冷化：超海洋中央部海山起源石灰岩の記録

*小福田 大輔¹、磯崎 行雄¹、猪郷 久義²、可児 智美³、石村 豊穂⁴ (1.東京大学総合文化研究科、2.自然史科学研究所、3.熊本大学自然科学研究科、4.産業技術総合研究所)

キーワード：G-L境界, 超海洋起源石灰岩, 海水準低下, 寒冷化, 炭素同位体, Sr同位体

For clarifying the global environmental changes relevant to the Guadalupian-Lopingian boundary (G-LB) extinction, i.e. the first major biodiversity drop during the Permian, litho-, bio-, and chemo- stratigraphy of δ¹³C_carb and ⁸⁷Sr/⁸⁶Sr were analyzed in the Middle-Upper Permian paleo-atoll limestone at Akasaka in central Japan, which was derived from a paleo-atoll complex deposited primarily in the low latitude in the mid-Panthalassa. Between the Capitanian (upper Middle Permian) black limestone (the Yabeina fusuline Zone) and the Wuchiapingian (lower Upper Permian) light gray limestone (the Codonofusiella-Reichelina Zone), a unique black-white striped limestone is intercalated, of which top marks the G-LB horizon. The major extinction occurred in the uppermost black limestone, large-tested fusuline and large bivalve that were adapted to low-latitude extremely warm conditions sharply became extinct. Most parts of the Akasaka Limestone consist of shallow marine wackestone/packstone deposited in low-energy settings of the subtidal zone likely within a lagoon on the top of a seamount. We newly identified 1) a remarkable hiatus with erosional features at the top of the striped limestone, 2) large-scale cross-beddings in the striped limestone immediately below the hiatus, and 3) the dominance of grainstone in the basal light gray limestone immediately above the hiatus. These lines of evidence altogether suggest that a remarkable sea-level drop has occurred around the G-LB in the mid-oceanic paleo-atoll complex, and that a cool climate has appeared in the Capitanian. The isotope stratigraphy for the Capitanian interval with extremely high δ¹³C_carb values over +5 ‰ and the extremely low ⁸⁷Sr/⁸⁶Sr ratios below 0.7070 indicate the high productivity in the superocean and the suppressed continental weathering on Pangea, respectively. Both isotope signatures can be concordantly explained by the appearance of a putative global cooling in the Capitanian. After all, the litho-, bio-, and chemostratigraphical records from the Permian mid-superocean positively suggest a possible link between the Capitanian global cooling and the end-Capitanian extinction.