[B-PT27] 原生代末／顕生代生物多様性変遷：絶滅と多様化

[BPT27-08] ペルム紀中期末における超海洋パンサラッサの窒素循環変動

*斎藤 誠史¹、西澤 学¹、上野 雄一郎²、小福田 大輔³、磯崎 行雄³、高井 研¹ (1.海洋研究開発機構、2.東京工業大学、3.東京大学)

To examine the changes in the oceanic N cycle and their possible relationships to the extinction at the end-Guadalupian (Middle Permian), we analyzed the N isotopic compositions (d¹⁵N) of the upper Guadalupian paleo-atoll limestone, accumulated on the top of a mid-oceanic seamount in the superocean Panthalassa, at Akasaka in central Japan. The d¹⁵N values of the limestone are substantially high (ca. +20 permil on average) throughout the analyzed interval. These values are the highest in the previously reported d¹⁵N records throughout the entire Phanerozoic. The substantially high d¹⁵N values suggest enhanced oceanic denitrification/anammox in the Capitanian (Late Guadalupian). Moreover, the present results revealed remarkably large d¹⁵N fluctuations in the analyzed limestone at Akasaka. We interpret that periodic expansion and reduction of the oxygen minimum zone (OMZ) in mid-Panthalassa caused the observed d¹⁵N fluctuations in the Capitanian. The suggested OMZ expansions may have been attributed to the high productivity 'Kamura event' in the surface oceans enhancing a biological pump. Chemostratigraphic correlations imply that the enhanced denitrification in the expanded OMZ may have been a global phenomenon in the Capitanian. Widespread developments of the anoxic deep-waters prior to the extinction may have stressed the shallow-marine biota by upwelling at the end-Guadalupian.