[M-IS23] [JJ] 古気候・古海洋変動

[MIS23-29] 浮遊性有孔虫飼育実験を基にした白亜紀無酸素事変時の海洋酸素環境推定

*黒柳 あずみ^1,4、豊福 高志²、長井 裕季子^2,3、木元 克典²、西 弘嗣¹、高嶋 礼詩¹、川幡 穂高⁴ (1.東北大学学術資源研究公開センター東北大学総合学術博物館、2.独立行政法人海洋研究開発機構、3.横浜国立大学、4.東京大学大気海洋研究所)

キーワード：無酸素事変、硫化水素、浮遊性有孔虫

The oceanic redox state is a critical determinant of the evolutionary history of life on Earth, and “anoxic events” have been proposed as one of the causal mechanisms for mass extinctions. During the mid-Cretaceous, oceanic anoxic events (OAEs) occurred several times with substantial turnover of planktonic foraminiferal species. However, the direct effects of the anoxic condition on planktonic foraminifera remain obscure. In this study, we cultured 6 species (n = 31) in all at three treatments: ~2 mg hydrogen sulfide (H₂S) L^-1 (H1 treatments), ~9 mg H₂S L^-1 (H2 treatments), and control (without H₂S). All planktonic foraminifera could not survive more than 48 hours. Furthermore, gametogenesis ratio of each H₂S treatments showed considerable low value (8% and 17%), and time to gametogenesis was also very short (less than one day) under H₂S occurrence. It revealed that foraminiferal biological response of anoxic with the presence of H₂S should be fundamentally different from that of the dysoxic (i.e., low dissolved oxygen; ~0.7 mg O₂ L^-1 or ~22 µmol O₂ L^-1). Our results also proposed the species-specific tolerance for H₂S and that if harmful influence of H₂S restricted in relatively short time (i.e., less than 24 hours) such as tidal cycle, some foraminifera (e.g., Neogloboquadrina dutertrei) might have the potential to survive even under the episodic/temporary occurrence of H₂S. Complete disappearance of planktonic foraminifera at Cretaceous OAE2 could result from the photic-zone euxinia (free H₂S), and presence/absence record of planktonic foraminifera could contribute to examine the detailed oceanic redox state in the photic zone around anoxic events.