[B-CG05] 地球史解読：冥王代から現代まで

[BCG05-05] Oxygen, not sulfate, as the driver of the Ediacaran Shuram excursion

★Invited Papers

*松浦 史宏^1,2、澤木 佑介³、小宮 剛³、牛久保 孝行⁴、清水 健二⁴、上野 雄一郎^2,4 (1.南京大学、2.東京工業大学、3.東京大学、4.海洋研究開発機構)

キーワード：シュラムエクスカージョン、硫黄同位体、エディアカラ紀、酸素

The Ediacaran period (635–541 Ma) is one of the key intervals of the earth history because of the appearance of the fossils of multicellular animal embryo, Ediacaran biota, and first calcifying metazoan. In addition to the appearance of metazoans, the Shuram excursion, the largest global negative δ¹³C_carb excursion in the earth history, has been related to the ocean oxidation in the late Ediacaran. The Shuram excursion has been considered to result from the activity of microbial sulfate reduction due to the increase of the difference of sulfur isotopic composition between sedimentary sulfate (carbonate associated sulfate: CAS) and sulfide (chromium reducible sulfur: CRS). However, recent studies revealed the isotopic composition of bulk pyrite, CRS, is a mix of the isotopic compositions of various origins of pyrite which are formed in water column or sediments at different depth. Thus, quantifying the isotopic composition of primary pyrite which likely retain microbial sulfur isotope fractionation is crucial to reconstruct paleo-oceanic sulfur cycle. If sulfate reducing bacteria caused the Shuram excursion, sulfur isotope fractionation by microbial sulfate reduction increased during the Shuram excursion concomitant with pyrite concentration increase. Here we carried out multiple sulfur isotope analyses of large pyrite grains (>100μm) by fluorination method combining with in-situ δ³⁴S analyses by SIMS in the Member 3 of Ediacaran Doushantuo Formation (Fm) using drilling core collected from Three Gorges area, South China. The method enables to quantify the sulfur isotope fractionation of microbial metabolism. The calculated microbial isotope fractionation in the middle to later part of the Shuram excursion was ³⁴ε = 55.7‰ and ³³λ = 0.5129, which is a typical sign of microbial sulfate reduction. The signature of microbial sulfur isotope fractionation was preserved in the core of the euhedral pyrite grains formed in sediments. Further, our results revealed the microbial sulfur isotope fractionation increased during the Shuram excursion. However, pyrite concentration decreased during the Shuram excursion, which cannot be explained by deficient iron supply. Thus, another factor should be contributed to the Shuram excursion other than microbial sulfate reduction. Previous studies showed that TOC concentration decreased and Ba concentration increased concomitant with the decrease of pyrite concentration during the Shuram excursion, which indicates buildup of oceanic dissolved O₂ during the Shuram excursion. Thus, we propose buildup of oceanic O₂ enhanced decomposition of dissolved organic matter, which decreased organic matter supply to microbial sulfate reduction zone and increased microbial sulfur isotope fractionation. Our new findings shed light on the significance of oxygen during the Shuram excursion, which is critical for considering coevolution of earth and life.