[B-PT27_28PM1] 顕生代生物多様性の変遷：絶滅と多様化

[BPT27-06] ペルム紀末大量絶滅後の海水モリブデン濃度の減少

*高橋 聡¹、山崎 慎一²、小川 泰正³、木村 和彦⁴、海保 邦夫⁵、吉田 武義⁵、土屋 範芳² (1.東京大学理学系研究科地球惑星科学専攻、2.東北大学環境科学研究科、3.秋田大学工学資源学研究科、4.宮城大学食産業学部、5.東北大学理学研究科地学専攻)

キーワード：モリブデン, 大量絶滅, ペルム紀, 三畳紀, 遠洋域深海相, パンサラッサ

The end-Permian mass extinction was the largest biotic catastrophe of the Phanerozoic, and evidence of global oceanic anoxia during this event has been reported (e.g. Wignall and Twitchett, 1996). Such anoxic?euxinic conditions have also been revealed by enrichments of redox-sensitive elements (Fio et al., 2010; Grasby et al., 2009, 2011; Algeo et al., 2012). Among redox-sensitive elements, uranium increased in sediments and finally result uranium drawdown, suggested by a decrease in sedimentary uranium isotope ratio (238/235U) and a increase in Th/U ratio from the shallow marine carbonates (Brennecka et al., 2011). In this presentation, we will show the possible evidence of Mo drawdown after the mass extinction event from the continuous deep-sea Permian-Triassic boundary section which located in the low latitude pelagic Panthalassa (Akkamori section-2; Takahashi et al., 2009). High resolution ICP-MS analysis using sedimentary rock samples from the study section (Takahashi et al., in review) indicates vertical distribution of UEF and MoEF (Enrichment factor of U and Mo), the Mo/U ratio. MoEF and UEF show a synchronous increase from the Upper Permian bedded chert to the overlying siliceous claystone, while the Mo/U ratio increases from 3.9 to 47.3 showing continuous elevation from the 1.0×modern seawater Mo/U ratio (1.0×SW) to 9.0×the modern ratio (9.0×SW). Accepting the previous study's criteria (Algeo and Tribovillard, 2009), increased Mo/U ratios that clearly exceed 9 (3×times the value of modern seawater [3.0×SW]) suggest the presence sulphidic bottom water at that time. Considering possibility of U drawdown suggested by Brenecka et al. (2011), decrease in seawater U concentration (possibly up to 1/7) would also help the rise of Mo/U ratio. Further elevations of MoEF and the Mo/U ratio reach values of more than 1000, and MoEF reaches values of several thousands from Upper Permian siliceous claystone to the basal 20cm end-Permian black claystone, indicating that sulphidic bottom water was increasingly developed and that Mo transportation by the particulate shuttle was activated. The particulate shuttle, proposed by Algeo and Tribovillard (2009), is a process by which Mn oxyhydroxides absorb molybdate oxyanions above the oxic?anoxic chemocline in the water column and then sink and finally dissolve on at or just below the sediment-water interface, releasing Mo to the sediments. Additionally, in such a developed sulphidic water column, syngenetic pyrite formation in the euxinic water column could possibly have contributed to Mo transportation to the sediment (Algeo and Maynard, 2004). Above the 20 cm horizon of the black claystone, MoEF decreases to values lower than 100 and the Mo/U ratio takes values of more than 3 but less than 20. These values could be interpreted to indicate that sulphidic bottom water was still present but that the particle shuttle had subsided to some extent after the time of the mass extinction. Low Mo/U values occur in earliest Triassic siliceous claystone bed, despite high MoEF and UEF values. Because MoEF and UEF are high, reducing bottom water conditions still existed. Thus, the decrease in Mo/U does not indicate a return to oxic conditions, but rather a Mo drawdown in the earliest Triassic seawater. The study examples of such trace-metal drawdown in geologic past have been reported by Algeo (2004) and Hetzel et al. (2009). In fact, the trend of low Mo/U values with high MoEF and UEF is consistent with that of the Mo-depleted seawater condition identified in the modern Black Sea (Algeo and Tribovillard, 2009), suggesting a drawdown of seawater Mo in the pelagic ocean.