[BCG10-P07] 放射性起源及び安定Sr同位体比から見積もるエディアカラ紀とカンブリア紀の大陸風化量
キーワード：安定ストロンチウム同位体比、エディアカラ紀、大陸風化
One of the most important biological evolution is the diversification of multi-cellular animals, and it occurred during the Ediacaran and the early Cambrian. Many researchers have a vague idea that life and Earth has co-evolved each other. The weathering influx from continents is thought to have a major influence on the change in seawater composition, and accordingly on biological evolution. The weathering influx can be estimated from the radiogenic Sr isotope ratio of carbonate rocks. The 87Sr/86Sr composition of seawater mainly depends on a mixing rate between non-radiogenic strontium derived from hydrothermal circulation occurring in oceanic crust and radiogenic input from continental weathering. Because of the large isotopic difference between them, the 87Sr/86Sr composition of seawater tracks the long-term changes in the continental weathering relative to the hydrothermal influx. Previous geochemical works indicated that some positive 87Sr/86Sr shifts are observed in Ediacaran and early Cambrian carbonate rocks. Increases of the 87Sr/86Sr ratios themselves, however, are induced by not only high weathering rate but also inactive hydrothermal circulation. Therefore, in view of stable Sr isotope ratios (δ88/86Sr), I tried to quantitatively estimate the continental weathering rate in the Ediacaran and early Cambrian.
The stable Sr isotope composition of carbonate rock changes mainly depending on δ88/86Sr value of seawater and isotope fractionation during precipitation of carbonate minerals. Enhanced input of Sr reduces seawater δ88/86Sr value close to input value, and accumulation of Sr in seawater leads a large isotope fractionation. As a consequence, in the current oceanic environment, both processes results in low δ88/86Sr value of carbonate rock. In the fossiliferous succession in South China, four positive 87Sr/86Sr shifts are seen in carbonate rocks deposited during the Gaskiers glaciation, the Shuram excursion, the earliest Cambrian, and the late Terreneuvian (e.g., Sawaki et al., 2008, 2010). The interval of the earliest Cambrian exhibits the largest 87Sr/86Sr shift within them, but is not accompanied with low δ88/86Sr values. This suggests that the positive 87Sr/86Sr shift during the earliest Cambrian possibly resulted from inactive hydrothermal circulation at oceanic crust or reflected a local event. In contrast, intervals of the Shuram excursion and the late Terreneuvian show negative shifts in stable Sr isotope ratios together with positive shifts in 87Sr/86Sr ratios. This suggests that continental weathering rates during these two periods were absolutely enhanced. The degrees of negative carbon isotope excursions of these two periods are greater than those of the other excursions in the studied section, which is likely attributed to the high continental weathering rates.
The stable Sr isotope composition of carbonate rock changes mainly depending on δ88/86Sr value of seawater and isotope fractionation during precipitation of carbonate minerals. Enhanced input of Sr reduces seawater δ88/86Sr value close to input value, and accumulation of Sr in seawater leads a large isotope fractionation. As a consequence, in the current oceanic environment, both processes results in low δ88/86Sr value of carbonate rock. In the fossiliferous succession in South China, four positive 87Sr/86Sr shifts are seen in carbonate rocks deposited during the Gaskiers glaciation, the Shuram excursion, the earliest Cambrian, and the late Terreneuvian (e.g., Sawaki et al., 2008, 2010). The interval of the earliest Cambrian exhibits the largest 87Sr/86Sr shift within them, but is not accompanied with low δ88/86Sr values. This suggests that the positive 87Sr/86Sr shift during the earliest Cambrian possibly resulted from inactive hydrothermal circulation at oceanic crust or reflected a local event. In contrast, intervals of the Shuram excursion and the late Terreneuvian show negative shifts in stable Sr isotope ratios together with positive shifts in 87Sr/86Sr ratios. This suggests that continental weathering rates during these two periods were absolutely enhanced. The degrees of negative carbon isotope excursions of these two periods are greater than those of the other excursions in the studied section, which is likely attributed to the high continental weathering rates.