[BCG10-P07] Quantitative estimation of continental weathering rate in the Ediacaran and Cambrian evidenced from radiogenic and stable Sr isotope ratios
Keywords:Stable Sr isotope ratio, The Ediacaran, Continental weathering
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.