| The Earth’s oxygen content and its change play crucial roles on life evolution.They remain open questions for decades. Sedimentary rocks contain large amount of redox information, which can be used for reflecting the Earth’s oxygen level. Previous studies have developed many geochemical proxies on resolving this problem, such as isotopes, trace elements and organic geochemistries. However, most of these proxies are based on data from shales, thus they have shortages of reflecting the minor change of oxygen in the early Earth. This problem can be resolved by using carbonates which were deposited under shallow waters.This study collected carbonate samples with different age. Which were then dissolved by HCl and the carbonate-bound As (CBAs) and Fe (CBFe) were determined. The depositional redox condition and the oxygen change trendy were investigated by analyzing the relationships between CBAs and CBFe, in combination with the basic prospective that the positive correlation between CBAs and CBFe indicates oxic deposition condition..The results showed that for carbonated aged 551-520 Ma, there is no any correlation between CBAs and CBFe, suggesting a relative anoxic deposition condition. While for carbonates those were deposited after 520 Ma, CBAs and CBFe are positively correlated, suggesting a relative oxic deposition condition. Furhtermore, it is found that the atmospheric oxygen level was low both during the age 1800-1500 Ma and 1200-800 Ma, while relatively high during 1500-1200 Ma. During 1200-800 oxygen was risen occasionally. Before 1500 Ma, the oceans were characterized with high As concentration.This study demonstrates that the relationship between CBAs and CBFe can be used for reflecting the redox change of shallow deposition environment under the conditionof low atmospheric oxygen level; there has been an obvious atmospheric oxygen rise during 1500-1200 Ma; the rise of oxygen and high ocean As concentration during 1500-1200 Ma may have deeply affected the evolution of life. |
PDF Full Text Request