碳酸盐岩中锰与铈分布相关性对沉积氧化还原条件的指示

Distribution relationship between Mn and Ce in carbonates as an indicator on sedimentary redox condition

浅水碳酸盐岩中包含大量指示沉积环境氧化还原条件的信息。文章利用基于同步辐射的X射线荧光微区元素面扫描(micro X-ray fluorescence spectroscopy,μXRF)及激光剥蚀电感耦合等离子质谱(laser ablation inductively coupled plasma mass spectroscopy,LA-ICP-MS)方法,对中二叠(瓜德鲁普)统茅口组生物灭绝层和非生物灭绝层碳酸盐岩中的微量元素进行了分析。μXRF分析结果显示,在非生物灭绝层Mn与Ce的微区分布显著相关,而在生物灭绝层两者的分布没有相关性;LA-ICP-MS分析结果显示,在非生物灭绝层Ce与Cr、Mn、Mo正相关,在生物灭绝层不相关。研究结果印证了前人关于"非生物灭绝期浅海富氧,生物灭绝期浅海缺氧"的论断。在氧化性海水中,锰氧化物通过氧化和吸附富集Ce、Cr及Mo,致使元素的微区分布具有相关性;在还原性海水中,锰氧化物还原分解,元素不再局部富集。研究发现,碳酸盐岩中Mn、Ce微区分布相关性可作为示踪沉积环境氧化还原条件的有效指标。

Shallow water carbonates contain massive information indicating sedimentary redox condition. In this study, trace elements in carbonates of mid-Permian Maokou Formation, with special regard to the mass extinction layer, were analyzed using synchrotron radiation-based micro X-ray fluorescence spectroscopy(μXRF) and laser ablation inductively coupled plasma mass spectroscopy(LA-ICP-MS) techniques. The μXRF results showed obvious positive relationship of the distribution of Mn and Ce in carbonate from non-extinction layer, while no such relationship was observed in carbonate from mass extinction layer. The LA-ICP-MS results showed positive relationships between Ce and other trace elements(Cr, Mn and Mo) in carbonate from non-extinction layer, while such relationships were not observed in carbonate from mass extinction layer. Our results verified the former inference that the global shallow oceans were oxic and anoxic during the non-extinction and extinction periods, respectively. Under oxic condition, Mn-oxides accumulate Ce, Cr and Mo through oxidation and adsorption;while under anoxic condition, such processes are not presented due to the reductive dissolution of Mn-oxides. It is found that the distribution relationship between Mn and Ce is an effective proxy for sedimentary redox condition.