铀同位素示踪早奥陶世特马豆克期海洋氧化还原状态演化  

Evolution of Oceanic Redox State during Early Ordovician Tremadocian Age Traced by Uranium Isotopes

作　　者：闵思雨 邱晨 栾晓聪 吴荣昌 邵庆丰 陈曦[1] 凌洪飞[1] MIN Siyu;QIU Chen;LUAN Xiaocong;WU Rongchang;SHAO Qingfeng;CHEN Xi;LING Hongfei(State Key Laboratory for Mineral Deposits Research,School of Earth Sciences and Engineering,Nanjing University,Nanjing 210023,China;Skate Key Laboratory of Palaeobiology and Stratigraphy,Nanjing Institute of Geology and Palaeontology,Chinese Academy of Sciences Center for Excellence in Life and Paleoenvironment,Nanjing 210008,China;Key Laboratory of Virtual Geographic Environment,Nanjing Normal University,Nanjing 210023,China)

机构地区：[1]内生金属矿床成矿机制研究国家重点实验室,南京大学地球科学与工程学院,南京210023 [2]中国科学院南京地质古生物研究所,现代古生物学和地层学国家重点实验室,中国科学院生物演化与环境卓越创新中心,南京210008 [3]虚拟地理环境教育部重点实验室,南京师范大学,南京210023

出　　处：《高校地质学报》2023年第2期147-160,共14页Geological Journal of China Universities

基　　金：国家自然科学基金(41603006);中国科学院战略性先导科技专项(B类)(XDB26000000)联合资助。

摘　　要：古海洋的氧化还原状态是影响动物演化的重要环境因素之一。奥陶纪特马豆克期处于奥陶纪生物大辐射爆发的前夕,该时期生物多样化水平已经逐渐升高,但前人对该时期海洋氧化还原状态演化仍缺乏深入研究。保存良好的碳酸盐岩地层中的铀同位素能够有效反映全球海洋整体的氧化还原水平。文章对湖北松滋响水洞剖面碳酸盐岩进行了系统的碳、氧同位素和铀同位素分析,结果显示在下奥陶统特马豆克阶底部出现一次显著的碳同位素正漂移事件—TSICE,同时δ^(238)U均值从-0.65‰±0.12‰增加到-0.26‰±0.15‰,表明该时期海洋经历了从缺氧(铀同位素质量平衡模型计算表明,全球海洋至少被2.56%的缺氧水体覆盖)到氧化程度增高的过程。这一环境变化过程与生物礁组成中的后生动物成分相对于微生物成分明显升高的过程相吻合。发生碳、铀同位素正漂移可能分别与海平面下降及海水表层温度降低而有利于海洋氧化水平提高相关。The oceanic redox state is one of the important environmental factors affecting evolution of animals in the ancient ocean.The Ordovician Tremadocian Stage was the‘eve’of the Great Ordovician Biodiversification Event(GOBE),during which the biodiversity was gradually increasing.However,there has been lack of researches on evolution of ocean redox state for this key interval.Studies have proved that the U isotope compositions recorded in well-preserved carbonates can effectively trace the redox state of the global ocean redox state.In this study,we conduct systematical analyses of carbon and oxygen isotope compositions and uranium isotope compositions of carbonate samples from the Xiangshuidong section in Songzi,Hubei Province.A significant positive carbon isotope excursion event—TSICE is identified at the base of the middle Tremadocian stage.The average value of δ^(238)U increased from -0.64‰±0.12‰ to -0.26‰±0.15‰across the TSICE,which indicates that the redox state of the global ocean changed from significantly anoxic(mass balance model results indicating>2.56% of the seafloor covered by anoxic seawater)to much more oxygenated before and after the TSICE.This process of environmental change is consistent with the process of increase in metazoan proportion relative to microorganism proportion in reef composition.The positive drift of carbon and uranium isotope compositions may have been related to the regression and sea-surface temperature decrease,respectively,which may have been conducive to oceanic oxidation.

关 键 词：铀同位素 氧化还原 早奥陶特世 特马豆克期 海洋氧化 

分 类 号：P619.4[天文地球—矿床学] S124.2[天文地球—地质学]