成冰纪南华盆地海水氧化还原性质的钨稳定同位素限定  

Redox condition of Cryogenian interglacial seawater in Nanhua Basin constrained by stable tungsten isotopes

作　　者：刘孟南 李涛 魏广祎 张飞飞[1] 陈骏[1] 李高军 Mengnan Liu;Tao Li;Guangyi Wei;Feifei Zhang;Jun Chen;Gaojun Li(Frontiers Science Center for Critical Earth Material Cycling,Nanjing University,Nanjing 210023,China;State Key Laboratory of Palaeobiology and Stratigraphy,Nanjing Institute of Geology and Palaeontology,Chinese Academy of Sciences,Nanjing 210008,China)

机构地区：[1]南京大学关键地球物质循环前沿科学中心,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,南京210008

出　　处：《科学通报》2023年第16期2133-2140,共8页Chinese Science Bulletin

基　　金：国家自然科学基金(41991321)资助。

摘　　要：成冰纪是大气和海洋氧化还原状态的关键转折期.重建成冰纪海洋氧化还原历史对解析早期动物演化、氧化还原敏感元素地球化学循环等诸多问题均具有重要意义.本研究针对具有重要全球对比意义和较好研究基础的南华盆地,尝试利用新近开发的钨稳定同位素体系,限定成冰纪间冰期陆架边缘海氧化还原环境.结果表明,南华盆地成冰纪间冰期黑色页岩层位的δ^(186)/^(184)W值明显高于陆源碎屑,指示具有较高δ^(186)/^(184)W值组分的加入.根据现代氧化海洋自生沉积钨稳定同位素组成,本研究认为,较高δ^(186)/^(184)W值端元来自海洋自生沉积组分.由于硫化海水中钨主要以硫代钨酸根形式存在,而硫代钨酸根具有较高的溶解度,硫化底层海水形成的沉积物中难以保存自生钨沉积组分.因此,钨稳定同位素组成表明,南华盆地成冰纪间冰期底层海水至少存在间歇性非硫化状态.结合已发表的铁组分和其他海水环境地球化学指标,本研究认为,南华盆地与当今波罗的海类似,间歇性通风导致底层硫化海水局部氧化,从而形成自生钨沉积.本研究展示了钨稳定同位素限定古海水氧化还原状态的巨大潜力.The redox condition of atmosphere and ocean is essential to understand evolution of the Earth’s system.Cryogenian is a critical period in the Earth’s redox history with massive Fe and Mn depositions and possible emergency of the earliest animals that may be linked to the changes in redox condition of the ocean.The redox condition of the Cryogenian seawater,especially for the seawater in Nahua Basin on the margin of Yangtze Block,however,are still under debates,despite the extensive researches conducted due to its great significance for global synthesis.The debates are mainly related to the black shale layer deposited in the early Cryogenian interglacial that contains deposits of Mn carbonate.The high pyriteassociated Fe fraction in the total reactive Fe and the high reactive Fe fraction in the total Fe of the black shale layer suggest euxinic deep seawater.However,the very heavy Fe isotopic composition of pyrite suggests ferric deep seawater.This work investigates the redox condition of the Cryogenian interglacial seawater in Nanhua Basin using stable tungsten isotopes.The rational is that the formation of highly dissolvable thiotungstate under euxinic deep water would prevent deposition of W from seawater while the accumulation of tungsten can be widely observed under none-euxinic seawater column associated with highly partial reactivity of tungstate.However,determining the authigenic deposition of tungsten from seawater based on proxies such as W/Th ratio can be largely complicated by changes of lithological source and sediment sorting.The stable tungsten isotopic composition may help to resolve the existence of authigenic tungsten because a detrital origin would have a upper continental crust-likeδ^(186)/^(184)W composition while the tungsten deposited from seawater would have a higher-than-UCCδ^(186)/^(184)W approaching that of the mean river water due to the preferential retention of light tungsten isotopes by Fe-Mn oxyhydroxides during weathering.The results show that Th/W ratio andδ^(186)/^(184)W of

关 键 词：成冰纪 钨稳定同位素 海洋氧化还原环境 铁组分 

分 类 号：P736.44[天文地球—海洋地质]