北冰洋门捷列夫海岭富锰棕色层稀土元素组成特征与来源初步分析

Composition and provenance analysis of rare-earth elements in the manganese-rich brown layers of the Mendeleev Ridge,Arctic Ocean

北冰洋深海广泛分布的富锰棕色沉积层(棕色层)是海冰、洋流、物源供给等多种因素共同作用的结果,本文基于西北冰洋门捷列夫海岭ARC07-E25岩芯沉积物稀土元素与微量元素含量、颜色反射率参数、粗颗粒组分和无机碳含量的变化特征,对该类沉积层中稀土元素的组成特征、形成机制和物质来源进行了综合分析。结果显示,沉积物稀土元素总含量(ΣREE)在122.37×10^−6~231.94×10^−6之间变化,北美页岩标准化配分模式显示出轻微的中稀土(MREE)富集以及由La、Ce、Nd主导的较强的轻稀土(LREE)优势。沉积物中ΣREE随着粗组分颗粒(如冰筏碎屑)的增多而呈现降低趋势,表明门捷列夫海脊沉积物中的稀土元素主要富集在细粒沉积物中。根据ΣREE在沉积物中的变化特征将E25岩芯沉积物划分为4种地层,反映出了冰期/间冰期的气候转变过程中温暖条件下形成的棕色层与寒冷条件下形成的浅灰绿色沉积层(灰色层)的岩性旋回中稀土元素组成的差异,由于两种沉积层在形成时受到不同的底层水氧化还原环境的控制,导致了铈(Ce)元素在氧化水体中会由Ce3+氧化为Ce4+并发生沉降,而在还原水体中则由Ce4+还原为Ce3+发生溶解,这一特性使LREE含量产生较大波动,进而影响到ΣREE,使之趋于在代表氧化条件的棕色层中升高而在代表还原条件的灰色层中降低。R型因子分析和物质来源判别结果显示,E25岩芯沉积物中稀土元素与亲碎屑元素(Nb、U、Th)有较好的相关性,主要来源于东西伯利亚海和新西伯利亚群岛的近岸侵蚀物质以及勒拿河物质的输入。

The Mn(manganese)-rich brown layers(brown layers)distribute widely in the Arctic Ocean seafloor which controlled by sea ice,surface currents,material supply and so on.In this study,we investigate the provenance and composition feature of brown layers based on changes of contents on rare earth and trace elements,coarse fraction,inorganic carbon and colour reflectance of core ARC07-E25 retrieved from the Mendeleev Ridge,western Arctic Ocean.Total rare earth elements(ΣREE)changes between 122.37×10^−6 to 231.94×10^−6,the north American Shale normalized records of the sediments generally show enrichment in medium rare earth element(MREE)and light rare earth element(LREE)which dominated by lanthanum(La),cerium(Ce)and neodymium(Nd),respectively.With the increase of coarse fraction,such as ice rafted detritus(IRD),the decrease trend ofΣREE indicates REE mainly enrich in fine grain fraction in the Mendeleev Ridge sediments.The differences of REE contents between brown and gray layers,which are classified in the core based on various characteristics ofΣREE,demonstrate distinct Ce anomalies in the brown layer and grey layer associated with different redox conditions:it will be oxidized from Ce3+to Ce4+in oxic brown layers and be reduced from Ce4+to Ce3+in gray layers.This behavior favors the large variation ofΣLREE content and affects theΣREE content in the sediments.Consequently,theΣREE contents are increasing in the brown layers representing oxidation condition and decreasing in the gray layer representing reduction condition.The results of R-type factor analysis show that the REE in sediments have a good correlation with the detritus elements(Nb,U and Th),and mainly derived from inshore erosion of the East Siberian continental shelf,New Siberian Islands and input of the Lena River.