浮游有孔虫钕同位素富集的控制因素及在重建洋流演化中的应用

Neodymium (Nd) isotopic composition extracted from the planktonic foraminifera has been widely used for reconstructing past deep ocean circulation changes, however, the origin of preserved Nd signals in planktonic foraminiferal tests and associated authigenic phases is an open debated issue. To better understand the exact mechanism that how Nd preserved in planktonic foraminifera within the water column where they settle through and the sediment where they were buried, in this study, we plan to carry out investigation of spatial Nd distribution of planktonic foraminifera which are obtained from the sediment traps, plankton tows and the marine sediment cores (include core-top and downcore samples) of the SCS. We will first determine the microscale element distribution in various phases of planktonic foraminifera. Analysis of elemental concentration and Nd isotopic composition will be performed on chemically cleaned planktonic foraminifera, the cleaning solution and uncleaned planktonic foraminifera as well. Combined the result from this study and the Nd isotopic composition distribution of modern seawater and benthic foraminifera of the SCS, we are seeking to better understand the possible mobility of Nd ions during changing redox conditions and to better understand that the possibility of using Nd isotope signal associated with planktonic foraminifera to trace past deep water source.

浮游有孔虫中的钕同位素近年来被广泛用于底层洋流演化的重建，然而对于浮游有孔虫中的钕同位素是否真实可靠的记录底层海水的钕同位素信号这一问题仍有争议。本项研究旨在通过对南海北部沉积物捕获器、浮游拖网中的现代浮游有孔虫样品、南海岩芯顶端以及岩芯中一定埋藏深度沉积物中的浮游有孔虫样品运用不同方法处理，测试浮游有孔虫碳酸钙壳体、铁锰氧化物覆层以及有机质覆层中的主微量元素分布以及钕同位素比值，结合南海北部现代海水Nd同位素分布以及底栖有孔虫Nd同位素比值特征，以期获得浮游有孔虫不同组份中的元素浓度分布图，了解影响浮游有孔虫吸收元素Nd的控制因素，以期回答浮游有孔虫Nd同位素比值在沉降至海底的过程中是否变化以及建立从浮游有孔虫中有效提取海水Nd同位素信息的方法。

浮游有孔虫钕同位素是古洋流示踪研究的重要替代性指标之一，然而目前学术界关于浮游有孔虫如何富集钕同位素的认识尚未明确。本项目通过对南海北部沉积物捕集器中的现生有孔虫和沉积物中的有孔虫开展元素地球化学特征和钕同位素的研究，获得了现生有孔虫主微量元素含量随时间和空间改变所发生的变化，同时还获得地质记录中有孔虫主微量元素含量。研究发现现生有孔虫中稀土元素含量随着深度的增加而逐渐增加，并且在冬季时达到峰值；沉积物有孔虫中的稀土元素含量明显高于现生有孔虫中的稀土元素含量。研究初步探讨了浮游有孔虫是如何获取海水稀土元素，以及浮游有孔虫钕同位素的古海洋学意义。项目还研究了末次冰期以来西太平洋海水钕同位素的变化，结果显示西太平洋海水钕同位素不仅受洋流变化的影响，还受到陆源碎屑物质的影响。. 总体来说，本项目研究获得的现生有孔虫稀土元素数据为了解有孔虫富集海水稀土元素的机制提供了重要的基础资料，为将来运用有孔虫钕同位素示踪研究古洋流演化提供了科学依据。结合沉积物有孔虫钕同位素研究结果，说明有孔虫记录的海水钕同位素来源比较复杂，能否反映洋流变化仍然具有不确定性，未来仍然需要进一步研究。