Assessment of seawater Nd isotope signatures extracted from foraminiferal shells and authigenic phases of Gulf of Guinea sediments
Assessment of seawater Nd isotope signatures extracted from foraminiferal shells and authigenic phases of Gulf of Guinea sediments
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DOI：
10.1016/j.gca.2013.07.029
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发表时间：
2013-11
影响因子：
5
通讯作者：
S. Kraft;M. Frank;E. Hathorne;S. Weldeab
中科院分区：
文献类型：
--
作者：
S. Kraft;M. Frank;E. Hathorne;S. Weldeab
有孔虫壳的放射性成因钕（Nd）同位素组成为研究过去水团来源和混合的变化提供了强有力的档案。然而，从有孔虫壳中提取的海水Nd同位素比率可能会受到污染物相（如有机物、硅酸盐或锰铁涂层）的影响，这些污染物的去除需要对样品进行严格的多步骤清洗。本文研究了在几内亚湾受河流沉积物输入强烈影响的陆架环境中，通过流动和批量清洗方法从浮游有孔虫中提取海水Nd同位素组成的效率。对还原和氧化清洗的单特异性浮游有孔虫样品以及还原清洗的混合底栖有孔虫进行了Nd同位素分析，并对非还原清洗的单特异性浮游有孔虫样品、脱碳化散装沉积物渗滤液的Fe-Mn涂层以及同一沉积物的残余碎屑分数进行了分析。完全清洗过的有孔虫样品的Al/Ca和Mn/Ca比值表明，批处理法和流式方法的清洗效率没有区别，两种方法获得的Nd同位素组成在误差范围内是相同的。此外，未经还原清洗的有孔虫样品与还原清洗过的有孔虫样品具有相同的Nd同位素组成。在尼日尔河口附近，有孔虫的Nd同位素组成与附近站点的海水Nd同位素组成一致。结合提取的Nd同位素特征和元素钙比，以及稀土元素的分布模式，推测浮游有孔虫Nd同位素特征反映了底水/孔隙水特征。大块脱碳化沉积物渗滤液（Fe-Mn涂层）的同位素组成与该地点的有孔虫数据有显著差异，可能反映了颗粒在附近河流中获得了铁锰/预形成的预形成/铁锰涂层。因此，在这种受河流影响的陆架环境中，应使用有孔虫壳来获得无偏的海底海水特征。
The radiogenic neodymium (Nd) isotope composition of foraminiferal shells provides a powerful archive to investigate past changes in sources and mixing of water masses. However, seawater Nd isotope ratios extracted from foraminiferal shells can be biased by contaminant phases such as organic matter, silicates, or ferromanganese coatings, the removal of which requires rigorous multiple step cleaning of the samples. Here we investigate the efficiency of Flow Through and batch cleaning methods to extract seawater Nd isotope compositions from planktonic foraminifera in a shelf setting in the Gulf of Guinea that is strongly influenced by riverine sediment inputs. Nd isotope analyses of reductively and oxidatively cleaned mono-specific planktonic foraminiferal samples and reductively cleaned mixed benthic foraminifera were complemented by analyses of non-reductively cleaned mono-specific planktonic foraminiferal samples, Fe–Mn coatings of de-carbonated bulk sediment leachates, and the residual detrital fraction of the same sediment.Al/Ca and Mn/Ca ratios of fully cleaned foraminiferal samples reveal indistinguishable levels of cleaning efficiency between the batch and the Flow Through methods and the Nd isotope compositions obtained from application of both methods are identical within error. Furthermore, non-reductively cleaned foraminiferal samples have the same Nd isotope composition as reductively cleaned foraminifera at our study sites. Close to the Niger River mouth the Nd isotope composition of the foraminifera agree with the seawater Nd isotope composition of nearby stations. Based on the combined extracted Nd isotope signatures and element to calcium ratios, as well as rare earth element distribution patterns, we infer that the planktonic foraminiferal Nd isotope signatures reflect bottom water/pore water signatures. The isotopic composition of the bulk de-carbonated sediment leachates (Fe–Mn coatings) differs significantly from the foraminiferal data at this site and probably reflects particles that acquired their ferromanganese/pre-formed pre-formed/ferromanganese coatings in nearby rivers. Therefore, in such river influenced shelf settings foraminiferal shells should be used to obtain unbiased bottom seawater signatures.