Nd/Ca ratios in plankton-towed and core top foraminifera

Planktic foraminifera have been used as recorders of the neodymium (Nd) isotopic composition of seawater, although there is still controversy over the precise provenance of the Nd signal. We present an extensive, multispecific plankton tow Nd/Ca data set from several geographic locations (SE Atlantic, NE Atlantic, Norwegian Sea, and western Mediterranean), together with core top samples from the Mediterranean region. The range of Nd/Ca ratios in plankton-towed foraminifera, cleaned only of organic material, from all regions (0.01-0.7 µmol/mol), is similar to previously published analyses of sedimentary foraminifera cleaned using both oxidative and reductive steps, with distribution coefficients (Kd) ranging between 4 and 302. For the Mediterranean, where core top and plankton tow data are both available, the range for plankton tows (0.05-0.7 µmol/mol) is essentially identical to that for the core tops (0.1-0.5 µmol/mol). Readsorption of Nd during cleaning is ruled out by the fact that the plankton tow samples underwent only an oxidative cleaning process. We find a relationship between manganese (Mn) and Nd in plankton tow samples that is mirrored by a similar correlation in core top samples. This relationship suggests that Fe-Mn coatings are of negligible importance to the Nd budgets of foraminifera as the Nd/Mn ratio it implies is over an order of magnitude greater than that seen in other Fe-Mn oxide phases. Rather, since both plankton tows and core tops present a similar behavior, the Nd/Mn relationship must originate in the upper water column. The data are consistent with the acquisition of Nd and Mn from the water column by binding to organic material and the fact that intratest organic material is shielded from both aggressive cleaning and diagenetic processes. Collectively, the results help to explain two abiding puzzles about Nd in sedimentary planktic foraminifera: their high REE contents and the fact that they record a surface water Nd isotopic signal, regardless of the cleaning procedure used.

浮游有孔虫（planktic foraminifera）已被用作海水钕（Nd）同位素组成的记录载体，但关于Nd信号的确切来源仍存在争议。我们呈现了一个广泛的、多物种的浮游拖网Nd/Ca数据集，该数据集来自多个地理位置（东南大西洋、东北大西洋、挪威海及地中海西部），同时还包含地中海区域的岩心顶部样本。所有区域仅去除有机物质的浮游拖网有孔虫的Nd/Ca比值范围（0.01-0.7 µmol/mol），与此前发表的、经氧化和还原步骤清洗的沉积有孔虫分析结果相似，其分配系数（Kd）介于4至302之间。在地中海区域（同时具备岩心顶部和浮游拖网数据），浮游拖网样本的Nd/Ca比值范围（0.05-0.7 µmol/mol）与岩心顶部样本（0.1-0.5 µmol/mol）基本一致。由于浮游拖网样本仅经过氧化清洗处理，因此排除了清洗过程中Nd的再吸附可能性。我们发现浮游拖网样本中锰（Mn）与Nd之间存在关联，而岩心顶部样本中也存在类似的相关性。这种关联表明，铁锰（Fe-Mn）涂层对有孔虫的Nd收支影响可忽略不计，因为其隐含的Nd/Mn比值比其他Fe-Mn氧化物相中的比值高出一个数量级以上。相反，由于浮游拖网和岩心顶部样本表现出相似的行为特征，Nd/Mn的关联必定源自上层水体。这些数据与以下两点相符：一是Nd和Mn通过与有机物质结合从水体中获取；二是有孔虫壳内有机物质不受强力清洗和成岩作用的影响。综合来看，这些结果有助于解释沉积浮游有孔虫中Nd的两个长期存在的谜题：其一为其较高的稀土元素（REE）含量；其二为无论采用何种清洗程序，它们均能记录表层水的Nd同位素信号。