Sm-Nd isotope and Mn/Ca ratios for cleaned Neogloboquadrina pachyderma from ODP Hole 105-647A (Table 1)

The neodymium (Nd) isotope composition of ancient seawater is a potentially useful tracer of changes in continental inputs and ocean circulation on timescales of a few ka. Here we present the first Nd isotope record for seawater using sedimentary foraminifera cleaned using standard oxidative-reductive techniques. The data, along with Mn/Ca ratios, suggest that cleaned foraminifera provide a reliable record of Nd in seawater and hold out the prospect of using Nd in foraminifera to examine changes in seawater that accompany glacial-interglacial climatic cycles. The principal potential problem to be overcome with the use of forams as records of trace elements in ancient seawater is their diagenetic Fe-Mn coatings. These contain large amounts of Nd and other trace elements but can be cleaned off using highly reducing reagents. Mn(Ca ratios for the majority of the cleaned sedimentary foraminifera analysed here lie within the range (10-100 µmol/mol) that has yielded success in studies of transition elements in forams. Mass-balance modelling suggests that for residual Mn/Ca ratios <100 µmol/mol, Nd added to the foram in the coating will never shift the measured Nd isotope composition significantly away from the seawater value acquired by the foram test in the water column. Additionally, Nd concentrations measured in cleaned sedimentary foraminifera are comparable with those for a modern sample that has never encountered diagenetic fluids. Finally, core-top planktonic foraminifera for two sites have Nd isotope compositions that are identical to local surface seawater. The data we present here for Labrador Sea forams over the past 2.5 m.y. are interpreted in terms of changes in the seawater isotopic composition. The data show a pronounced shift from epsilon-Nd values of ~-12 to ~-19 in the period 2.5-1.5 Ma. This change is interpreted to result from the initiation of Northern Hemisphere glaciation and the increased derivation of Labrador Sea Nd via ice-rafting from Archaean terranes in central Canada. In combination with stable isotope and foraminiferal relative species abundance data, the new Nd data are consistent with the surface hydrography of the Labrador Sea being dominated by a fluctuating balance between cold, polar waters containing unradiogenic Nd and warm, subtropical waters containing more radiogenic Nd. The major change in Labrador Sea Nd that is observed in the past 2.5 Ma can, on its own, account for the change in the Nd isotope composition of North Atlantic Deep Water over the same time period.古海水的钕（Nd）同位素组成是一种潜在的有效示踪剂，可用于示踪千年尺度下的陆源输入变化与大洋环流演化。本文首次报道了采用标准氧化还原技术（oxidative-reductive techniques）清洗的沉积有孔虫（sedimentary foraminifera）所获取的海水Nd同位素记录。本次研究获得的数据集与锰钙（Mn/Ca）比值数据共同表明，经清洗后的有孔虫可可靠记录海水Nd同位素信号，为利用有孔虫中的Nd元素探究冰期-间冰期气候旋回（glacial-interglacial climatic cycles）伴随的海水变化提供了可行途径。以有孔虫作为古海水微量元素记录载体的主要潜在问题在于其表面的成岩铁锰包膜（diagenetic Fe-Mn coatings）：这类包膜富含大量Nd及其他微量元素，但可通过强还原剂予以去除。本次分析的多数经清洗沉积有孔虫的Mn/Ca比值处于10~100 μmol/mol区间，该区间已被证实可有效用于有孔虫过渡金属元素研究。质量平衡模型（mass-balance modelling）结果显示，当残余Mn/Ca比值低于100 μmol/mol时，包膜附着的Nd不会显著改变有孔虫壳体在水体中获取的海水Nd同位素组成。此外，经清洗的沉积有孔虫中测得的Nd浓度，与未经历成岩流体作用的现代样品浓度相当。最后，两个站位的表层沉积浮游有孔虫的Nd同位素组成与当地表层海水完全一致。针对过去2.5 Ma的拉布拉多海有孔虫Nd同位素数据，本文结合海水同位素组成变化进行了解译。数据显示，在2.5~1.5 Ma期间，εNd（epsilon-Nd）值从约-12显著偏移至约-19。该变化被认为是北半球冰川作用（Northern Hemisphere glaciation）启动以及加拿大中部太古宙地体（Archaean terranes）的浮冰搬运（ice-rafting）为拉布拉多海提供更多Nd源所致。结合稳定同位素（stable isotope）与有孔虫物种相对丰度数据，新的Nd同位素数据与拉布拉多海表层水文学特征相符：其水文状态受含低放射成因Nd的寒冷极地水与含较高放射成因Nd的温暖亚热带水之间的动态平衡调控。本次观测到的过去2.5 Ma拉布拉多海Nd同位素组成变化，本身即可解释同期北大西洋深层水（North Atlantic Deep Water）Nd同位素组成的演化过程。