Neodymium and hafnium boundary contributions to seawater along the West Antarctic continental margin
Neodymium and hafnium boundary contributions to seawater along the West Antarctic continental margin
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DOI：
10.1016/j.epsl.2014.03.008
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发表时间：
2014-05
影响因子：
5.3
通讯作者：
J. Rickli;M. Gutjahr;D. Vance;M. Fischer-Gödde;C. Hillenbrand;G. Kuhn
中科院分区：
文献类型：
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作者：
J. Rickli;M. Gutjahr;D. Vance;M. Fischer-Gödde;C. Hillenbrand;G. Kuhn
结合阿蒙森海海湾的五个底部水样，在南极绕极流以南和罗斯海以东的三个水柱剖面中测量了钕和铪同位素和元素浓度（Sm、Nd、Hf、Zr）。钕和铪似乎都是从海湾沉积物中释放出来的。就 Nd 而言，这反映在放射性同位素组成（ε Nd 高达− 5.4）和高度升高的浓度（高达 41 pmol/kg）中。另一方面，铪同位素相对于公海地点仅发生了非常轻微的变化，并且边界释放最明显地表现为浓度升高（> 1 pmol/kg 对 0.7 pmol/kg）。玛丽伯德海山还存在本地铪和铷输入，导致铪同位素组成为放射性 as-3.1，铪在当地底层水中转变为放射性较低的组成。新数据与文献数据的汇编揭示了南极洲对下环极深水（LCDW）和南极底水（AABW）中 Nd 同位素组成的影响的一致观点。相对于 LCDW（平均 ε Nd=− 8.4），扇区特定的 Nd 添加将大西洋扇区形成的 AABW 转变为放射源性较低的同位素成分（平均 ε Nd=− 9），而太平洋扇区的 AABW 则转变为放射源性较高的值（平均 ε Nd=− 7）。太平洋区域 LCDW 随深度向放射源性更强的 ε Nd 演化可能反映了 AABW 的混合，但此外，也受到在玛丽伯德海山观察到的与斜坡的边界交换的控制。数据集中的铪同位素相对均匀，大多数样本的范围在 ε Hf=+ 2 和+ 3.8 之间，不包括靠近玛丽伯德海山的深水中放射性成分较少的情况。然而，太平洋区域的 Hf 同位素组成比大西洋区域的放射性稍低，相对于平均值 + 3.8，对应于平均值 + 3。这可能反映了从南极洲到太平洋区域的非放射性 Hf 输入，这些输入通过可逆清除而垂直均匀化。两个部门之间 LCDW 中的 Hf 同位素异质性可能表明 Hf 的海水停留时间比 Nd 短，这与两种元素的溶解-颗粒相分配一致。
Neodymium and hafnium isotopes and elemental concentrations (Sm, Nd, Hf, Zr) have been measured in three water column profiles south of the Antarctic Circumpolar Current in, and to the east of the Ross Sea, in conjunction with five bottom water samples from the Amundsen Sea Embayment. Neodymium and hafnium both appear to be released from sediments in the Embayment. In the case of Nd, this is reflected in radiogenic isotope compositions (ε Nd up to− 5.4) and highly elevated concentrations (up to 41 pmol/kg). Hafnium isotopes, on the other hand, are only very slightly altered relative to the open ocean sites, and boundary release is most prominently indicated by elevated concentrations (> 1 pmol/kg versus∼ 0.7 pmol/kg). There is also a local input of both Hf and Nd at the Marie Byrd Seamounts, which leads to Nd isotope compositions as radiogenic as− 3.1, and hafnium shifted to less radiogenic compositions in local bottom water. A compilation of the new data with literature data reveals a consistent view of the influence of Antarctica on the Nd isotope composition in Lower Circumpolar Deep Water (LCDW) and Antarctic Bottom Water (AABW). Sector specific Nd addition shifts AABW formed in the Atlantic sector to less radiogenic isotope compositions (average ε Nd=− 9) relative to LCDW (average ε Nd=− 8.4), whereas AABW in the Pacific sector is shifted to more radiogenic values (average ε Nd=− 7). The evolution towards more radiogenic ε Nd with depth in LCDW in the Pacific sector is likely to reflect admixture of AABW but, in addition, is also controlled by boundary exchange with the slope as observed at the Marie Byrd Seamounts. Hafnium isotopes are relatively homogeneous in the data set, ranging between ε Hf=+ 2 and+ 3.8 for most samples, excluding less radiogenic compositions in deep waters close to the Marie Byrd Seamounts. The Hf isotope composition in the Pacific sector is, however, slightly less radiogenic than in the Atlantic, corresponding to an average of+ 3 relative to an average of+ 3.8. This probably reflects unradiogenic Hf inputs from Antarctica to the Pacific sector, which are vertically homogenized by reversible scavenging. The Hf isotope heterogeneity in LCDW between both sectors is likely to indicate a shorter seawater residence time for Hf than for Nd, which is consistent with the dissolved–particulate phase partitioning of both elements.