Significant gaps in our understanding of the oceanic cycling of neodymium (Nd) and the other rare earth elements (REEs) remain despite decades of research. One important observation which has not been adequately explained is that the concentration of dissolved Nd typically increases with depth, similar to nutrient profiles, while Nd isotopes appear to reflect conservative water mass mixing in the intermediate and deep ocean; this has been termed the “Nd paradox”. Here we present a detailed study of the dissolved Nd isotopic composition across a section at 40°S in the South Atlantic, collected by UK GEOTRACES cruise (section GA10). The South Atlantic represents a natural laboratory for our understanding of spatial controls on ocean geochemistry, because of the large variability of inputs, spatial differences in particulate cycling, and horizontal advection and mixing at depth between major northern- and southern-sourced water masses. This variability has also made the South Atlantic a critical region subject to intense investigations that aim at reconstructing past changes in ocean processes, such as changes in biological productivity and deep ocean circulation.

Our Nd isotope results from the GA10 section provide observational data show the signal of water mass mixing and reversible scavenging. In the surface ocean (0–600 m), Nd isotopic compositions are distinct between different surface ocean currents and spatially can be tied to various continental sources. In the intermediate ocean (600–2500 m), the vertical Nd isotope distribution exhibits distinct signals of different water masses by horizontal advection, including upper North Atlantic Deep Water and Antarctic Intermediate Water formed in the Atlantic Ocean or the Indian Ocean. The Nd isotope distribution also reflects influence of reversible scavenging that smears the signals downwards in the water column (i.e., offset to more radiogenic values). In the deep ocean below 2500 m, Nd isotope distribution largely follows conservative water mass mixing model. Nd concentration in the deep ocean, however, deviates from conservative mixing and increases constantly with depth. We also observe that Nd isotopes appear to be shifted towards the composition of overlying water masses. These observations suggest that reversible scavenging of Nd onto organic and other types of particles is a major vertical process throughout the water column. We also suggest that this process can resolve the “Nd paradox” of decoupling of Nd concentration and isotopic composition due to mixing dynamics. Because abyssal water masses already have a high Nd concentration, a given amount of Nd added from the vertical process has less of an effect on Nd isotopic compositions in deep water masses than it does for intermediate water masses which have comparatively low Nd concentration.

尽管进行了数十年的研究，但我们对钕 (Nd) 和其他稀土元素 (REE) 的海洋循环的理解仍然存在重大差距。一项尚未得到充分解释的重要观察结果是，溶解 Nd 的浓度通常随着深度而增加，类似于营养成分，而 Nd 同位素似乎反映了中海和深海中保守的水团混合；这被称为“Nd悖论”。在这里，我们详细研究了由英国 GEOTRACES 巡航（GA10 部分）收集的南大西洋 40°S 部分的溶解 Nd 同位素组成。南大西洋是我们了解海洋地球化学空间控制的天然实验室，因为输入的巨大可变性、颗粒循环的空间差异、以及来自北方和南方的主要水团之间深度的水平平流和混合。这种变化也使南大西洋成为一个关键地区，需要进行大量调查，这些调查旨在重建海洋过程的过去变化，例如生物生产力和深海环流的变化。

我们来自 GA10 剖面的 Nd 同位素结果提供了观测数据，显示了水团混合和可逆清除的信号。在表层海洋（0-600 m）中，不同表层洋流之间的 Nd 同位素组成不同，并且在空间上可以与各种大陆来源联系起来。在中间海洋（600-2500 m），垂直的Nd同位素分布通过水平平流表现出不同水团的不同信号，包括在大西洋或印度洋形成的北大西洋上层深水和南极中水。Nd 同位素分布也反映了可逆清除的影响，这种清除在水体中向下涂抹信号（即偏移到更多的放射成因值）。在 2500 m 以下的深海中，Nd 同位素分布很大程度上遵循保守的水团混合模型。然而，深海中的 Nd 浓度偏离保守混合并随着深度不断增加。我们还观察到 Nd 同位素似乎向上覆水团的组成移动。这些观察结果表明，Nd 可逆地清除到有机和其他类型的颗粒上是整个水体中一个主要的垂直过程。我们还建议这个过程可以解决由于混合动力学导致的 Nd 浓度和同位素组成解耦的“Nd 悖论”。因为深海水团已经有很高的 Nd 浓度，