Neodymium isotopic characterization of Ross Sea Bottom Water and its advection through the southern South Pacific, POLARSTERN cruise ANT-XXVI/2

Since the inception of the international GEOTRACES program, studies investigating the distribution of trace elements and their isotopes in the global ocean have significantly increased. In spite of this large-scale effort, the distribution of neodymium isotopes (143Nd/144Nd) and concentrations ([Nd]) in the high latitude south Pacific is still understudied. Here we report dissolved Nd isotopes and concentrations from 11 vertical water column profiles from the south Pacific between South America and New Zealand. Results suggest that Ross Sea Bottom Water (RSBW) is represented by an epsilon-Nd value of ~ -7, and is thus more radiogenic than Circumpolar Deep Water (epsilon-Nd ~ -8). RSBW and its characteristic epsilon-Nd signature can be traced far into the SE Pacific until progressive mixing with ambient Lower Circumpolar Deep water (LCDW) dilutes this signal north of the Antarctic Polar Front (APF). The SW-NE trending Pacific-Antarctic Ridge restricts the advection of RSBW into the SW Pacific, where bottom water density, salinity, and epsilon-Nd values of -9 indicate the presence of bottom waters of an origin different from the Ross Sea.
Neodymium concentrations show low surface concentrations and a linear increase with depth north of the Polar Front. South of the APF, surface [Nd] is high and increases with depth but remains almost constant below ~1000 m. This vertical and spatial [Nd] pattern follows the southward shoaling density surfaces of the Southern Ocean frontal system and hence suggests supply of Nd to the upper ocean through upwelling of Nd-rich deep water. Low particle abundance dominated by reduced opal production and seasonal sea ice cover likely contributes to the maintenance of the high upper ocean [Nd] south of the APF.
The reported data highlights the use of Nd isotopes as a water mass tracer in the Southern Ocean, with the potential for paleocenaographic reconstructions, and contributes to an improved understanding of Nd biogeochemistry.

自国际GEOTRACES计划启动以来，针对全球海洋中痕量元素及其同位素分布的研究数量显著增加。尽管开展了如此大规模的研究工作，南太平洋高纬度海域的钕同位素（neodymium isotopes，143Nd/144Nd）与钕元素浓度([Nd])的分布特征仍未得到充分研究。本研究报道了南美洲与新西兰之间南太平洋海域的11个垂直水柱剖面的溶解态钕同位素与元素浓度数据。结果显示，罗斯海底层水（Ross Sea Bottom Water, RSBW）的ε-Nd值约为-7，其放射性较环极深层水（Circumpolar Deep Water, CDW，ε-Nd≈-8）更强。罗斯海底层水及其特征性的ε-Nd信号可追踪至东南太平洋远域，直至与周围的下层环极深层水（Lower Circumpolar Deep Water, LCDW）发生渐进式混合，最终在南极极锋（Antarctic Polar Front, APF）以北削弱该示踪信号。呈西南-东北走向的太平洋-南极海岭限制了罗斯海底层水向西南太平洋的平流输送；该区域的底层水密度、盐度以及-9的ε-Nd值表明，其底层水来源与罗斯海不同。
钕元素浓度表现为表层浓度较低，且在极锋以北随深度呈线性升高。在极锋以南，表层[Nd]浓度较高且随深度升高，但在约1000米以深基本保持恒定。这种垂直与空间分布模式与南大洋锋面系统的密度面向南变浅的特征相一致，因此表明富钕深层水上涌为上层海洋提供了钕元素来源。以减少的蛋白石生产和季节性海冰覆盖为主导的低颗粒物丰度，可能是维持极锋以南上层海洋高[Nd]浓度的关键因素。
本研究报道的数据凸显了钕同位素作为南大洋水团示踪剂的应用潜力，可用于古海洋学重建，同时也有助于增进对钕元素生物地球化学循环的理解。