Testing the fidelity of neodymium isotopes as a paleocirculation tracer in the Southeast Indian-Southern Ocean

在现代开阔海洋中，海水的钕（Nd）同位素值通常被解释为水团的混合。因此，Nd被认为是“准保守”示踪剂。在这种“准保守”行为中隐含的假设是，沿水体流道沿着不存在显著的Nd源。这是使用海洋沉积物中的Nd同位素作为过去海洋环流示踪剂的基本原理。近年来，一些研究的Nd源，汇，和地球化学循环的挑战，这一传统的智慧，并提出了额外的来源，如：（i）交换与微粒，修改海水Nd同位素比值，或（ii）增加孔隙水Nd与沉积物的反应的影响。为了成功地使用Nd同位素作为水团示踪剂代理，必须了解这些限制。因此，本项目将检查这些替代来源（和汇）的钕，以评估其对海水钕的准保守行为的影响。这项研究将有助于限制钕同位素作为过去海洋环流示踪剂的局限性。资金支持博士后研究人员的职业发展，为研究生提供现场经验，并为本科生提供研究经验。项目调查人员将收集构成现代Nd循环不同成分的样品（例如，过滤海水、水柱颗粒、孔隙沃茨和并行的井下沉积物样品）。SEISO巡航轨道将：（i）覆盖南大洋起源的主要水团（南极模态水、南极中层水、上绕极深水和下绕极深水），（ii）广泛地经过东南印度洋脊，（iii）对亚南极洲前缘正北部的区域取样，亚南极洲模态水今天正在形成。海水和孔隙水Nd同位素和稀土元素将进行分析，从而允许表征南大洋水团，海水Nd同位素的影响，从潜在的反应脊岩石，和孔隙水Nd通量对底层水Nd同位素的影响。除了提高对钕循环的理解外，该项目的数据还将为解释研究巡航的其他数据集提供重要信息，并将引起广泛的气候科学家以及地球化学建模者，化学和生物海洋学家以及极地科学家的兴趣。

In the modern open ocean neodymium (Nd) isotope values of seawater are often explained by the mixing of water masses. As such, Nd is recognized as a "quasi-conservative" tracer. Implicit in this "quasi-conservative" behavior is the assumption that no significant source of Nd exists along the water mass flowpath. This is the basic principle behind using Nd isotopes archived in marine sediments as a tracer of past ocean circulation. In recent years, several studies of Nd sources, sinks, and biogeochemical cycling has challenged this conventional wisdom and proposed additional sources of Nd, such as: (i) exchange with particulates that modifies seawater Nd isotope ratios, or (ii) addition of pore water Nd influenced by reactions with sediment. To successfully use Nd isotopes as a water mass tracer proxy, it is essential to understand these limitations. Therefore, this project will examine these alternative sources (and sinks) of Nd to evaluate their impact on the quasi-conservative behavior of seawater Nd. This research will help to constrain the limitations of Nd isotopes as a tracer of past ocean circulation. Funding supports career development for a postdoctoral researcher, provides field experience for a graduate student, and provides research experience for an undergraduate student.Project investigators will collect samples that constitute the different components of modern Nd cycle (e.g., filtered seawater, water column particulates, pore waters and concurrent down hole sediment samples) during a cruise in the Southeast Indian-Southern Ocean (SEISO). The SEISO cruise track will: (i) cover major water masses of Southern Ocean origin (Antarctic Mode Water, Antarctic Intermediate Water, Upper and Lower Circumpolar Deep Water), (ii) extensively transit over the southeast Indian Ridge, and (iii) sample the region just north of the Subantarctic Front, where Subantarctic Mode Water is forming today. Seawater and pore water Nd isotopes and Rare Earth Elements will be analyzed, thereby allowing characterization of the Southern Ocean water masses, the effects on seawater Nd isotopes from potentially reactive ridge rocks, and the influence of pore water Nd flux on bottom water Nd isotopes. Besides improving the understanding of Nd cycling, data from this project will provide important information for interpreting other datasets from the research cruise, and will be of interest to a wide range of climate scientists as well as biogeochemical modelers, chemical and biological oceanographers, and polar scientists.