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

在现代远洋中，海水的钕(ND)同位素值往往是由水团的混合作用解释的。因此，ND被认为是一种“准保守”示踪剂。在这种“准保守”行为中，隐含着这样一种假设，即沿水团流动路径不存在重要的ND源。这就是使用海洋沉积物中存档的ND同位素作为过去海洋环流的示踪剂的基本原理。近些年来，一些关于ND源、汇和生物地球化学循环的研究对这一传统观点提出了挑战，并提出了更多的ND来源，例如：(I)与改变海水ND同位素比率的颗粒物交换，或(Ii)受与沉积物反应影响的孔隙水ND的加入。为了成功地使用ND同位素作为水团示踪剂，了解这些限制是至关重要的。因此，本项目将研究这些ND的替代源(和汇)，以评估它们对海水ND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。资助支持博士后研究人员的职业发展，为研究生提供实地经验，为本科生提供研究经验。项目调查人员将在东南印度洋-南大洋(SEVEO)巡航期间收集构成现代ND循环的不同组成部分的样本(例如，过滤后的海水、水柱颗粒、孔隙水和同时存在的海底沉积物样本)。地震巡航轨道将：(1)覆盖起源于南大洋的主要水团(南极模式水、南极中间水、上、下绕极深水)，(2)广泛穿越东南印度洋海脊，以及(3)采样亚南极模式水今天正在形成的亚南极锋面以北的区域。将分析海水和孔隙水的Nd同位素和稀土元素，从而确定南大洋水团的特征，潜在活性脊岩对海水Nd同位素的影响，以及孔隙水Nd通量对底层水Nd同位素的影响。除了增进对ND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生物地球化学建模人员、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.