Modeling the Ocean Distribution of Neodymium Isotopes: Testing the Bottom-Up Hypothesis

钕（Nd）同位素已被提出作为过去海洋环流的示踪剂;然而，决定现代海洋中Nd同位素分布的过程仍然没有得到很好的理解。以前的工作假设Nd是在靠近大陆的海洋浅水区的水柱和沉积物之间交换的，但是应用这个过程的模型很难再现Nd同位素的观测分布。这项研究将使用计算机模型来测试一个新的想法：自下而上的假设，这表明钕从深海沉积物输入到海洋中的任何地方的水柱，包括远离大陆的深海平原。将对最后一个冰河时期的海洋进行模拟，以探索此时海洋环流变化的物理和化学影响。这项研究可能会改变我们重建过去海洋深层环流的能力，这对于更好地了解碳和其他生物和气候重要元素的循环非常重要。 PI计划发表一篇包含模型描述、用户手册和代码的论文，并为模型的现有用户和GEOTRACES社区的潜在新用户举办网络研讨会。 该项目将包括国际合作者杜博士的参与。 一名研究生和一名本科生将作为该项目的一部分得到支助和培训。 学生将接受分析技术和建模方面的培训。 将在已建立的海洋环流和海洋地球化学全球三维模式中实施钕同位素。结构上不同的模型版本将产生的溶解和颗粒钕同位素分布的清除，边界交换和底栖通量的作用进行探索。对表层沉积物Nd同位素的新测量将填补现有数据库中的空白，该数据库将用于模拟海底通量的同位素组成。将利用GEOTRACES观测数据对模型进行全面评价，以检验以下假设：沿海底沿着的Nd普遍通量是水柱中Nd的主要来源。以往的研究假设交换仅沿沿着大洋上边缘和保守的行为在内部，这表明钕同位素是一个没有速率信息的水团示踪剂。与此相反，海底通量假说意味着深海流速是一个重要的影响钕同位素。这表明，从沉积物中的Nd同位素测量可以用来推断底层水流的速度和深海翻转循环率。对末次冰期最大期（约20，000年前）海洋的模拟将检验这一想法，并评估使用Nd同位素重建深海环流的能力。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Neodymium (Nd) isotopes have been proposed as a tracer of past ocean circulation; however, the processes that determine the distribution of Nd isotopes in the modern ocean are still not well understood. Previous work has assumed Nd is exchanged between the water column and sediments in shallow parts of the ocean near continents, but models applying this process have difficulties reproducing observed distributions of Nd isotopes. This study will use a computer model to test a new idea: the bottom-up hypothesis, which suggests input of Nd from deep sea sediments to the water column everywhere in the ocean, including the deep abyssal plains far away from continents. Simulations of the ocean during the last ice age will be performed to explore the physical and chemical impacts from changes in ocean circulation at this time. This study may transform our ability to reconstruct deep circulation in past oceans, which is important to better understand cycling of carbon and other biologically and climatically important elements. The PIs plan to publish a paper with the model description, user manual, and code, as well as hold webinars for existing user of the model and potentially new users from the GEOTRACES community. The project would include the participation of Dr. Du, an international collaborator. One graduate student and one undergraduate student would be supported and trained as part of this project. The students would be trained in analytical techniques and modeling. Neodymium (Nd) isotopes will be implemented in an established global three-dimensional model of ocean circulation and biogeochemistry. Structurally different model versions will be generated to explore the roles of scavenging, boundary exchange and benthic flux on dissolved and particulate Nd isotope distributions. New measurements of surface sediment Nd isotopes will fill gaps in the existing database, which will be used to model the isotopic composition of the benthic flux. Models will be comprehensively evaluated with GEOTRACES observations to test the hypothesis that a ubiquitous flux of Nd along the sea floor is the dominant source of Nd in the water column. Previous studies assume exchange only along upper ocean margins and conservative behavior in the interior, which suggests Nd isotopes are a water mass tracer without rate information. In contrast, the benthic flux hypothesis implies abyssal flow rate to be an important influence on Nd isotopes. This suggests that Nd isotope measurements from sediments could be used to infer rates of bottom water flow and the abyssal overturning circulation rate. Simulations of the Last Glacial Maximum (~20,000 years ago) ocean will test this idea and evaluate the ability to use Nd isotopes to reconstruct abyssal circulation.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.