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

钕（Nd）同位素被认为是过去海洋环流的示踪剂；然而，决定Nd同位素在现代海洋中分布的过程仍然没有得到很好的理解。以前的工作假设在靠近大陆的浅海部分的水柱和沉积物之间有Nd交换，但是应用这一过程的模型很难再现观测到的Nd同位素分布。这项研究将使用一个计算机模型来测试一个新的想法：自下而上的假设，该假设认为，Nd从深海沉积物输入到海洋的任何地方的水柱中，包括远离大陆的深海平原。将对最后一个冰河期的海洋进行模拟，以探索此时海洋环流变化所产生的物理和化学影响。这项研究可能会改变我们重建过去海洋深层循环的能力，这对于更好地理解碳循环和其他生物和气候重要元素非常重要。pi计划发表一篇包含模型描述、用户手册和代码的论文，并为该模型的现有用户和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。作为该项目的一部分，将支持和培训一名研究生和一名本科生。学生们将接受分析技术和建模方面的培训。钕（Nd）同位素将在一个已建立的全球海洋环流和生物地球化学三维模型中实施。将生成结构上不同的模型版本，以探索清除、边界交换和底栖生物通量对溶解和颗粒Nd同位素分布的作用。对地表沉积物Nd同位素的新测量将填补现有数据库的空白，该数据库将用于模拟底栖生物通量的同位素组成。将利用GEOTRACES观测资料对模型进行全面评估，以验证沿海底普遍存在的Nd通量是水柱中Nd的主要来源这一假设。以前的研究假设交换只在上层海洋边缘进行，而在海洋内部则是保守的，这表明Nd同位素是一种没有速率信息的水质量示踪剂。相比之下，底栖生物通量假说表明深海流速对Nd同位素有重要影响。这表明沉积物的Nd同位素测量可以用来推断底部水流速率和深海倾覆循环速率。对末次冰期极大期（~2万年前）海洋的模拟将验证这一观点，并评估使用Nd同位素重建深海环流的能力。该奖项反映了美国国家科学基金会的法定使命，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.