Collaborative Research: Investigation of paired uranium and chromium isotope behavior during authigenic metal uptake into continental margin sediments

海洋沉积物岩心中的化学测量可以用来重建过去海水成分、环境条件和气候的变化。为了做到这一点，我们必须准确地了解沉积物的化学成分是如何确定的，以及它与环境条件是如何相关的。沉积物中铀(U)和铬(Cr)等痕量金属的浓度被用来重建海洋生物生产、底层水氧浓度或两者的过去变化。潜在的比这些元素的绝对浓度更有价值的是它们不同同位素比率的变化。为了充分理解和使用这一解释过去的工具，这项研究将涉及对现代海洋沉积物中铀和铬同位素化学的详细调查。该项目将资助一名研究生、一名博士后研究员和几名本科生。外展活动将与罗格斯大学海洋科学外展项目协调。塞弗曼和她的实验室成员将领导一年一度的气候变化青少年峰会，并将向学生和教师介绍与这项研究直接相关的主题。该计划旨在激发约200名纽约-新泽西州高中生对科学、技术、工程和数学(STEM)的兴趣。该计划将通过动手活动和讲座向年轻人介绍科学概念，并支持学生在当地社区规划和实施与科学相关的服务项目。该项目的主要目标是推进铀和铬同位素作为古氧化还原代用品的刻度和发展。与简单的金属丰度相比，自生金属同位素替代物具有明显的优势，因为它们可以提供有关金属吸收机制的额外信息，而且它们不需要对可变的质量积累速率进行校正。目前对海洋沉积物中U和Cr同位素组成的测量，结合实验结果，提供了对与不同沉积汇相关的主要同位素效应及其吸收机制的第一手了解。然而，由于这两种金属的自生吸收主要发生在沉积物包裹体中，因此详细调查早期成岩改造过程中的同位素效应对于充分利用这些同位素体系作为古代物至关重要。调查人员将在一组具有典型大陆边缘代表性的地点测量铀和铬的同位素组成。对于这两种金属来说，这些类型的沉积物代表着一个主要的海洋沉没；因此，这项研究的结果将填补限制全球同位素质量平衡努力的一个重大空白。通过将沉积物与孔隙水测量配对，研究人员将量化与金属固存相关的特定同位素分馏，以减少沉积物。他们将把同位素测量与元素在这些环境中的基本生物地球化学行为的详细研究结合起来，这是目前缺乏的，特别是对铬来说。深度的累积同位素效应将与沉积边界条件进行比较，例如有机碳埋藏率和底水氧合作用，这将促进全球沉淀池的参数化。核心论点是，这两种元素将表现出与有机物质循环(U)和金属氧化物循环(Cr)直接相关的可预测行为，它们配对的同位素系统将被证明是追踪海洋净氧合作用的强大组合。

Chemical measurements in ocean sediment cores can be used to reconstruct past variations in the composition of seawater, environmental conditions, and climate. In order to do so we must understand exactly how the chemical composition of sediments is set and how it relates to environmental conditions. The concentrations of trace metals such as uranium (U) and, to a lesser extent, chromium (Cr) in sediments have been used to reconstruct past variations in ocean biological production, bottom water oxygen concentrations, or both. Potentially even more valuable than absolute concentrations of these elements are variations in the ratios of their different isotopes. In order to fully understand and use this tool for interpreting the past, this study will involve a detailed investigation of uranium and chromium isotope chemistry in modern ocean sediments. One graduate student, one postdoctoral researcher, and several undergraduate students will be funded through this project. Outreach activities will be coordinated with the Rutgers Marine Sciences outreach program. Severmann and her lab members will lead the annual Climate Change Teen Summit and will introduce the students and teachers to themes related directly to the research. This program is aimed at stimulating the interest of about 200 NY-NJ high school students in science, technology, engineering and math (STEM). The program will introduce young people to scientific concepts through hands-on activities and talks, and support students in planning and implementing science-related service projects in their local communities. The primary goal of this project is to advance the calibration and development of U and Cr isotopes as paleo-redox proxies. Authigenic metal isotope proxies offer a distinct advantage over simple metal abundances as they can provide additional information regarding the mechanism 
of metal uptake, and they do not require correction for variable mass accumulation rates.
 Current measurements of U and Cr isotope compositions in marine sediments, combined with experimental results, provide a first-order understanding of the dominant isotope effects associated with 
different sedimentary sinks and their uptake mechanism. However, since authigenic uptake for 
both metals takes place predominantly within the sediment package, a detailed investigation
 of the isotope effects during early diagenetic transformation is essential for fully exploiting 
these isotope systems as paleoproxies. The investigators will measure U and Cr isotope compositions in a suite of sites that are representative 
of typical continental margins. For both metals, these types of sediments represent a dominant
 oceanic sink; results from this study will thus fill a major gap in efforts to constrain the global isotope mass balance. By pairing sediment with porewater measurements the investigators will quantify the specific isotope fractionations associated with metal sequestration into 
reducing sediments. They will pair isotope measurements with detailed investigation
 of the elements' fundamental biogeochemical behavior in these settings, which currently is lacking, especially for Cr. The cumulative isotope effect at depth will be compared
 to depositional boundary conditions such as organic carbon burial rates and bottom water oxygenation, which will facilitate parameterization of sedimentary sinks globally. The central thesis is that these two elements will exhibit predictable behavior that is directly tied to organic
matter cycling (U) and metal oxide cycling (Cr) and that their paired isotope systems will 
prove to be a powerful combination for tracing net oxygenation in the ocean.