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用于重建过去海水成分、环境条件和气候的变化。为了做到这一点，我们必须确切地了解沉积物的化学成分是如何确定的，以及它与环境条件的关系。沉积物中铀（U）和（在较小程度上）铬（Cr）等微量金属的浓度已被用于重建海洋生物生产、底层水氧浓度或两者的过去变化。可能比这些元素的绝对浓度更有价值的是它们不同同位素比例的变化。为了充分理解和利用这一工具来解释过去，这项研究将涉及现代海洋沉积物中铀和铬同位素化学的详细调查。本项目将资助一名研究生、一名博士后研究员和几名本科生。外联活动将与罗格斯大学海洋科学外联方案协调。Severmann和她的实验室成员将领导年度气候变化青少年峰会，并将向学生和教师介绍与研究直接相关的主题。该计划旨在激发约200名NY-NJ高中学生对科学，技术，工程和数学（STEM）的兴趣。该计划将通过实践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指标的校准和开发。自生金属同位素代理提供了一个明显的优势，简单的金属丰度，因为它们可以提供更多的信息，关于机制#8232;的金属吸收，他们不需要校正可变的质量积累率。与其他文件#8232;目前测量的U和Cr同位素组成在海洋沉积物中，结合实验结果，提供了一个一阶的理解与
不同的沉积汇及其吸收机制的主导同位素效应。然而，由于自生吸收#8232;这两种金属主要发生在沉积物包，详细调查#8232;在早期成岩转化的同位素效应是必不可少的充分利用#8232;这些同位素系统作为古代理。调查人员将测量一组典型大陆边缘代表性地点的铀和铬同位素组成。对于这两种金属，这些类型的沉积物代表了一个占主导地位的#8232;海洋汇;这项研究的结果将填补一个主要的空白，努力限制全球同位素质量平衡。通过将沉积物与孔隙水测量配对，研究人员将量化与金属螯合相关的特定同位素分馏到#8232;还原沉积物中。他们将把同位素测量与这些环境中元素基本地球化学行为的详细调查结合起来，这是目前缺乏的，特别是对Cr。将比较深度处的累积同位素效应
这将有助于对全球沉积汇进行参数化。中心论点是，这两种元素将表现出可预测的行为，直接与有机物质循环（U）和金属氧化物循环（Cr）有关，并且它们的成对同位素系统将被证明是追踪海洋中净氧合的强大组合。

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.