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

NSF Award Abstract:

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.