Collaborative Research: Dating the Recent Arctic: Refining a comprehensive suite of fallout radioisotope analyses for improved age constraints in Arctic marine sediments

Seafloor sediments hold histories of certain processes occurring on land and in the ocean. Radioisotopes can be used as tracers to study some of these histories—for example, the history of how rapidly sediment accumulates on the seafloor or the history of contaminants released into the environment. Interestingly, these two examples can be used together because of nuclear proliferation during the mid-1900s. During the development of atomic weapons, abundant atmospheric testing was done by nations globally. Fallout of certain isotopes including Cs-137 and Pu-241 made its way into the oceans and into marine sediments. Geologists and oceanographers studying a variety of marine processes use Cs-137 as a tracer to verify signals of other isotopes that allow us to determine the age of seafloor sediments. However, the Cs-137 signal is nearly minimized at this point and has been weak in the Arctic since the time of its introduction due to atmospheric circulation patterns. Given this limitation, it seems timely to use novel applications of modern isotopic detection technologies such as neutron activation and various types of radiation detection which have not been historically applied to marine sediments. For example, we aim to better detect Cs-137 through activation of Cs-135 (which can be accomplished with a university research reactor). In this project we will leverage five total detection techniques on seabed sediment samples from different polar environments to provide the research community with an updated “toolkit” of approaches for measuring ages of Arctic seafloor sediments. This work will enable marine geologists to form a better understanding of seafloor sedimentation and contaminant burial. Radionuclide tracers are widely used to constrain ages and accumulation rates of seabed sediments. Dating Arctic seabed sediments is a critical and current research question owing to rapid changes in shallow coastal environments—for example, accelerating coastal erosion and changing river hydrology are sending more sediments into the coastal ocean, and diminishing annual sea ice is leading to a stronger wave environment and in turn increased seabed reworking. Creating geochronologies for Arctic seabed sediments is challenging, however, because of low activities of Pb-210 and Cs-137, as well as mobility and diminishing detectability of Cs-137. While subfields of geology, chemistry, nuclear science, and archaeology have used a variety of approaches to exploit natural and fission-produced isotopes for research, these methods have not been comprehensively combined in marine sedimentology to tackle challenging problems like quantifying Arctic seabed sedimentation. This research will test a suite of methods that can be applied to existing sediment cores at the Marine and Geology Repository at Oregon State University to build a toolkit for dating Arctic seafloor sediments. These methods include direct measurements of fission-produced Am-241 by gamma detection on modern ultra-pure germanium as well as sodium-iodide detectors; amplification of Am-241, Cs-135, and Cs-137 signals through neutron activation; and Inductively Coupled Plasma Mass Spectroscopy (ICP-MS) analyses of sediments for a suite of isotopes. We will conserve on sample collection costs by utilizing existing cores to build a toolkit for dating Arctic seafloor sediments. Broader impacts of this research are to: (1) combine existing methods from disparate subfields (and test new ways to apply those methods) to assemble a robust toolkit for the marine geology community; (2) strengthen the expertise in Arctic research communities regarding the use of these isotopic tools; and (3) take the lessons from this work into graduate and undergraduate classrooms and public outreach events to inform students, kids, and community members about natural and anthropogenic nuclear fallout, and the novel and interesting ways that this knowledge can be applied to better understand the changing Arctic. 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.