Award: OCE-1830168

Overview: 227Ac is a naturally-occurring radioactive isotope with a 22 year half-life, primarily entering the water column from deep ocean sediments, where it is formed by its long-lived parent 231Pa. The input from the sediments causes water column 227Ac decay to exceed its parent 231Pa decay rate. This excess 227Ac decays as it is transported, and its distribution serves as a clock for defining the rates of mixing and current flow in the deep ocean. Transport rates that are calculated based on 227Ac may be applied to other solutes, including nutrients and metals that are important to ecosystem dynamics. Scientific Merit: This project measured 227Ac distributions in the deep Pacific as part of the US GEOTRACES program on transect GP15, along 152W from the Alaska margin to Tahiti. The highest 227Ac concentrations are found near the bottom, its source for the overlying water, and generally decrease with distance from the seabed. In addition to water column measurements, sediment samples were collected and analyzed to determine profiles of the naturally occurring radioisotopes 227Ac, 228Ra, and 210Pb. These profiles were used to calculate rates of bioturbation (mixing of sediment by organisms), and inputs of 227Ac to the bottom waters. With these constraints for input, analysis of the water column data indicates that transport in this region is largely lateral along layers of constant density, rather than vertical, across these stratified layers. These results for transport appear consistent with estimates previously made, using seawater density distribution and physical models for expected flow. Irregular topography of the sea floor provides 227Ac input that varies with water depth, sometimes creating irregularity in the water column profiles, and also illustrating the importance of the lateral transport. In addition to the work on 227Ac, we also collaborated with groups at Woods Hole and Univ. South Carolina in their studies of 228Ra distribution as a tracer of flow and mixing in the upper ocean. Broader Impacts: Quantifying the biogeochemical dynamics of the deep sea is necessary to understand the ocean carbon cycle. Defining these dynamics requires knowledge of boundary exchanges and water column transport patterns and rates. Dynamics for actinium can be applied to interpret concentration fields of other, biogeochemically important solutes. This project provided training for one Ph.D. student, a marine veteran, who has completed his dissertation and is now employed as a scientist for a company evaluating coastal sites for wind farms, marine cables and pipelines. In addition, 3 undergraduate students (2 female and 1 male) assisted with lab work; two are now pursuing careers in hydrology and in climate science. The third student has now completed a Ph.D. in geochemistry, and she is doing post-doctoral work. Analytical refinements in the Radecc technique developed under this funding are proving useful for analyses of radium and actinium isotopes by other groups. Last Modified: 08/16/2024 Submitted by: DouglasEHammond