Award: OCE-1436958

Overview: 227Ac is a naturally-occurring radioactive isotope with a 22 year half-life. Its distribution may serve as a clock for defining the rates of mixing and sediment-water exchange in the ocean. The rates that are calculated based on 227Ac may be applied to other solutes, including nutrients and metals that are important to ecosystem dynamics. This proposal requested funding to measure 227Ac profiles on the GEOTRACES Arctic transect, a section extending from Dutch Harbor, through the Bering Sea and across the Western Basin to near the North Pole. Scientific Merit: Despite having a half-life that is well suited for the study of both vertical and lateral transport in the deep ocean, few 227Ac measurements have been made in the deep sea, with only one profile measured in the Arctic. Recent advances in instrumentation have facilitated this analysis, and the synergy provided by the GEOTRACES program provides an ideal opportunity to obtain additional data. Prior to this cruise, model calculations indicated that Arctic sediments should supply actinium at a rate that would provide a clear signal of the input and mixing. Bioturbation, the mixing activities of bottom-dwelling organisms, was expected to enhance this input. Results from our study indicated very different dynamics. Shallow water Arctic sediments must be characterized by a high biotubation rate, sufficient to create a measurable signal in surface waters. However, the analytical uncertainty in surface waters is sufficiently large that it obscures concentration gradients in the surface water, limiting its value for finding mixing rates in this part of the water column. In deep waters, there is only a very modest input of actinium from the bottom sediments, indicating that bioturbation in these deeper water sediments must be minimal. The low rate of bioturbation undoubtedly reflects the low productivity in the surface waters, limiting the food supply for bottom-dwelling organisms. Profiles of actinium in the water column indicate that horizontal transport in the deep Arctic appears to be at least as important as vertical transport in governing actinium distribution. In addition to the Arctic work, an opportunity to participate in a cruise of opportunity to the North Pacific allowed new insights to be developed into the dynamics of actinium and silicic acid in these deep waters. Silic acid is an essential nutrient for diatom growth, a phytoplankton group that plays a key role in the dynamics of the ocean carbon system. 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. While the water column actinium signals were less pronounced than expected, they provide evidence that benthic activity in shallow waters is very high, and it is extremely low in deep waters. This observation provides a benchmark for present-day behavior that may be compared to future behavior as a rapidly-changing Arctic evolves. This project also provided training for 3 students who are now pursuing careers in Earth and Ocean Science. Analytical refinements in the Radecc technique developed under this funding will be useful for future analyses of radium and actinium isotopes in other settings, including other deep sea basins and possibly groundwater hydrology. Last Modified: 10/06/2019 Submitted by: Douglas E Hammond