Award: OCE-1458424

Intellectual Merit We have utilized the long-lived Ra isotopes (228Ra and 226Ra) to determine the water mass ventilation timescales and deep water residence times in the Western Arctic. We combined the Arctic GEOTRACES dataset with previous measurements of radium isotopes in the Western Arctic (including those measured during the NSF-funded Shelf-Basin Interactions project) to identify the 228Ra/226Ra activity ratios and salinities associated with the main water masses in the upper water column of the Western Arctic (e.g. surface meltwater and meteoric water, Atlantic water, Chukchi shelf water). This compilation of data spans the last 15 years, and by using the difference in the 228Ra/226Ra activity ratios between the different waters masses we determined that the ventilation time of the halocline with respect to inputs from the Chukchi Shelf is less than ~20 years. We have also used this combined dataset of Ra isotopes to study the seasonal changes in 228Ra over the Chukchi Shelf and in the Chukchi shelfbreak jet, which will improve our ability to use this isotope as a tracer of changing shelf inputs in the Arctic. This effort involves a collaboration with physical oceanographers at WHOI who have created a model of Chukchi Sea circulation. Preliminary results show good agreement between the model and observations. Next steps will include using the model to investigate how the release of radium will change in the future as a result of changing temperatures and ice coverage over the Chukchi Shelf. Because radium has a sediment source, this experiment will provide a useful prediction for how the Chukchi shelf source of other sediment-derived materials, such as carbon and trace metals, will change in the future. The short-lived 223Ra and 224Ra isotopes were low in the Arctic basin, with the exception of excess 224Ra (224Ra not supported by the decay of its parent isotope 228Th), which was high in surface waters in the Transpolar Drift and the Canada Basin. Because we only have measurements of dissolved 228Th and not particulate 228Th in surface waters, it is likely that this excess 224Ra is coming from particulate 228Th. However, the relationship between 228Th, 224Ra, and ice should be further studied to examine whether ice-rafted sediments and/or interactions between 228Th and the ice may be impacting surface water activities of these isotopes. Water column activities of excess 223Ra and 224Ra (corrected for activities supported by their parent isotopes, 227Ac and 228Th, respectively) decreased away from their shelf source, as expected, with some higher activities measured at depth indicating a benthic source. Broader Impacts: Two female undergraduate students were supported during three academic years and summers on this grant. Moore has presented seminars at East China Normal University, Shanghai, and Xiamen University based on the Arctic data. Last Modified: 05/28/2019 Submitted by: Willard S Moore