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
Modified by: Willard S Moore