Biogeochemical behaviours of barium and radium-226 in the Pacific Ocean

Barium and radium-226 are not just proxies for nutrients and ocean circulation but are themselves marine biogeochemical tracers. Since the 1970s, scientists have noted a strong correlation between these Group II elements. Despite their similarities, barium and radium-226 have distinct sources and biogeochemical processes affecting their concentrations. Barium primarily comes from rivers and groundwater, while radium-226 originates from deep-sea sediments through thorium-230 decay. Both elements are removed from surface waters by incorporation into barite, then released at greater depths. Additionally, radium-226 is removed by radioactive decay, with a half-life of around 1,600 years, adding complexity to its behaviour.

Le Roy and her colleagues (2024, see reference below) collected barium and radium-226 data from the Pacific Ocean along 152 °W during GEOTRACES GP15. By combining this data with a statistical model of water mass distributions, they distinguished between biogeochemical processes and ocean circulation on the observed distributions. Their study yielded three main findings:

• In the upper ocean, barium and radium-226 undergo significant biogeochemical removal, likely due to microcrystalline barite formation during organic matter breakdown. The greatest removal occurs in high productivity regions, such as the North Pacific near the Alaskan Margin, and in the subsurface where most barite forms.
• In the deep Pacific, barium and radium-226 levels are among the highest globally, due to initial enrichments from Antarctic Bottom Water and accumulation through sediment interactions. Slow circulation in this region further allows these elements to accumulate via the dissolution of sinking particles.
• Despite deep Pacific enrichment, barium and radium-226 show distinct benthic patterns. Radium-226 has additional sources, such as sediment-bound thorium-230 decay and hydrothermal venting.

Mapping these distributions and quantifying non-conservative processes, Le Roy et al. (2024) constructed a basin-wide mass balance for the Pacific Ocean. This revealed a surface barium deficit and a deep radium-226 excess, indicating the need for further studies on barium replenishment in the North Pacific and the fate of radium-226 exported from this region.

Reference:

Le Roy, E., Charette, M.A., Henderson, P.B., Shiller, A.M., Moore, W.S., Kemnitz, N., Hammond, D.E., & Horner, T.J. (2024). Controls on dissolved barium and radium‐226 distributions in the Pacific Ocean along GEOTRACES GP15. Global Biogeochemical Cycles, 38(6), e2023GB008005. Access the paper: 10.1029/2023GB008005.