Comparing methods for measuring 210Po in seawater: Fe(OH)3 can extract less 210Po than Co-APDC and thus overestimate POC export fluxes

Authors/Creators

• Harold Carrasco, Edith Cowan UniversityFollow
• J. Kirk Cochran
• Beat Gasser
• Inés Sanz-Alvarez
• Montserrat Roca-Martí
• Christina Heilbrun
• Nicholas S. Fisher
• Jana Friedrich
• Pere Masqué, Edith Cowan UniversityFollow

Author Identifier (ORCID)

Harold Carrasco: https://orcid.org/0009-0001-6080-7661

Pere Masqué: https://orcid.org/0000-0002-1789-320X

Abstract

Over six decades of research, two methods have been used to extract the natural radionuclides ²¹⁰Pb and ²¹⁰Po from seawater- the precipitation of iron hydroxide (Fe(OH)3) and coagulation of cobalt ammonium pyrrolidine dithiocarbamate (Co-APDC). Although the two methods had been intercompared for ²¹⁰Pb and shown to agree, they were never intercompared for ²¹⁰Po until Roca-Martí et al. (2021) , in which only unfiltered samples were used. Here we document that Fe(OH)3 may be less effective in extracting ²¹⁰Po from a seawater sample than Co-APDC. We hypothesize that cycling of ²¹⁰Po through the marine food web can produce a fraction of dissolved ²¹⁰Po complexed with organic ligands and not efficiently precipitated by Fe(OH)3. We compare the Fe(OH)3 and Co-APDC methods through field sampling at the DYFAMED site in the Mediterranean Sea and complementary laboratory incubation experiments using phytoplankton. Lower dissolved ²¹⁰Po activities are extracted with Fe(OH)3 relative to Co-APDC immediately after sampling at times of high primary productivity. No difference between the methods is found for ²¹⁰Pb. Time-series experiments with stored filtered, acidified samples show that Fe(OH)3 can yield higher activities of ²¹⁰Po after extended storage (∼2 months), approaching values obtained with Co-APDC immediately after sample collection. In laboratory incubation experiments with diatoms and coccolithophores, exudates produced by diatoms show higher ²¹⁰Po activities using the Co-APDC method; no difference between the methods is apparent for coccolithophores. Direct comparison of the methods during a time of high productivity at the DYFAMED site suggests that POC export fluxes based on the Fe(OH)3 method can be overestimated by ∼50% in the primary productivity zone and by much larger extents in the mesopelagic. Additional field study results from the Northeast Atlantic and Pacific Oceans support these observations. We recommend the Co-APDC method be used for marine studies involving ²¹⁰Po, especially in biologically active waters and if samples are processed soon after collection.

Keywords

Methods comparison, natural radionuclides, POC flux, polonium-210

Document Type

Journal Article

Date of Publication

3-1-2026

Volume

275

Publication Title

Marine Chemistry

Publisher

Elsevier

School

Centre for Marine Ecosystems Research / School of Science

Funders

US National Science Foundation (OCE-2219285) / “La Caixa” Foundation (100010434, LCF/BQ/PI24/12040022) / Ramón y Cajal Program (RYC2023-045355-I)

Creative Commons License

This work is licensed under a Creative Commons Attribution-Noncommercial 4.0 License

Comments

Carrasco, H., Cochran, J. K., Gasser, B., Sanz-Alvarez, I., Roca-Martí, M., Heilbrun, C., Fisher, N. S., Friedrich, J., & Masqué, P. (2026). Comparing methods for measuring 210Po in seawater: Fe(OH)3 can extract less 210Po than Co-APDC and thus overestimate POC export fluxes. Marine Chemistry, 275, 104618. https://doi.org/10.1016/j.marchem.2026.104618