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

Over six decades of research, two methods have been used to extract the natural radionuclides 210 Pb and 210 Po 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 210 Pb and shown to agree, they were never intercompared for 210 Po 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 210 Po from a seawater sample than Co-APDC. We hypothesize that cycling of 210 Po through the marine food web can produce a fraction of dissolved 210 Po 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 210 Po 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 was found for 210 Pb. Time-series experiments with stored filtered, acidified samples show that Fe(OH) 3 can yield higher activities of 210 Po 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 showed higher 210 Po activities using the Co-APDC method; no difference between the methods was 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 210 Po, especially in biologically active waters and if samples are processed soon after collection.