Quantifying Po-210/Pb-210 Disequilibrium in Seawater: A Comparison of Two Precipitation Methods With Differing Results

The disequilibrium between lead-210 (Pb-210) and polonium-210 (Po-210) is increasingly used in oceanography to quantify particulate organic carbon (POC) export from the upper ocean. This proxy is based on the deficits of Po-210 typically observed in the upper water column due to the preferential removal of Po-210 relative to (21)0Pb by sinking particles. Yet, a number of studies have reported unexpected large Po-210 deficits in the deep ocean indicating scavenging of Po-210 despite its radioactive mean life of similar to 200 days. Two precipitation methods, Fe(OH)(3) and Co-APDC, are typically used to concentrate Pb and Po from seawater samples, and deep Po-210 deficits raise the question whether this feature is biogeochemically consistent or there is a methodological issue. Here, we present a compilation of Pb-210 and Po-210 studies that suggests that Po-210 deficits at depths >300 m are more often observed in studies where Fe(OH)(3) is used to precipitate Pb and Po from seawater, than in those using Co-APDC (in 68 versus 33% of the profiles analyzed for each method, respectively). In order to test whether Po-210/Pb-210 disequilibrium can be partly related to a methodological artifact, we directly compared the total activities of Pb-210 and Po-210 in four duplicate ocean depth-profiles determined by using Fe(OH)(3) and Co-APDC on unfiltered seawater samples. While both methods produced the same Pb-210 activities, results from the CoAPDC method showed equilibrium between Pb-210 and Po-210 below 100 m, whereas the Fe(OH)(3) method resulted in activities of Po-210 significantly lower than Pb-210 throughout the entire water column. These results show that Po-210 deficits in deep waters, but also in the upper ocean, may be greater when calculated using a commonly used Fe(OH)3 protocol. This finding has potential implications for the use of the Po-210/Pb-210 pair as a tracer of particle export in the oceans because Po-210 (and thus POC) fluxes calculated using Fe(OH)(3) on unfiltered seawater samples may be overestimated. Recommendations for future research are provided based on the possible reasons for the discrepancy in Po-210 activities between both analytical methods.