Quantifying 210Po/210Pb Disequilibrium in Seawater: A Comparison of Two Precipitation Methods With Differing Results
Montserrat Roca‐Martí, Viena Puigcorbé, Maxi Castrillejo, Núria Casacuberta, Jordi García-Orellana, J. Kirk Cochran, Pere Masqué
Abstract
The disequilibrium between lead-210 ( 210 Pb) and polonium-210 ( 210 Po) is increasingly used in oceanography to quantify particulate organic carbon (POC) export from the upper ocean. This proxy is based on the deficits of 210 Po typically observed in the upper water column due to the preferential removal of 210 Po relative to 210 Pb by sinking particles. Yet, a number of studies have reported unexpected large 210 Po deficits in the deep ocean indicating scavenging of 210 Po despite its radioactive mean life of ∼ 200 days. Two precipitation methods, Fe(OH) 3 and Co-APDC, are typically used to concentrate Pb and Po from seawater samples, and deep 210 Po deficits raise the question whether this feature is biogeochemically consistent or there is a methodological issue. Here, we present a compilation of 210 Pb and 210 Po studies that suggests that 210 Po 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 210 Po/ 210 Pb disequilibrium can be partly related to a methodological artifact, we directly compared the total activities of 210 Pb and 210 Po 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 210 Pb activities, results from the Co-APDC method showed equilibrium between 210 Pb and 210 Po below 100 m, whereas the Fe(OH) 3 method resulted in activities of 210 Po significantly lower than 210 Pb throughout the entire water column. These results show that 210 Po 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 210 Po/ 210 Pb pair as a tracer of particle export in the oceans because 210 Po (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 210 Po activities between both analytical methods.