Improving 210Po low level measurements in seawater

Alvaro López Rodríguez, Beatriz González González, Cristina García Prieto, José Luis Más Balbuena, Frédéric André Corentin Le Moigne, Sarah Lou Carolin Giering, Santiago Hurtado Bermúdez, Juan Mantero Cabrera, María Villa Alfageme

Ocean is the largest sink of atmospheric carbon, atmospheric CO2 is synthesized by surface phytoplankton into particle organic carbon (POC) that is exported from the ocean surface to depth, where it can be stored for years. An accurate quantification of downward POC flux is crucial for making reliable predictions of present and future atmospheric CO2 concentrations. A method based on the measurement of the disequilibrium of the radioactive pair 210Pb - 210Po in the seawater column is commonly used to estimate the POC export flux; as 210Po is preferentially scavenged by sinking particles in relation to its parent 210Pb, due to their different biogeochemical behaviour. To quantitatively measure the preferential 210Po scavenging it is necessary to isolate polonium from the aqueous matrix. This is a challenging step in seawater matrices due to its high content in salt content and organic matter, to which 210Po is strongly bound.

We propose an accurate, robust, and easy-to-perform onboard during cruises method for measuring ²¹⁰Po and ²¹⁰Pb in seawater using FeSO₄ (Fe²⁺) to coprecipitate polonium as Fe(OH)₂, with ²⁰⁹Po as a radioactive tracer. The method was first tested on groundwater and coastal seawater samples from southern Spain and validated with open-ocean samples from two campaigns around the PAP-Site Observatory (48.83° N, 16.5° W) in the North Atlantic. Replicate samples were also processed employing a traditional method based on the co-precipitation using Fe(OH)₃ (Fe3⁺). The results show that the novel method coprecipitation method using FeSO₄ (Fe²⁺) yields activities of ²¹⁰Po and 210Pb that are consistent with previously reported values. Applying this method, identical activity concentrations of 210Po and 210Pb were observed in deep ocean waters. In these waters no preferential scavenging of 210Po relative to 210Pb should occur, so the secular equilibrium is expected. Contrastingly, when coprecipitation is used with Fe(OH)₃ (Fe3⁺), the activity of ²¹⁰Po is underestimated by an average of 40 ± 5% in both coastal and open ocean samples, compared to coprecipitation with Fe²⁺. Furthermore, unexpected 210Po deficits relative to 210Pb were measured in deep ocean waters and secular equilibrium was not detected, evidencing that 210Po is underestimated using this technique.