Improving ²¹⁰Po low level measurements in seawater

The ocean is one of the largest sinks of atmospheric carbon. Surface phytoplankton absorb CO₂, forming the marine snow that sinks as a downward flux of particulate organic carbon (POC). A fraction of this POC flux reaches the deep ocean, where it can be stored for long periods of time. Quantifying this export flux enables us to estimate the ocean carbon sequestration. One of the most widely used methods of estimating the POC flux is measuring the disequilibrium of the radioactive pair ²¹⁰Pb-²¹⁰Po in the seawater column. We present an accurate, robust method, easy-to-perform onboard during cruises, for measuring ²¹⁰Po and ²¹⁰Pb in seawater using FeSO₄ (Fe²⁺) to co-precipitate polonium. This is a challenging step in seawater matrices, due to its high content in salt and organic matter, to which ²¹⁰Po is strongly bound. The method was validated with open-ocean samples collected on the PAP-Site Observatory. Replicate samples were processed employing a traditional method based on the co-precipitation with Fe(OH)₃ (Fe³⁺). The co-precipitation method using FeSO₄ yields activities of ²¹⁰Po and ²¹⁰Pb that are consistent with previously reported values. In contrast, when co-precipitation is done using Fe(OH)₃²¹⁰Po is underestimated by an average of 40 ± 5% compared to co-precipitation with FeSO₄. Identical activity concentrations of ²¹⁰Po and ²¹⁰Pb, i.e. secular equilibrium, were expected in deep ocean waters, indicating the cease of the POC export. This was found using the FeSO₄ method, but a systematic deviation from the secular equilibrium was detected when precipitating with Fe(OH)₃, evidencing the ²¹⁰Po underestimation.