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)3 210Po 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.