Measuring 210Po in Seawater: A Comparison of Methods

The natural radionuclides lead-210 and its granddaughter polonium-210 are present dissolved in seawater and have multiple applications for understanding ocean processes, including determining how fast other similar elements are removed from the oceans and as indicators of the sinking of particulate organic carbon from the photic zone. The latter process is an important control on the level of carbon dioxide in the atmosphere and is thus important to measure. Significantly, the two radionuclides have very different geochemical behaviors- lead is taken up on particle surfaces while polonium can be incorporated into organic matter and recycled with it. From the 1970’s to the present, these radionuclides have been measured on many oceanographic cruises. Two methods have been used to extract them from seawater for measurement. One involves precipitating iron hydroxide from a water sample; the other involves precipitation of a colloidal chelating agent added to the samples. Historical and recent results suggest that both methods give comparable results for lead-210, but the latter method can produce higher concentrations of polonium-210 than the former. This research hypothesizes that polonium-210 in seawater can be associated with dissolved organic matter, especially in the upper water column where organic matter cycling is high, and this can prevent its extraction from a seawater sample by precipitation of iron hydroxide. The aim of this research project is to determine under what circumstances the two methods give comparable results for polonium-210 concentrations in seawater. This research will compare the two methods at a long-established oceanographic site (DYFAMED) in The Mediterranean Sea, where prior results are available for these radionuclides. The work will involve field sampling and a series of laboratory experiments with the iron hydroxide method. This will enable recommendations of best practices for utilization of the methods going forward, even if different methods are used on different oceanographic campaigns such as GEOTRACES and EXPORTS. The project will support one graduate student and outreach events at American Natural History Museum, New York. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.