Award: OCE-2219285

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 previous results show that both methods give comparable results for lead-210, but the latter method can produce higher concentrations of polonium-210 than the former. Our research project hypothesized 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 was to determine under what circumstances the two methods give comparable results for polonium-210 concentrations in seawater. We compared the two methods on freshly collected water samples at a long-established oceanographic site (DYFAMED) in the Mediterranean Sea, where prior results were available for these radionuclides. The work involved field sampling and a series of laboratory experiments with the iron hydroxide method. We found that when biological productivity was high, the iron hydroxide method extracted significantly less polonium-210 from a seawater sample. This effect was evident in the euphotic zone, where light is present, (approximately the upper 100 meters of the water column) but could extend into the mesopelagic (the “Twilight Zone”) down to 1000 meters. These results support our hypothesis that polonium can be associated with dissolved organic matter in a manner that prevents its co-precipitation with iron hydroxide. This research has enabled recommendations of best practices for utilization of the methods going forward, even if different methods are used on different large-scale oceanographic campaigns such as GEOTRACES and EXPORTS. Our results will help produce better estimates of the sinking flux of particulate organic carbon using the relationship between polonium-210 and lead-210 in seawater.

Last Modified: 03/31/2026
Modified by: J Kirk Cochran