Award: OCE-1736591

This project measured the distributions of two naturally-occurring radioactive isotopes- polonium-210 and lead-210- in air and water samples on an oceanographic expedition from Alaska to Tahiti. The expedition was part of a large, international research program called GEOTRACES, which is designed to map concentrations of trace elements and their isotopes in the oceans. Radionuclides such as polonium-210 and lead-210 are useful because they can be readily measured and have sources and rates of decay that are known. For example, lead-210 is added to the surface oceans from the atmosphere, where it is produced from the radioactive decay of radon-222, a noble gas that emanates from continental rocks and soils. Lead-210 is also produced internally in the oceans through the radioactive decay of radium-226, its grandparent in the uranium decay series. In turn, lead-210 undergoes radioactive decay to polonium-210. The two radioelements, polonium (Po) and lead (Pb) have very different rates of decay (or half-lives) as well as different chemical behaviors in seawater. Polonium is incorporated into organic matter and cycled with it as it decomposes, while lead is chemically reactive and can be taken up on particle surfaces. The relationship between Po-210 and Pb-210 and Pb-210 and Ra-226 can be used to derive rates for a variety of oceanographic processes. Both the Po-210/Pb-210 and Pb-210/Ra-226 couples are very useful in determining the rate at which particulate organic carbon (POC) sinks out of the photic zone, where it is produced, into the deep ocean. This process, termed the "biological pump", effectively removes carbon from the upper ocean and thus the atmosphere from which it has been added to the surface ocean. Understanding the biological pump is critical to our understanding of the role of the oceans in the global carbon cycle. The results from this research project showed higher rates of POC export in the highly productive Subarctic (northern) portion of the transect and near the equator, where upwelling brings nutrient-rich water to the surface and produces high organic matter production. The respective radionuclide pairs are also useful in determining their rates of uptake onto particles and removal from the water column. The Pb-210/Ra-226 results show that a Pb atom spends less than 10 years in the upper water column before removal by sinking particles and much longer in the deeper ocean, where particle concentrations are low. These results can be used to model the distributions of stable (non-radioactive) Pb and similarly behaving trace elements in the ocean. Last Modified: 11/21/2022 Submitted by: J. Kirk Cochran