Award: OCE-1736612

GEOTRACES is an international program which aims to map the concentrations and fluxes of key trace elements and their isotopes (TEIs), and to identify the processes controlling TEI distributions in the oceans. These chemical species play important roles in the ocean as nutrients, tracers of current and past oceanographic processes, and as contaminants from human activity. Their biogeochemical cycling has direct implications for research in such diverse areas as the carbon cycle, climate change, and ocean ecosystems. Many processes in the ocean cannot be directly observed and as such, tracers are used to provide important constraints on their rates and pathways. Radionuclides are useful tracers because they can be readily measured and they have well known sources and decay rates. In this study, we measured the distributions of two naturally occurring radioactive isotopes (Polonium-210 or Po-210, and Lead-210 or Pb-210) in air and water samples from a transect of the Pacific Ocean from Alaska to Tahiti, French Polynesia in 2018. The pair of isotopes are tracers for several processes affecting the distribution and fate of carbon and trace elements in the marine environment. In the ocean, Pb-210 has two sources: 1) atmospheric deposition of Pb-210 produced from the radioactive decay of Rn-222 emanating from continental rocks and soils, and 2) Pb-210 produced internally by the decay of its grandparent, Ra-226, dissolved in seawater. Po-210 is produced by the radioactive decay of Pb-210. The two isotopes have different chemical behaviors in seawater: polonium is incorporated into organic matter while lead is chemically reactive and taken up on particle surfaces. The specific processes addressed in this project were as follows: the sources and atmospheric deposition of micronutrients and trace elements, particle scavenging, and the transfer (or export) of particulate organic carbon from the surface layer to the deep ocean. Sources of aerosol particles were identified using Pb-210 (continental source) and Be-7 data from a related project. Be-7 is an isotope with an upper troposphere source. The results point to a significant continental influence in aerosols near Alaska and in the North Pacific, while in the equatorial and South Pacific, aerosols are supplied to the marine boundary layer largely from the upper troposphere. The sources affect the composition of aerosol particles and ultimately play a role in ocean chemistry. For example, continental sourced aerosols will deliver more lithogenic elements such as aluminum and titanium to the area of deposition. In the water column, Po-210 and Pb-210 are useful in determining the rate at which particulate organic carbon sinks out of the photic zone. This process, termed the ?biological pump?, removes carbon from the upper ocean and thus the atmosphere from which it has been added to the surface ocean. Understanding this process is critical to our understanding of the role of the oceans in the global carbon cycle. Results from this project showed higher rates of POC export in the highly productive subpolar portion of the transect and near the equator, where upwelling brings nutrient-rich water to the surface. Last Modified: 01/04/2023 Submitted by: Mark P Stephens