Award: OCE-1233688

We analyzed samples collected during a research expedition between Peru and Tahiti for naturally-occurring radionuclides of thorium and protactinium. We interpret the radionuclide distributions to inform us about rates of processes that supply and remove dissolved substances in the ocean, with an emphasis in certain elements that serve as essential nutrients for marine ecosystems, such as iron, zinc and cobalt. Although our emphasis is on nutrient elements, are results also inform us about the processes that remove contaminant metals from the ocean, such as lead. We find that the supply of these elements by dissolution of continental mineral aerosols (dust) deposited on the ocean is much less than we found in a previous study in the subtropical Atlantic Ocean. This is as expected because the sources of dust are much weaker in the eastern Pacific than they are in the eastern Atlantic, downwind from the Sahara desert. However, an unexpected finding was that despite the much lower rates of supply from dust, the concentrations of these elements in the Pacific Ocean were not very different from those measured in the Atlantic. This informs us that the rates of removal of these elements are much slower in the Pacific than in the Atlantic Ocean. In the case of dissolve iron, an essential nutrient whose scarcity in the ocean is thought to limit the growth of many organisms, the rate of removal was more than ten times slower than in the Atlantic. We attribute these differences to two factors: 1) these elements are removed faster from the Atlantic Ocean because they adsorb to dust particles as they settle through the water column to the sea bed, and 2) for elements like iron, that are essential to life but scarce in the ocean, especially in the Pacific Ocean, marine ecosystems have developed strategies to efficiently recycle and retain the iron, thereby slowing its removal. We also studied three aspects of the processes that remove these elements from the ocean. Principal findings include: 1) the elements are removed near the coast of Peru much faster than far from land, in the open ocean, because both higher biological productivity and erosion from the nearby continent supply many more particles to near-shore waters, and it is these particles that remove the dissolved elements by adsorption to the particle surfaces; 2) removal of the elements is strongly enhanced in the eastern half of the study area below depths of about 2500 meters, and although this feature is widespread, the specific processes responsible are not obvious; and 3) particulate iron and manganese oxides formed from hot solutions emitted from hydrothermal vents on the East Pacific Rise can be traced for 4000 km to the west of the Rise, and these particles are responsible for removing dissolved particle-reactive elements from the water column over the entire extent of the feature. Each of these observations provides important constraints on the rates and mechanisms (processes) of removal of dissolved chemicals in the ocean. Last Modified: 03/28/2018 Submitted by: Robert F Anderson