The open ocean often lacks the vital micronutrients, e.g. iron, that are required for primary production. While the biological requirements for these micronutrients are relatively low, their deficiency creates great expanses of low productivity in the surface ocean. Sources of micronutrients to the ocean may include rivers, bottom sediments, and hydrothermal vent systems but the surface ocean is effectively cut off from these margin sources. In the case of iron oxygenated seawater is an environment which causes rapid oxidation and precipitation of the dissolved fraction thereby removing it from the water column and reducing the overall bioavailability. The deposition of continental dust and anthropogenic emissions from land sources to the surface ocean therefore provides a major and direct source of micronutrients to areas of the ocean that other sources are unable to reach. 

The Eastern Tropical Pacific Ocean is just such a region. Near the South American continent, micronutrients are supplied by margin sediments, upwelling of deep water, and riverine systems supporting some of the most productive fisheries in the world. But as one moves west, the impact of those sources decreases resulting in surface waters that do not contain enough nutrients to support significant primary production. With relatively less land mass present in the Southern Hemishphere relative to the Northern Hemisphere, the ambient concentration of atmospheric aerosols (lithogenic and/or anthropogenic) is likewise lower in the south. 

Upon deposition to the surface ocean, aerosol material may quickly dissolve into an aqueous form which is potentially bioavailable. Bioavailable forms of micronutrients are accessible to organisms to support primary production which supports the overlying food web. We employ a leaching method that has been designed to replicate this process using collections of atmospheric aerosols. This method provides a precise characterization of the chemical composition of the soluble aerosol fraction as well as a measure of the fractional solubility of particular elements on a per cent basis. This estimate of fractional solubility is of fundamental importance to global atmospheric deposition models.

This study involved aerosol collection on an eight week research cruise between Ecuador and Tahiti. As expected, there was a significant gradient in the concentration of atmospheric aerosols moving offshore of South America. These findings reinforce that the open ocean, distant from margins, is subject to micronutrient limitation. Aerosol solubility varies by the element of interest. Aerosol iron solubility was low (~1.5%) further exacerbating the potential for iron to act as a biolimiting nutrient in the central South Pacific Ocean.

This project supported a postdoctoral researcher who conducted most of the analyses associated with the work. Results have been shared with the scientific community through presentations at scientific conferences and several manuscripts are in the final stages of preparation. These manuscripts will be submitted for publication in peer-reviewed journals. The data is also available for free access on the internet and has been included in the 2017 Intermediate Data Product produced by the international GEOTRACES program.

Last Modified: 03/14/2017
Modified by: Clifton Buck