Atmospheric fluxes of mineral dust-derived soluble trace elements to the ocean using thorium isotopes (ThorMap)

Biological productivity (the growth of phytoplankton) is limited by the availability of iron (Fe) in at least 30% of the ocean. Fe is so insoluble in seawater that the large amounts entering from rivers cannot be transported far from the continental margins. The supply of Fe from dust falling on the ocean becomes the primary way to add Fe (and other elements important to life such as phosphorus) to the open ocean. The pattern and flux of Fe from the atmosphere to the surface ocean is therefore important for ocean ecosystems, and for the global carbon cycle (because ocean life consumes carbon).
Despite this importance, the flux of dust and of its incorporated metals to the ocean is poorly known. It is challenging to measure this flux directly, and other observational approaches require quite fundamental assumptions, which limit accuracy. At present, therefore, most estimates of dust flux rely on atmospheric models, and are generally considered to be uncertain by a factor of ten, particularly in remote regions.
In the proposed work, we will assess and use a new approach to quantify the inputs of dust and its associated micronutrients to the ocean. This approach relies on measurements of two biologically inactive, partially soluble components of dust: thorium (Th) and aluminium (Al). Two isotopes of Th are used in this assessment. 232Th, is present in continental rocks. If found dissolved in the open ocean, 232Th must have been recently added by dissolution of dust transported from the continents. Another isotope, 230Th, is formed within seawater by the decay of a uranium isotope. Its concentration in seawater reflects a competition between this known rate of formation, and removal due to its insoluble nature. We can therefore use 230Th to assess the removal rate of Th, including 232Th, from seawater. The 232Th removed must be replaced by input from dust to maintain the observed 232Th concentrations, so we can calculate the input of dust.
There are two main challenges to the reconstruction of dust fluxes from Th isotopes. One is that the solubility of Th in dust, a critical term in the flux calculation, is not well known. Our new results indicate that Th is amongst a small group of elements whose solubility is very little impacted by transport of dust through the atmosphere, while the solubilities of Fe, Al and several other biologically active elements are all altered greatly during transport.
Using aerosol samples collected on a series of research cruises, and at a sampling tower on Bermuda, we will assess the solubility of Th, the controls on how that varies during atmospheric transport, and its relationship to changes in Al and Fe solubility. We will also conduct laboratory studies on desert dust parent soils aimed at better understanding the unusual Th solubility in dust aerosols. Dust fluxes can also be calculated from dissolved Al concentrations, but these estimates are affected by changes in Al solubility during atmospheric transport.
The second challenge is that we do not know how far 232Th from the continents might travel after input at the coast. We will address this by incorporating 232Th into an ocean model. Such models have a proven ability to reconstruct 230Th, and we will develop them to also model 232Th, and to indicate where 232Th is dominated by coastal inputs rather than by dust. These models will also be used to assess the uncertainty in using Th isotopes to reconstruct dust inputs.
A large number of observations of Th isotopes in seawater has recently been measured during an international programme: GEOTRACES. We will add data from two further cruises, to complete a detailed coverage of Th and Al measurements for the Atlantic Ocean.
Combined use of the Th and Al tracers will therefore allow us to produce robust maps of dust inputs (from Th) and soluble Fe inputs (by taking account of the changes in solubility during transport using Al) for the Atlantic (with associated maps of uncertainty).

This study will benefit national and international agencies that are concerned with the impacts of desert dust on the environment and climate function, and the effects of this on society. Specifically, we will work together with the World Meteorological Organization's Global Atmosphere Watch (GAW) programme, who seek to improve understanding of dust transport through the atmosphere under present conditions and how that might change in the future.

The motivation for their work goes well beyond the area of nutrient supply and carbon cycling in the ocean that is the subject of our research study, because airborne dust affects weather and climate directly by absorbing radiation from the sun, affects human health (e.g. by transporting pathogens rapidly over great distances) and degrades visibility, severely affecting transport systems. Thus the study of dust forms part of GAW's mission to provide reliable scientific information for policymakers, support international conventions and contribute to improve the understanding of climate change and long-range transboundary air pollution.

Dust transport is very patchy and sporadic, making it difficult to study accurately. Our goal to significantly improve knowledge of how much dust is transported to the oceans is actually of great benefit to all other aspects of the study of dust in the atmosphere. Determining accurately how much dust is entering the oceans and where it is deposited, will aid GAW's efforts to understand how much dust is in the atmosphere and by what routes it is transported.

Our impact goals are therefore to interact with GAW's Total Atmospheric Deposition and Aerosols Scientific Advisory Groups as our work progresses. We will provide information on the methods used and the final products of our research, as both of these are likely to beneficial to them.