Project EARTH-17-GMH1: Quantification of dust and iron fluxes to the surface ocean with thorium isotopes

Supervisors:  Gideon Henderson, Yves Plancherel and Alan Hsieh

Atmospheric transport of dust from the continental crust to the oceans is a fundamental component of the earth system and its geochemical cycles.  Perhaps most significant is the role that such dust transport plays in adding the limiting micronutrient, Fe, to the surface ocean.  This Fe plays a critical role in controlling biological productivity, and therefore influences the ocean uptake of carbon and the vibrancy of ocean ecosystems.  Quantifying the flux of dust and its constituent metals to the surface ocean is challenging.  Satellite observations allow detailed assessment of the atmospheric loading of aerosols, but do not constrain the flux at which aerosols are removed and deposited to the surface.  Direct observations of such fluxes are sparse and most estimates are therefore based on atmospheric modelling (e.g. Mahowald et al. 2009).  These models are sophisticated, but their accuracy remains uncertain and the flux of dust to many regions of the ocean is not known to better than a factor of ten.

At Oxford we have pioneered development of a new geochemical approach to quantify oceanic dust fluxes.  This relies on the combined use of two isotopes of thorium; a highly insoluble metal in seawater.  Th-232 is present in continental rocks, so its presence in the open ocean signifies recent addition by dust.  Th-230, meanwhile, forms continuously from decay of 234U in seawater and constrains the removal rate of thorium.  The use of these isotopes to assess dust fluxes was first suggested by Hsieh et al. (2011), and has been followed up in two other studies (Hayes et al 2013; Deng et al. 2014).

Ph.D. research on this novel tracer could focus on its application to the Atlantic Ocean through new measurements of Th isotopes, and/or through modelling of thorium isotopes in the ocean to test and refine the tracer.  We have a good collection of seawater samples in hand, and agreements to collect more (offering potential sea-going opportunities and international networking within the GEOTRACES Programme;  We have also demonstrated computer models of insoluble metals in seawater, and have used them to conduct a first test of the limitations on the thorium tracer that might be imposed near ocean margins where there is direct, non-atmospheric, input of 232Th.  Samples and expertise are therefore in place to enable a graduate student to take the potential of the Th dust tracer and turn it into the first quantified maps of dust input to the ocean.   Such maps would be of interest to workers in many areas of the earth and ocean sciences.
This project would suit those interested in geochemistry and climate or ocean science.  It could involve lab work (in clean lab; with mass spectrometers; and on ships) and/or computer modelling, depending on the skills and interests of the student.   The research could also be taken in a number of other fruitful directions including the application of other U-series nuclides to assess the rate key processes in the ocean (231Pa, 228Ra, etc.).

Further Reading:

Deng, F., Thomas, A.L., Rijkenberg, M.J.A. and Henderson, G.M. (2014) Controls on seawater 231Pa, 230Th and 232Th concentrations along the flow paths of deep waters in the Southwest Atlantic. Earth Planet. Sci. Lett. 390, 93-102.

Hayes, C.T., Anderson, R.F., Fleisher, M.Q., Serno, S., Winckler, G. and Gersonde, R. (2013) Quantifying lithogenic inputs to the North Pacific Ocean using the long-lived thorium isotopes. Earth Planet. Sci. Lett. 383, 16-25.

Hsieh, Y.-T., Henderson, G.M. and Thomas, A.L. (2011) Combining seawater Th-232 and Th-230 concentrations to determine dust fluxes to the surface ocean. Earth Planet. Sci. Lett. 312, 280-290.

Mahowald, N.M., et al. (2009) Atmospheric Iron Deposition: Global Distribution, Variability, and Human Perturbations. Annual Review of Marine Science 1, 245-278.