Quantifying the Impact of Dust Deposition to the Southern Ocean using Dissolved Aluminium Concentrations

Supervisors

Andrew Bowie (UTas), Ed Butler (CSIRO), Tom Trull (UTas)

(PhD only)

Project Outline

This project will be suitable for graduates with degrees in Chemistry (preferably analytical), any Earth or Environmental Science discipline, Oceanography/Marine Science. A strong interest in the biogeochemistry of trace elements in the ocean would be desirable. Students should meet normal University PhD entry requirements

It is becoming increasingly important to understand the role that Aeolian deposition plays in supplying trace elements to the surface ocean, and consequentially the role that such episodic supply plays in moderating biological processes. Periodic input of dust laden with iron, an element essential for photosynthesis[1], is thought to stimulate phytoplankton growth, resulting in a shift in the dominant phytoplankton species composition and an increase in carbon fixation[2], with consequent effects on atmospheric CO2. This coupling is particularly evident in the iron-deficient Southern Ocean, where there is mounting evidence that dust particles from the arid Australian desert[3] provide a vital link in the planet's climate control system, and provide a key climate feedback loop linking the lithosphere, atmosphere and ocean[4]. Indeed, the polar ice core record shows that during the last ice age, average dust input to the oceans was tenfold greater than today[5], with high dust periods being closely associated with abnormally low atmospheric CO2 and temperature.

Despite being the most abundant metallic element in the Earth's crust (8.1% by weight), oceanic aluminium (Al) concentrations are extremely low predominantly due to its extremely short residence time (2-5 yr)[6]. A knowledge of its surface water distribution is thus extremely useful in that it can be used to identify the location and magnitude of inputs of continentally-derived dusts to the ocean. Vertical profiles of Al are probably controlled by dissolution of dust particles (at the sea-surface) and bottom sediments (at the ocean depths) balanced against scavenging by particulate matter. Recent work has also shown Al distributions to show large inter-ocean variability.

Methodology

To map Al distributions on a large scale requires the development of analytical methodology that is sensitive, precise, rapid, minimises the risk of sample contamination and operable on-board research vessels. Through this project, the candidate will initially develop and optimise a shipboard analytical method for the determination of Al in seawater. Recent successful methods have coupled flow injection analysis (FIA) with in-line preconcentration and fluorometric detection, and thus this approach will be an obvious starting point. FIA methods for Al have been based on the well-documented lumogallion chemistry[7,8] and the preconcentration abilities of resin-immobilised 8-hydroxyquinoline[9]. Sub-nanomolar detection limits have been achieved. The optimised system will be tested against a series of archived Southern Ocean and Atlantic samples. The validated method will then be deployed alongside other shipboard analytical methods for trace elements on major Southern Ocean research expeditions, for the real-time determination of Al.

Wider implications

Such Al data will be essential to investigate trace elemental ratios in the surface ocean. Dissolved Al released into surface waters through dust deposition may be used as a proxy to estimate atmospheric deposition into the ocean[10]. This will allow us to construct models which estimate the flux of aeolian iron to the surface ocean. Al/Fe ratios will provide an indicator on the solubility (and hence bioavailability) of iron from such dust, and aid in examining the role of iron-rich red dust from Australia in the climate system of the globally-important Southern Ocean.

It is also envisaged that this study may be extended to other elements which are strongly coupled to dust deposition (e.g. Mn), and are bio-essential for phytoplankton growth (e.g. Zn, Co, Cu). Similar FIA methods have been reported, based on a variety of detection systems (e.g. fluorescence, chemiluminescence, spectrophotometry), which may be developed by the candidate in parallel to the Al methodology.

Training

The successful applicant will join an active team within the University of Tasmania and CSIRO Marine Laboratories that is working on important aspects of trace elemental biogeochemistry in the Southern Ocean. The student will be trained in state of the art ultra-clean analytical procedures for use both at sea and on land. Further development of sampling procedures and analytical protocols will be made as required for the low level trace metal analysis. The successful candidate will also participate in a student focused seminar series, and receive additional training from the Research Higher Degrees Unit at the University in the range of skills required for a successful postgraduate career.

References

[1] Boyd, P.W. et al. (2000). A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron fertilisation of waters, Nature 407, 695-702.

[2] Lenes, J.M. et al. (2001). Iron fertilization and the Trichodesmium response on the West Florida shelf. Limnology and Oceanography 46, 1261-77.

[3] Gabric, A.J., Cropp, R., Ayers, G.P., McTainsh, G., Braddock, R. (2002). Coupling between cycles of phytoplankton biomass and aerosol optical depth as derived from SeaWiFS time series in the Subantarctic Southern Ocean. Geophysical Research Letters 29, art-1112.

[4] Jickells, T. D. and L. J. Spokes (2001). Atmospheric iron inputs to the oceans, in "The Biogeochemistry of Iron in Seawater", edited by D. Turner and K. A. Hunter, Wiley, Chapter 4, p 85.

[5] Edwards, P.R., Sedwick, P.N., Morgan, V., Boutron, C.F. and Hong, S. (1998). Iron in ice cores from Law Dome, East Antarctica: implications for past deposition of aerosol iron. Annals of Glaciology 27, 365-70.

[6] Orians, K.J., Bruland, K. W. (1986). The biogeochemistry of aluminium in the Pacific Ocean. Earth and Planetary Science Letters 78, 397-410.

[7] Hydes, D.J., Liss, P.S. (1976). Fluorimetric method for the determination of low concentrations of dissolved aluminium in natural waters. Analyst 101, 922-931.

[8] Resing, J.A., Measures, C.I. (1994). Fluorometric determination of Al in seawater by flow-injection analysis with in-line preconcentration. Analytical Chemistry 66, 4105-4111.

[9] Landing, W.M., Haraldsson, C., Paxeus, N. (1986). Vinyl polymer agglomerate based transition metal cation chelating ion-exchange resin containing the 8-hydroxyquinoline functional group. Analytical Chemistry 58, 3031-3035.

[10] Measures, C.I., Vink, S. (2000). On the use of dissolved aluminium in surface waters to estimate dust deposition to the ocean. Global Biogeochemical Cycles 14, 317-327.

Contact

A/Prof Tom Trull or tel +61 3 6226 2988