Natural iron fertilisation of oceans around Australia : linking terrestrial aerosols to ocean biogeochemistry

• Main Author: Perron, MMG
• Format: Thesis
• Language: English
• Published: 2020
• Subjects: Aerosol; Labile Fe; Solubility; Southern Ocean; Australia; Atmospheric source; Leaching protocol; Model prediction; Antarctic; Kerguelen; Heard Island; Indian; Heard; McDonald Islands; Big Ben; Antarc*; Antarctica
• Online Access: https://eprints.utas.edu.au/35207/; https://eprints.utas.edu.au/35207/1/Perron_whole_thesis.pdf

Half of the world oceans’ productivity is limited by the availability of the micronutrient iron (Fe), including marine phytoplankton in high nutrient low chlorophyll regions and diazotrophic bacteria (nitrogen fixers) in low latitude regions. Iron is therefore a key regulator of global climate through its control on the biogeochemical cycling and drawdown of atmospheric carbon dioxide. One major source of iron to marine ecosystems is atmospheric transport of aerosols from desert dust, biomass burning, volcanoes and anthropogenic emissions. Recent efforts have been made through international research programs such as GEOTRACES and SOLAS to gather critical biogeochemical data on aerosols. However, the small and sparsely located aerosol sources present in the Southern Hemisphere (SH), compared to the Northern Hemisphere, remain largely understudied. Such paucity of field observations induces large uncertainties in model predictions of atmospheric Fe deposition, and the subsequent response of marine biological productivity in this region. Australia has often been considered to be one of the largest dust sources to the SH oceans in past interglacial periods, with strontium and neodymium isotopic signatures of Australian soil found in East Antarctic ice cores, but this is based on only a small dataset. This thesis reports data from an unprecedented ship-board atmospheric sampling effort in the vast marine region surrounding Australia and in the Southern Ocean (SO), between the latitudes 33 - 66 S, and the longitudes 71° - 150° E). A further obstacle to better understand the oceanic deposition of atmospheric Fe is the current lack of standard laboratory procedures for the analysis of key chemical parameters in aerosols. Thus, there are large discrepancies in global aerosol datasets (GEOTRACES Intermediate Data Products and SOLAS Implementation Product). After an extensive review and assessment of existing laboratory techniques, a 3-step leaching protocol was developed and assessed. It defined and quantified the concentrations and solubilities (soluble and labile fractions) of a range of bio-essential trace metals (including Fe), as well as key chemical tracers of dust source (aluminium and titanium) and anthropogenic emissions (lead and vanadium) in aerosols. This leaching protocol was then applied to a large set of aerosol samples, providing the first continental-scale insight of Fe properties in aerosols collected in ocean waters surrounding Australia’s coast. Further analysis of key atmospheric tracers (levoglucosan and radon concentrations) and the use of atmospheric tools (air-mass back-trajectories and satellite fire detection) highlighted a striking contrast between rather low Fe solubility (average L\(_{Fe}\)=6%) measured downwind major dust sources to the west of Australia compared to enhanced Fe solubilities (average L\(_{Fe}\)=14 - 22%) found in aerosols from more industrial regions to the east and south west of Australia. Moreover, surprisingly high Fe solubilities (>20%) in all aerosols from northern Australia were attributed to the frequent bushfire activity previously reported in this region. These results suggest that Australia contributes an important atmospheric source of labile Fe to the southern Indian Ocean and to the Arafura and the Timor seas, north of Australia. In regions farther offshore, along the atmospheric dust transport path south of the Australian continent, external sources of Fe (and other trace metals) to surface SO waters were much scarcer. Aerosol sources may, however, trigger disproportionate responses from anaemic (i.e., iron-deplete) phytoplankton in this region. Atmospheric sampling along the GEOTRACES oceanographic section GIPY06 (SR3 section along 104ºE, [132.0ºE, 150.0ºE, 66.5ºS, 42.8ºS] Marine National Facility - MNF, Australia) allowed the measurement of a 1000-fold gradient (13 – 22235 pg m\(^{-3}\)) in Fe concentration between aerosols collected close to Tasmania and Antarctica (high values) compared to open SO aerosols (low values). Prevailing air masses along this section (derived from the HYSPLIT trajectory analysis tool) successively originated from Australian, South America and southern Atlantic Ocean, and finally from Antarctica. Long-range atmospheric transport, over thousands of kilometers, was associated with high aerosol Fe solubilities (22 - 100%), likely resulting from increased atmospheric processing and as well as a mixture of atmospheric air-masses of various origins. A second atmospheric study was undertaken in the SO around the high iron high productivity waters near volcanically-active Heard and McDonald Islands (HIMI, HEOBI voyage [71.3ºE, 147.5ºE, 54.2ºS, 31.9ºS] MNF), a region where new internal lateral Fe inputs from the Kerguelen plateau have been observed. Atmospheric input of Fe-rich aerosols was identified at proximity to Heard Island (~2000 pg m\(^{-3}\)), which was attributed to emissions from the island’s erupting volcano “Big Ben”. However, such emissions were associated with relatively low soluble Fe fractions in aerosols (up to 9%). Aerosols collected in the HIMI region only comprised soluble Fe (as defined by our 3-step leaching protocol) which may highlight solubility-enhancing interactions with acidic volcanic emissions near the island and intense atmospheric processing over long-range transport in the open SO. The findings presented in this thesis significantly contribute to a better understanding of Febearing aerosol geochemistry in the SH. Finally, these new data on Fe solubilities and concentrations in various oceanic regions close to Australia and further away in the Southern Ocean was implemented into a biogeochemical model to fine-tune the model projections on atmospheric Fe over the SH oceans. This work also highlighted the need for defining standard laboratory procedures to gather coherent and comparable aerosol data globally. An ongoing sampling effort along new and repeat oceanic transects would also enhance identification of key recurring aeolian deposition features (rather than single sporadic events) to aid with climatological assessment of the effect of trace metals delivered by aerosol deposition on marine productivity.