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

Perron, MMG ORCID: 0000-0001-5424-7138 2020 , 'Natural iron fertilisation of oceans around Australia : linking terrestrial aerosols to ocean biogeochemistry', PhD thesis, University of Tasmania.

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Abstract

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 LFe=6%)
measured downwind major dust sources to the west of Australia compared to enhanced Fe
solubilities (average LFe=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.

Item Type: Thesis - PhD
Authors/Creators:Perron, MMG
Keywords: Aerosol; Labile Fe; Solubility; Southern Ocean; Australia; Atmospheric source; Leaching protocol; Model prediction
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Copyright 2019 the author

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