Mineral dusts are a major source of micronutrients (e.g. Fe) that limit phytoplankton growth in the open ocean, in particular in the so-called “High Nutrient Low Chlorophyll” (HNLC) oceanic zones. The southern Ocean is by far the largest of all HNLC regions and thus has the potential to greatly enhance the biological CO2 pump at the global scale. As the aerosol fluxes and sources in the southern Ocean are not well constrained and the potential impact of anthropogenic airborne particles may be larger than expected, a multidisciplinary study is being carried out on dust-bearing snow samples collected in NE Antarctica. Our goals are multiple: (i) determine the mineralogy, morphology and chemical composition of these dusts and, (ii) quantify, by using heavy stable isotopic signatures, the origin and the relative contribution of desert-derived, volcanic and anthropogenic particles in the dust, (iii) estimate the bioavailable fraction of bio-essential elements such as Fe through chemical extraction.
Snow samples were collected at four sites: two a few kilometres from the sea, at the summit of the Derwael Ice Rise (about 200km North of the Belgian Princess Elisabeth Station) and two other locations in a continental area (~225km inland) around the Princess Elisabeth station in the Sør Rondane Mountains. Three litres of snow from each site were melted and filtrated on 0.2 µm poresize NucleporeÓ polycarbonate filters in an ISO 5-class clean room. Subsequently, a series of single particle analyses were performed by (i) FEG-SEM (Field Emission Gun-Scanning Electron Microscopy) to determine particles-size distribution of dust; (ii) automated-SEM-EDS (Energy Dispersive Spectroscopy) to estimate the chemical composition of individual dust particles; (iii) TEM-SAED analysis (Transmission Electron Microscopy Selected Area Electron Diffraction) to identify the minerals present. In addition, trace element compositions of the bulk samples were analyzed by high-resolution ICP-MS.
Preliminary data in elemental composition and mineralogy indicate similarities between dust samples and the rock-forming minerals from the Sør Rondane Mountains suggesting a local dust source. However, major distinctions can be made between coastal and inland dust samples: (i) mineralogical distributions are very distinct; (ii) a large proportion of the quartz and feldspath dust particles exhibit surprisingly enrichment in iron (less than 20% of particles for inland samples and up to 80% for coastal samples), which is probably due to surface Fe-rich coating/aggregates; (iii) coastal samples are heavily enriched (vs. upper continental crust reference values) in Pb and Ni. These Fe, Pb and Ni enrichment trends tend to suggest an external and distal source of dust at the coast, potentially impacted by anthropogenic activities.
To complement those preliminary results, new sampling campaign will take place in December 2014 at the same locations to acquire large quantity of snow. In addition, passive dust collectors will be placed for a period of one year along a transect connecting the Sør Rondane Mountains and the Derwael Ice Rise. The perspectives of this new sampling campaign aim to fulfil our analysis plan through isotopic analyses and chemical extractions.
This in-depth characterisation study will improve our knowledge and understanding of dusts reaching the coast of NE-Antarctica, which represent a proxy of the dust materials supplied to the Southern Ocean.
by Aubry Vanderstraeten, PhD student