Low Source-Inherited Iron Solubility of South American Dust Limits Fertilization Potential of the Southern Oceans

COSENTINO, NICOLÁS J.; SIMONELLA, LUCIO; GAIERO, D. M.; PALOMEQUE M.E; MONTES M.L.; CROOT P.L

New Orleans

Congreso; AGU Fall Meeting 2021; 2021

AGU

Mineral dust can impact the global climate through fertilization of iron-deficient ocean waters and subsequent CO2 capture. From sink to source, this mechanism is controlled by various phytoplankton pathways of iron assimilation, post-depositional interactions between iron in dust particles and the ocean chemistry, atmospheric processing, and the inherited iron geochemistry of dust-emitting surface sediments. While southern South America has a small contribution to global natural dust emission, it is the greatest source of dust to the southern oceans (<45ºS), the most extensive of the iron-deficient oceans. To assess the fertilization potential of South American dust, we characterized the iron geochemistry and solubility of dust-emitting surface sediments from the Puna-Altiplano plateau (~19ºS) to southernmost Patagonia (~54ºS), as well as dust samples collected close to these sources. Specifically, we obtained grain-size distributions through laser diffraction, semi-quantified the size-resolved mineralogy through powder X-ray diffraction, determined the total iron content and performed iron leaching experiments, and constrained iron speciation through Mössbauer spectroscopy. We find that while total iron content in <63-μm surface sediments and bulk dust (4 ± 2 wt%, 1σ) is close to the global average, the fractional iron solubilities (FFS) in pure water (0.05 ± 0.05%) and seawater (0.03 ± 0.04%) are very low compared to analogous measurements globally. In contrast, FFS in 1% HNO3 solution (5 ± 4%) is similar to global measurements of mineral aerosols under non-acidic leaching conditions. Preliminary analyses show that the main controls influencing iron solubility are clay content (direct), grain size (inverse) and iron speciation (lower solubilities for higher fractions of iron as hematite). The spatial uniformity in low values of iron solubility of South American dust-emitting surface sediments and close-to-source dust has important implications for the present-day fertilization potential of mineral aerosols derived from this region, which is characterized by non-acidic atmospheric conditions that limit solubility enhancement due to atmospheric processing.