Volcanic emissions in the Southern Ocean: an efficient and unexpected source of iron for this remote area

20 September 2021

Southern Ocean marine productivity participates in regulating the global climate through mitigating atmospheric CO₂ content. The Heard and McDonald Islands (HIMI) are two volcanically-active islands on the Northern Kerguelen plateau, in the Southern Indian ocean. There, seasonal blooms of phytoplankton evidence local iron (Fe) supply mechanisms to seawater which remain poorly understood. Perron and her colleagues (2021, see reference below) revealed increased atmospheric Fe loadings (average T_Fe = 1.8 ng m^-3), more than twice the Southern Ocean aerosol concentrations, in the vicinity and up to 500 km downwind from HIMI. Such high aerosol Fe concentrations were associated with volcanic emissions on Heard Island, as evidenced by similar atmospheric signature in aerosols compared to igneous rocks from the island’s volcano Big Ben. Interestingly, this freshly emitted Fe was more soluble than what is commonly observed in the mineral aerosols. Volcanic emission can therefore represent a significant source of bioavailable Fe to open ocean anaemic ecosystems. Such source is currently poorly or unaccounted for in biogeochemical models, likely causing models to underestimate marine productivity over the Southern ocean and its feedbacks on climate.

**Figure:** The figure shows an increase in the concentration of iron (larger point size), a vital nutrient for marine ecosystems, measured in the atmosphere near the volcanic islands of Heard and McDonald (HIMI, A6-A10) compared to aerosols over the Southern Ocean (A3-A5 and A12-A13). Iron solubility (colour scale) near the islands was also high (yellow-sih colour) which means that volcanic Fe emitted from HIMI is more likely to be used by marine phytoplankton compared to other crustal sources of Fe like dust and soil indicating, which commonly show solubilities < 3%. The stars on the figure indicate the location of the volcanic rocks collected and used to characterise volcanic emissions of Fe. The dashed lines represent the prevailing atmospheric air-masses influencing each regions studied (HIMI, the remote Southern Ocean and Australia coastlines).

Reference:

Perron, M. M. G., Proemse, B. C., Strzelec, M., Gault-Ringold, M., & Bowie, A. R. (2021). Atmospheric inputs of volcanic iron around Heard and McDonald Islands, Southern ocean. Environmental Science: Atmospheres. Access the paper: https://doi.org/10.1039/D1EA00054C

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