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Final Activity Report Summary - MARIRON (Modelling the marine iron cycle: Past and future biogeochemical-climate feedbacks)

Iron is known to be a limiting nutrient for phytoplankton growth over large areas of the oceans, thus moderating the oceanic biological pump's ability to sequester carbon dioxide (CO2) in the deep ocean. The major source of iron to the open ocean is via the aeolian pathway. Thus, a change in the flux of aeolian iron to the ocean should affect the ability of the ocean to absorb CO2.

Ice core reconstructions between dust flux and carbon dioxide show an inverse relationship. During times of high dust flux, CO2 concentrations are low and vice-versa. This led to the implication of changes in aeolian iron as a driver in CO2 changes during Earth's history and as a possible mitigator of anthropogenic-induced rising CO2.

In order to test the effect that changes in aeolian iron could have on CO2 concentrations in the past and the future, we added a parameterisation of oceanic iron cycling to the Bern three-dimensional global ocean biogeochemical and circulation model. Dust records from Dome-C, Antarctica were used to scale dust deposition in the model over four Antarctic warm events of the last glacial period. Our results suggested that changes in dust flux to the Southern Ocean played a limited role in modulating CO2 variations. The impact of iron fluxes on CO2 was dependent on parameter values chosen for the iron-binding ligand.

In order to test the efficacy of iron fertilisation as a carbon mitigation solution, we increased aeolian iron fluxes by 100 times in various regions of the ocean for 1 000 years. Atmospheric CO2 was most sensitive to iron additions in the Southern Ocean, resulting in a CO2 reduction of 10 ppmv. This reduction though was a drop in the bucket compared to the 110 ppmv increase in CO2 from 1850 to today.

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