Projections of atmospheric soluble iron deposition under CMIP6 emission scenarios.

Abstract:

The atmospheric supply of iron to the oceans is fundamental to oceanic primary production and carbon dioxide uptake. Ocean productivity depends specifically upon the soluble or bioavailable fraction, which is poorly constrained by measurements. Recent studies suggest roughly a doubling of soluble iron deposition to the ocean since preindustrial climate conditions due to a combination of higher dust and combustion iron emissions along with more efficient atmospheric processing. In our study, we will assess past, present, and future soluble iron deposition using an advanced iron cycle module implemented into the EC-Earth Earth System Model. Mineral dust is calculated on-line considering an updated mineralogy dataset and particle size distribution at emission. The model takes into account primary emissions of insoluble and soluble Fe forms, associated with dust minerals and combustion aerosols. Dissolution processes due to atmospheric acidity and organic ligands, for both Fe-containing dust and combustion aerosols, are treated in the model as kinetic processes, accounting for 1) a proton-promoted, 2) an oxalate-promoted Fe dissolution (with oxalate calculated on-line), and 3) a photo-reductive dissolution. We run time-slice simulations using the atmosphere-chemistry model configuration using past, present, and future sea surface temperature and sea ice concentration climatologies obtained from EC-Earth CMIP6 coupled model experiments. We have chosen three different CMIP6 future scenarios representing different socio-economic pathways (SSP 1, 2, and 3) and different forcing levels by the end of the century (RCP 2.5, 4.5, and 7.0, respectively). We address the uncertainties attributed to the natural dust cycle, by perturbing the dust emissions in our past and future experiments. Our setup allows estimating the soluble iron deposition into the ocean while quantifying the contribution from natural and anthropogenic sources under a range of scenarios.