Evaluating the impact of improved dust representation and atmospheric iron chemistry in marine primary production and subsurface iron stocks

The impact of dust deposition on the fertilisation of marine ecosystems has been studied for decades. Despite the relevance of this air-sea interaction, aerosol chemical transformation, deposition over the ocean, and the eventual influence on ocean biogeochemistry (including carbon export) are poorly represented in most Earth System Models (ESM). For instance, the deposition of soluble iron (the chemical iron forms that phytoplankton can uptake) is often estimated in ESM as a constant fraction of deposited dust. This type of simplistic formulation underrepresents the interannual and spatial variability of the aeolian input of nutrients in marine ecosystems. 

In this work, we present a reconstruction of global ocean biogeochemistry for the last 30 years, where we evaluate the impact of newly produced iron deposition fields derived from the state-of-the-art atmospheric model EC-Earth3-Fe, which explicitly resolves the mineralogy of dust sources, includes a detailed representation of the atmospheric Fe dissolution processes and accounts for the contribution of other sources of Fe, such as anthropogenic combustion and biomass-burning. When compared to a standard run using climatological atmospheric inputs and constant dissolution rates the new simulation shows a contrasted response of marine primary production where production increases above 10% in large areas of the Pacific and the South Atlantic, while other smaller regions show an equivalent decrease. 

We also analysed the impact of the monthly resolved historical reconstruction of dust deposition (i.e., atmospheric model forced with observed meteorology) on the primary production’s interannual variability. Results showed no immediate impact of dust deposition variability on marine primary production. However, we found a replenishment of the subsurface stock of dissolved iron associated with the increase in dust deposition over the Equatorial Atlantic, the Indian Ocean and the subtropical Pacific. As this subsurface stock is one of the main seasonal inputs of iron through winter vertical mixing, it can induce delayed responses in marine ecosystems. Ongoing work is evaluating this hypothesis and comparing the simulated vertical distribution of dissolved iron in the water column against observations acquired by the GEOTRACES program.

In this presentation, we will also describe the efforts made in the new project BIOTA to understand how changes in aerosol transformation and deposition interact with the projected increase in upper ocean stratification, potentially enhancing the relative importance of aeolian nutrient inputs.