Sensitivity of soluble iron deposition to soil mineralogy uncertainty

Abstract

Mineral dust emitted from arid and semi-arid areas has several effects on the Earth system (e.g., perturbation of the radiative budget, interaction with cloud processes, implications on ocean and land biogeochemical cycles). Mineral dust aerosols are mixtures of different minerals whose relative abundances, particle size distribution, shape, surface topography, and mixing state influence their interaction with the Earth system. However, Earth System Models (ESMs) typically assume that dust aerosols have a globally uniform composition, neglecting the known variations in the sources’ mineralogical composition. This work investigates the sensitivity of a key biogeochemical cycle, the iron (Fe) cycle to uncertainties in the description of soil mineralogy in dust-producing areas. Airborne mineral dust is the primary input of Fe to the open ocean. Fe constitutes a fundamental micro-nutrient for marine biota in its soluble form. It is, in fact, the limiting nutrient in remote regions of the open ocean known as High Nutrient Low-Chlorophyll (HNLC) regions (e.g., the Southern Ocean), where the Fe supply occurs mainly through atmospheric deposition. Ocean productivity relies on the availability of limiting nutrients. Hence, the ocean’s ability to capture atmospheric CO2 in HNLC regions highly depends on the atmospheric deposition of soluble Fe. Fe abundance in soils is usually set to 3.5% [1], and its solubility is considered to be less than 0.1% [2]. However, both observations and modeling studies suggest that the solubility of Fe from dust increases downwind of the sources [3]. A primary mechanism leading to this increase in Fe solubility is acidic (proton-promoted) dissolution. Low pH conditions in aerosol water favor Fe dissolution by weakening Fe-O bonds of Fe oxides in dust [4]. Other physical and chemical mechanisms that enhance Fe solubilization involve photochemical reduction and organic ligand (e.g., Oxalate) processing [5]. Modeling the global dust mineralogical composition presents critical challenges. First, soil mineralogy atlases for dust modeling are derived by extrapolating a sparse set of mineralogical analyses of soil samples that are particularly scarce in dust source regions. Moreover, atlases are based on measurements following the wet sieving technique that tampers the undisturbed parent soil size distribution by breaking coarse particles and replacing them with smaller ones [6]. In this work, we assess the implications of soil mineralogy uncertainties on bio-available Fe delivery to the open ocean by using a state-of-the-art ESM, EC-Earthv3, where a detailed atmospheric Fe cycle and two different data sets that characterize the soil composition over dusty areas have been implemented [7] [8].