Experimental study investigating the abiotic dissolution kinetics of iron during lithogenic particle-water interactions in high-energy regions

Recent oceanic elemental budget calculations have highlighted the importance of considering continental margins as a source of elements to the oceans, allowing elemental and isotopic budget balancing, which are imbalanced without consideration of this source. Quantification of source fluxes of the essential micronutrient iron (Fe) to the oceans has received much attention in recent decades. Rivers and continental runoff are an important source of Fe to the oceans; however, they deliver Fe mainly in the particulate form, most of which is deposited on the adjacent continental shelf. These (mostly lithogenic) particles may be reworked by internal and coastal waves, sediment resuspension and diagenetic processes, leading to the dissolution of Fe into seawater. The supply of Fe from non-reductive dissolution of lithogenic particles may be significant on a global scale due to the predominance of oxic shelf regions. However, to properly address the so-called boundary-exchange, process studies at these source regions are lacking.

Here we address the abiotic processes leading to dissolution of Fe caused by agitation during high-energy continental freshwater transport (rivers, ravines, torrents) and on wave impacted coastal regions, such as beaches. To explore these processes, as a first approximation, we have investigated the dissolution kinetics of isolated pure minerals (olivine, pyroxene and plagioclase) in distilled water and synthetic seawater, in shaken closed system batch reactors. Fe results are supported by Mn, Cu, Ni, Cu, Zn and Pb concentrations. Preliminary results suggest simultaneous release and removal processes occurring throughout the experiments, with release processes predominating during the start of the experiments and removal processes predominating towards the end. Stages and degree of release differ between freshwater and seawater experiments. Secondary mineral phases were detected in particles recovered at the end of the experiments, which may play an important role in the removal of Fe from solution (direct incorporation and/or adsorption). Fe isotopic ratios are used to interpret geochemical processes. Our data are fitted to a model that considers mineral dissolution kinetics, combined with Fe²⁺ oxidation rates and Fe³⁺ precipitation rates.