Impact of wildfires ash deposition on iron binding humic substances concentrations in surface waters: Results from a dissolution experiment

Wildfires contribute significantly to biomass burning. The deposition of ash from wildfires into surface ocean waters is a source of iron (Fe), namely pyrogenic Fe, and may enhance primary production in Fe-limited domains. However, due to the low solubility of Fe and the operational definition of its dissolved fraction, a portion of the dissolved Fe (DFe) released during ash dissolution may reprecipitate as authigenic inorganic colloids. This process can lead to an overestimation of the bioavailable pyrogenic DFe. To remain in a soluble form, Fe must be complexed with organic ligands capable of undergoing biochemical processes such as bacterial degradation, direct uptake, or photoreduction, leading to potentially bioavailable forms of DFe. Among the diverse range of iron-binding ligands, humic-type ligands (LFeHS) are important. LFeHS are ubiquitous in seawater, soluble, and may lead Fe to a bioavailable form. LFeHS are ubiquitous in seawater, soluble, and keep Fe in a bioavailable form. Here we present results from dissolution experiments. Ash samples collected in 2009 after wildfire events in the Spanish Mediterranean region were put in contact with non-euxinic, filtered Mediterranean surface seawater in a 7-day batch experiment. Four deposition fluxes were tested. The concentrations of DFe, fluorescent dissolved organic matter (FDOM), LFeHS, and the amount of Fe complexed by humic-type ligands were measured. Our results indicate that ash dissolution induces an increase in LFeHS, proportional to the ash concentration in the experimental medium. FDOM measurements confirm a time-dependent increase in humic-type material of terrestrial origin. Additionally, the observed increase in protein-like FDOM (C4) suggests that ash deposition enhances the modification of dissolved organic matter by bacteria. Using a simple kinetic model, we determined the dissolution rate constant for the tested ash. This constant can be incorporated into global oceanic models such as PISCES or REcoM to improve predictions of pyrogenic Fe bioavailability and its impacts on marine ecosystems.