Fe(II) Biogeochemistry in Coastal Waters

Ocean acidification and warming modify iron (Fe) redox cycling by altering reaction kinetics, speciation, and complexation processes that control Fe bioavailability in coastal waters. These drivers arise from both anthropogenic CO₂ emissions and natural volcanic inputs, which coexist in the ocean and allow the investigation of Fe(II) oxidation under contrasting chemical regimes.

Within the FeRIA project (PID2021-123997NB-I00), Fe(II) oxidation dynamics were investigated at coastal sites influenced by volcanic CO₂ emissions (Fuencaliente and Tazacorte, La Palma) and at sites mainly affected by anthropogenic CO₂ (El Hierro and Gran Canaria). Although both systems experience reduced pH, volcanic environments introduce additional chemical species that influence Fe complexation and redox reactivity.

Fe(II) oxidation rates exhibited strong spatial variability and were controlled by the combined effects of physi-cochemical parameters (pH, temperature, salinity, dissolved oxygen) and organic ligands. Lower pH consistently decreased Fe(II) oxidation kinetics, favouring longer Fe(II) lifetimes, while increasing temperature enhanced oxidation rates. Dissolved and particulate organic matter exerted a key control through complexation, either stabilising Fe(II) and inhibiting oxidation or promoting electron transfer depending on ligand composition and functional groups.

These results highlight the kinetic balance between acidification, warming, and organic complexation in regulating Fe(II) persistence. They also assess whether volcanic CO₂–impacted marine systems capture the dominant kinetic and complexation processes controlling Fe(II) oxidation under future anthropogenic ocean acidification.