Two years of monitoring the Fe(II) oxidation rate constants in coastal seawater affected by the La Palma eruption

The eruptive process that took place in La Palma, Canary Islands, Spain, in September 2021 was the longest in the island records. The eruption lasted 85 days, had a major social and environmental impact and gave rise to the Tajogaite volcano. During this time, the lava formed two lava deltas on the west coast of the island (Tazacorte). Lava entered the sea during four different time periods. Previous research by González-Santana et al., (2024) presented the evolution of the iron size fractionation during the Tajogaite eruption, demonstrating a long term fertilization effect. In this work, we expand the research with Fe(II) oxidation kinetics studies after the post-eruptive phase to study the regeneration and the evolution of the zone. For this study, 7 coastal cruises were carried out between January 2023 and December 2024 in the proximities of the formed deltas. During each cruise, seawater samples were collected at 10 surface stations.

Iron is an essential trace metal in the development of life. Its speciation plays a key role in its bioavailability. Monitoring the oxidation of iron(II) in this environment is an opportunity to improve our understanding of its behaviour in coastal and volcanic environments. These environments are characterized by high organic compounds concentrations and variability. Iron can be complexed by organic ligands that affect its speciation. Understanding how iron impacts biogeochemical cycles in marine ecosystems is crucial. These behaviours are still poorly constrained, particularly in relation to factors like organic matter composition, and seasonally influenced variables in surface waters.

The experimental methodology is based on the Direct Flow Injection Analysis by Chemiluminescence (FIA-CL) method (Santana-González et al., 2018). Analysis at fixed temperature, pH and oxygen saturation conditions (T=15ºC, pH=8 and Sat[O₂]=100%) were carried out to remove the effects of these factors on the iron speciation. Results show a great variability between sampling points, with sections affected by groundwater discharge. Analysed oxidation rate constants (k ') were between 0.021 and 0.300 min^-1 (t_1/2 between 2.3 and 33.4 min, inversely), while theoretical calculations were of 0.048 min^-1 (t_1/2=14.6 min). Colloidal sized particles are thought to slow down oxidation rates. The colloidal-sized effect was demonstrated by performing the analyses using samples collected with different pore-sized filters (unfiltered, 0.2μm and 0.02μm), at different temperatures (T= 5, 10, 15 and 20ºC) and at constant pH=8, which allowed for the calculation of the activation energy (Ea) of each sample, around 120 kJ·mol^-1. Analysis at different pH conditions (pH=7.5, 7.8 and 8) and at a fixed temperature (T=15ºC) were performed to characterise pH dependent processes occurring in the colloidal-sized fraction.

Acknowledgments: This work was funded by the FeRIA project (PID2021-123997NB-I00) from the Ministerio de Ciencia e Innovación (Spain). A. Bullón-Téllez participation was funded by the PhD grant (ULPGC2023-2-01).

References:

González-Santana et al., (2024). Hot spot volcano emissions as a source of natural iron fertilization in the ocean. Science of The Total Environment, 957, 177638. https://doi.org/10.1016/j.scitotenv.2024.177638

Santana-González et al., (2018). Fe(II) oxidation kinetics in the North Atlantic along the 59.5° N during 2016. Marine Chemistry, 203, 64-77. https://doi.org/10.1016/j.marchem.2018.05.002