Subaerial lava as a source of coastal hydrothermal iron

The interaction between lava and seawater has received significant interest, since multiple reactions can take place which act as sources or sinks of elements, consequently changing the chemical properties of the surrounding seawater. However, most of these studies are based on underwater hydrothermal vent sites. On the 19th of September 2021, 6 km away from the coast, the Tajogaite volcano in La Palma, Canary Islands started its eruptive phase.

Lava flows reached the shoreline during three events on September 28th, November 10th, and November 22nd, strongly affecting the seawater properties as they interacted with the seawater. The evolution of surface and water column physicochemical properties (temperature, salinity, carbonate system variables, dissolved oxygen), iron concentrations (soluble (sFe), dissolved (dFe) and total dissolvable (TdFe)) concentrations, and iron ligands were characterized during 13 visits to the frontal zone of the newly formed deltas. A large volume of hot (50°C measured on November 12th) and high salinity seawater promoted pH values in the frontal zone of 7.0 with important decreases in alkalinity and total dissolved inorganic carbon. These waters were also characterised by high iron concentrations reaching 18 nM, 117 nM, and 2024 nM for sFe, dFe, and TdFe, respectively. The affected waters travelled in the top meter of the water column over 1 km away from the coast, naturally fertilising the surrounding photic layer.