Hot spot volcano emissions as a source of natural iron fertilization in the ocean.

Dust deposition, river runoff and glacial melt are the main sources of trace metals to the surface ocean. In the Canary Islands, deposition is dominated by dry dust deposition from the Saharan desert. However, during 85 days, from 19 September to 13 December 2021, the main source of trace metals on the island of La Palma changed drastically. The eruption of the Tajogaite volcano released tonnes of volcanic ash. Concurrently, several lava flows reached the coastal waters. Volcanic activity on land became the main source of iron (Fe) into the coastal waters. Fe concentrations in seawater reached over 1900 nmol L-1. ∼99 % of the iron was found in the particulate phase (particles >0.2 μm wide; 1920 ± 50 nmol L-1). Colloids, smaller size particles (0.02 μm < colloids < 0.2 μm) represented ∼0.7 % of the iron pool (14.5 ± 0.5 nmol L-1). While the truly soluble phase represented ∼0.03 %. However, the soluble phase presented concentrations reaching 0.66 ± 0.05 nmol L-1, which is ∼10 times higher than generally found in open Atlantic Ocean waters. The results show how the Fe size fractionation evolved during the eruption, from a dominance of large particle phases to smaller-sized compounds.