Volcano eruptions can be climate-relevant. Airborne volcanic ash which reaches an ocean increases the likelihood of algal blooms. Dissolving in sea water, the iron contained in the ash–a natural fertilizer–fosters algal growth. While spreading, the algae extract CO2 from the atmosphere and absorb it into their biomass with a positive effect on the climate. This is all due to a fascinating natural phenomenon: Upon being ejected from volcanic craters, ash iron is insoluble, whereas it dissolves in the sea. The hows and whys of this modulation were unexplored to date.
It is impossible to take specimens during a volcanic eruption. Therefore, in trying to recreate volcanic activity, my colleagues from the CliSAP Cluster of Excellence and I rely on computational models. We employ equations to simulate the physical and chemical processes which take place within the hot cloud of ash and gas. On this basis we can investigate, for instance, what conditions cause changes in the chemical compounds of an ash cloud. Is the varying temperature inside the volcanic plume decisive? Or are the plate tectonics underneath the volcano–which trigger eruptions in the first place–a crucial factor?
In fact, a series of processes within the volcanic cloud contributes to changing the chemistry of the iron and to literally dissolving it. Initially, volcanoes spew large quantities of ash particles and gases at about 1000 degrees centigrade. Volcanic ash, however, cannot be compared with common ash. It is best conceived as consisting of the most minuscule lava fragments. My model indicated what happens exactly when a cloud of smoke cools off on its rapid passage into the atmosphere. Its steam condenses at temperatures just below 100 degrees centigrade, forming a kind of shell around the ash particles. The thus created droplets absorb the surrounding gases, such as hydrogen chloride and hydrogen fluoride. Hence, the droplets turn into acid, for example, into the highly corrosive hydrochloric acid which attacks the surface of the ash particles. On account of this etching process, iron ions are released from the ash particles and become soluble in water.
My model helped me find out which processes enhance the iron solubility of volcanic ash. Now, I aim to improve the model to conduct investigations that provide a more detailed insight. Our current research, for instance, is still based on the assumption of a dry milieu. Yet, when volcanoes erupt in tropical regions, the humid climate influences condensation. Clarifying whether more or less iron is released from the ash under humid conditions could be significant, as the intensity of the marine algal bloom will vary accordingly.