A different ocean acidification hazard—The Kolumbo submarine volcano example

Abstract

1039 Detailed knowledge of the geochemistry of CO 2 , the signature molecule of the 21 st century, is a modern day requirement for almost all geochemists. Concerns over CO 2 driven contemporary climate change, its relationship to past climates in Earth history, skills required for geologic CO 2 sequestration, and the rapid emergence of ocean acidifi cation as an environmental threat are all prime subject matter for the literate geoscientist today. In this issue of Geology, Carey et al. (2013, p. 1035) describe a new, interesting, and quite powerful natural example of the intersection of these concerns in describing the build-up of a large body of acidic, dense CO 2 rich sea water in the shallow crater of the Kolumbo volcano close to the Mediterranean island of Santorini. They present this fi nding in the context of a geochemical hazard to humans and as a natural test bed for CO 2 sequestration leakage from shallow injection. How real are these concerns, how similar are the situations to known threats, what strategies could be taken, and how useful an analog is this fi nding to the broad discussion over world-wide ocean acidifi cation or the specifi c concerns over leakage from geologic CO 2 sequestration? Kolumbo last explosively erupted in A.D. 1650, and ~70 people were killed by volcanic gases (Fouque, 1879) so plainly the potential is real. The build-up of CO 2 in the crater at the time of this earlier eruption is not known, but some signifi cant concentration must surely have been present. The population of Santorini has likely increased since then, and thus some concern is clearly justifi ed. But what would it take to destabilize the CO 2 -rich waters, how large is the chemical reservoir compared to known hazards, and how might this threat be detected? The fi nding of submarine volcanos close to populated areas, and discussion of hazards from their eruptions, is not new, with the 2011‐ 2012 eruption of El Hierro, Canary Islands (see the Geology Research Focus by Schmincke and Sumita [2013]) a prominent recent example. But Carey et al. focus not only on the sporadic eruptive events, but also on the physical stability of the large (2.0 ◊ 10 5 m 3 STP; 395 metric tons) pool of excess CO 2 accumulated within the crater only 500 m below sea level, and draw attention to the similarity with the deadly abrupt release of CO 2 gas from the volcanic lakes Monoun and Nyos in Cameroon, Africa (Sigurdsson et al., 1987). Clearly there are some similarities: in both locations there is a crater-contained pool of CO 2 -enriched water made dense, and thus held in place, by the dissolution of the gas under hydrostatic pressure. This arises simply due to the fact that the partial molal volume of CO 2 in sea water (~30 ml/mol) is much less than its molecular weight (44 g/mol), so that dissolution creates a dense fl uid. The effect is well known, and dense plumes of CO 2 -enriched water have been readily created in experimental investigations of deep seafl oor CO 2 sequestration scenarios (Brewer et al., 2005).