Origin of carbon dioxide emanation from the 1979 Dieng eruption, Indonesia: Implications for the origin of the 1986 Nyos catastrophe

In February 1979, CO 2 emanations accompanying a phreatic eruption killed 142 people at Dieng volcano, Central Java. The gas emitted was nearly pure carbon dioxide, with subordinate amounts of methane and sulfur compounds. The δ 3C of the carbon dioxide was −4.0‰, identical to that of magmatic CO 2 from the nearby Merapi volcano and close to that of mantle-derived carbon. This, and data on helium supports the hypothesis that magmatic carbon dioxide was involved. Lower δ 13C, ranging from −4.6 in −8.4‰ were measured in the fumarole gases from the area. The lightest CO 2 occurs in samples in which methane is more abundant and which are poorer in 13C and thus may partly derive from biogenic carbon. The su34 S 32 S ratio of sulfur in all samples suggests a volcanic origin. It is proposed that magmatic carbon dioxide, accumulated beneath the Dieng volcanic complex, was the source of the lethal gas, the effusion of which was triggered by the pressure release generated by the phreatic eruption. The total CO 2 discharge of the 1979 Dieng event might have approached 0.1 km 3, i.e. close to the lower output estimated for the 1986 Nyos catastrophe. The Dieng example demonstrates that expansion and then effusion of pure magmatic carbon dioxide, accumulated at shallow levels beneath volcanoes, may account for a major hazard from phreatic eruptions, be it a trigger or only a consequence of the eruptions.