Anomalous gas emission offshore from Panarea Island (Aeolian Arc, Italy)

Abstract

Panarea Island is located in the eastern sector of the Aeolian arc at only 20 km from the active volcano of Stromboli. Its recentmost volcanic products date back to about 50 ka but a probable submarine eruption occurred there in 126 B.C. Near to volcanic islets 3 km E of Panarea, on 3 November 2002 an impressive submarine gas emission began from several points at a depth 8-20 m. The strongest gas emission, with jet velocity of 1 m/s, occurred from a small depression, possibly produced by a weak submarine phreatic explosion. Most of the gas was emitted from NE-SW trending fissures, whose direction matches with the main tectonic lineament of the Panarea- Stromboli sector. Only a very weak local seismicity was recorded. Gas was sampled and the physico-chemical characteristics of the seawater (T, pH, Eh, dissolved O2,) were measured in two campaigns carried out in November and in December. Gas mostly consists of CO2 (95-98.2 vol % ) and H2S (0.7-2%). Compared with previous chemical data on the submarine fumaroles of the same area, the 2002 emission is characterized by a marked increase of H2 and a decrease of the CH4/CO ratio. These data indicate the presence of high-temperature components with a likely magmatic provenance raised from a pressurized system through newly opened fractures. The sea water was chemically modified over a wide area, as indicated by the low pH and Eh values. In the first months of 2003 the gas emission started to decline, but it was still clearly visible four months after its beginning. This phenomenon recalled the attention on the volcanic hazard related to a possible submarine eruption near Panarea, where over 10,000 people live in the tourist season.