Active hydrothermal discharge on the submarine Aeolian Arc

In November 2007 we conducted a water column and seafloor mapping study of the submarine volcanoes of the Aeolian Arc in the southern Tyrrhenian Sea aboard the R/V Urania. On 26 conductivity-temperature-depth casts and tows we measured temperature, conductivity, pressure, and light scattering and also collected discrete samples for helium isotopes, methane, and pH. The He-3/He-4 isotope ratio, an unambiguous indicator of hydrothermal input, showed a clear excess above background at 6 of the 10 submarine volcanoes surveyed. Marsili seamount had the highest anomaly, where the He-3/He-4 ratio reached a delta He-3 value of 23% at 610 m depth compared with background values of similar to 5%. Smaller but distinct delta He-3 anomalies occurred over Palinuro, Enarete, Eolo, Sisifo, and Secca del Capo. Although hydrothermal emissions are known to occur offshore of some Aeolian subaerial volcanoes, and hydrothermal deposits have been sampled throughout the arc, our results are the first to confirm active discharge on Marsili, Enarete, Eolo, Sisifo, and Secca del Capo. Samples collected over Lametini, Filicudi North, Alicudi North, and Alcione had delta He-3 near the regional background values, suggesting either absence of, or very weak, hydrothermal activity on these seamounts. Hydrocasts between the volcanoes revealed a consistent delta He-3 maximum between 11% and 13% at 2000 m depth throughout the SE Tyrrhenian Sea. The volcanoes of the Aeolian arc and the Marsili back arc, all <1000 m deep, cannot contribute directly to this maximum. This deep He-3 excess may be a remnant of tritium decay or may have been produced by an unknown deep hydrothermal source.