Geochemical and isotopic changes in the fumarolic and submerged gas discharges during the 2011-2012 unrest at Santorini caldera (Greece)

A geochemical survey of fumarolic and submerged gases from fluid discharges located in the Nea Kameni and Palea Kameni islets (Santorini Island, Greece) was carried out before, during, and after the unrest related to the anomalously high seismic and ground deformation activity that affected this volcanic system since January 2011. Our data show that from May 2011 to February 2012, the Nea Kameni fumaroles showed a significant increase of H-2 concentrations. After this period, an abrupt decrease in the H-2 contents, accompanied by decreasing seismic events, was recorded. A similar temporal pattern was shown by the F-, Cl-, SO42-, and NH4+ concentrations in the fumarolic condensates. During the sharp increase of H-2 concentrations, when values up to 158 mmol/mol were measured, the delta C-13-CO2 values, which prior to January 2011 were consistent with a dominant CO2 thermometamorphic source, have shown a significant decrease, suggesting an increase of mantle CO2 contribution. Light hydrocarbons, including CH4, which are controlled by chemical reactions kinetically slower than H-2 production from H2O dissociation, displayed a sharp increase in March 2012, under enhanced reducing conditions caused by the high H-2 concentrations of May 2011-February 2012. The general increase in light hydrocarbons continued up to July 2012, notwithstanding the contemporaneous H-2 decrease. The temporal patterns of CO2 concentrations and N-2/Ar ratios increased similarly to that of H-2, possibly due to sealing processes in the fumarolic conduits that diminished the contamination related to the entrance of atmospheric gases in the fumarolic conduits. The compositional evolution of the Nea Kameni fumaroles can be explained by a convective heat pulse from depth associated with the seismic activation of the NE-SW-oriented Kameni tectonic lineament, possibly triggered by either injection of new magma below Nea Kameni island, as apparently suggested by the evolution of the seismic and ground deformation activity, or increased permeability of the volcanic plumbing system resulting from the tectonic movements affecting the area. The results of the present study demonstrate that the geophysical and geochemical signals at Santorini are interrelated and may be precursory signals of renewed volcanic activity and encourage the development of interdisciplinary monitoring program to mitigate the volcanic risk in the most tourist-visited island of the Mediterranean Sea.