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

Abstract 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 H2 concentrations. After this
period, an abrupt decrease in the H2 contents, accompanied by
decreasing seismic events, was recorded. A similar temporal
pattern was shown by the F−, Cl−, SO4
2−, and NH4
+ concentrations
in the fumarolic condensates. During the sharp
increase of H2 concentrations, when values up to 158 mmol/
mol were measured, the δ13C–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 H2 production from
H2O dissociation, displayed a sharp increase in March 2012,
under enhanced reducing conditions caused by the high H2
concentrations of May 2011–February 2012. The general
increase in light hydrocarbons continued up to July 2012,
notwithstanding the contemporaneous H2 decrease. The temporal
patterns of CO2 concentrations and N2/Ar ratios increased
similarly to that of H2, 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.

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