Statistical analysis reveals spatial and temporal anomalies of soil CO2 efflux on Mount Etna volcano (Italy)

Extensive geochemical surveys were carried out on the Western flank of Mt. Etna volcano for the determination of soil CO2 effluxes, in order to study the relationship between soil gas anomalies, faults and volcanic activity. The areas of Santa Maria di Licodia (SML) and W-Rift (WR) were selected, because of their importance within the volcano-tectonic framework of Etna. Two gas surveys were performed in each area in different periods (November 2005 and May 2006 in SML, September 2007 and June 2008 in WR), with 2140 measurements in total. In each survey, data were log-normally distributed and were statistically different from the other surveys, therefore their standard normal form was used to compare them. Log probability plots revealed five populations of data in each survey, due to varying degrees of mixing between biogenic and magmatic CO2, and indicated anomalous CO2 effluxes for values > 36 g m−2 d−1. Magmatic output was 39.2 t d−1 in November 2005, 15.8 t d−1 in May 2006, 98.4 t d−1 in September 2007 and 234.1 t d−1 in June 2008. Natural Neighbor interpolation of standardized data produced distribution maps that showed some clustering of anomalous values along directions possibly related to hidden faults compatible with volcanic or regional structural trends. Analysis of magmatic CO2 emissions in time suggested a possible influence from seasonal variations, but comparison with volcanic activity of Etna also indicated a volcanic influence accompanying the 2008-2009 flank eruption.