Geochemical Surveys on Specchio di Venere Lake, Pantelleria Island, South Italy

Abstract

Lake Specchio di Venere is an endorheic saline lake within a calderic depression on Pantelleria, a quiescent volcanic island in the Sicily Channel, between Tunisia and South Italy. This endorheic basin has been formed through upwelling of the water table, and that it is continuously fed by the thermal springs situated on its shores. In the shore of Lake Specchio di Venere, CO2 fluxes and concentrations were measured with the accumulation chamber method for a total of 136 measurements. Furthermore a vertical profile of main water chemical-physical characteristics (EC, pH, Eh and T) and a bathymetric survey have been made. Flux measurements for the whole surveys gave values ranging from 1 to 4700 gCO2 m-2 day-1 and a median of 21.8 gCO2 m-2 day-1. Organic activity and root respiration contributes can be distinguished on a probability plot evidencing statistically distinct populations. In this case a threshold of 30g m-2 day-1 to separate background (organic) from anomalous (magmatic/geothermal) values was chosen. The total CO2 output of the anomalous degassing areas was estimated through statistical method considering only values above the anomaly threshold. We have obtained a total output of about 0.349 kg s-1 over an area of about 0.103 km2. CO2 concentrations in soils ranged from 0.035 (atmospheric value) up to 95% and two statistically distinct populations. The spatial distributions of CO2 concentrations closely resemble those of the CO2 fluxes. At sites where concentration and flux anomalies are both present, the C isotopic composition of CO2, shows the imprint of the magmatic/geothermal isotopic marker (5±1 ‰). The lake waters were inspected with one vertical profile. All analysed parameters (T, EC, pH, Eh) did not show any significant variation with depth. Such results exclude the presence at that time of any thermal or chemical stratification of the lake. Also the analysis of dissolved gases did not evidence anomalous gas accumulations in the lake waters. A bathymetric survey has been carried out using a fish-finder sonar coupled to a GPS. The obtained morphology of the lake bottom is quite irregular. The southern sector is characterized by very shallow waters (< 1 m), whereas the maximum depth of about 13 m is reached in the northern area. Tectonic seems to strongly influence the submerged morphology: a main lineation NW-SE oriented, coupled with a secondary conjugated direction SE-NW, is clearly revealed by the arrangement of the depth contour lines.