Comparison between mechanisms of CO2 degassing from El Chichon volcanic lake, México, and Specchio di Venere lake, Pantelleria, Italia.

Abstract:

We present results of the CO₂ diffuse emission from the surface of two volcanic lakes: El Chichón (EC) in Mexico and Specchio di Venere (SV) on Pantelleria Island, Italy. Both lakes are drainless, have similar sizes (~2x10⁵ m²) and similar input-output dynamics. However, they are drastically different in water chemistry. The SV lake is alkaline (pH >9) and of a high near constant salinity, whereas EC lake is acidic (pH 2.3) and of a low variable salinity. In the vicinity of both lakes there are thermal grounds with steam vents and hot springs and a high CO₂ soil flux. The SV lake has high alkalinity (~70 meq/L), whereas the EC lake is characterized by high concentration of dissolved CO₂.
CO₂ flux measurements from the surface of both lakes were made with the “floating” accumulation chamber. During the flux measuring, gas samples were taken for carbon isotopic analysis. Soil flux measurements were also made in the crater of El Chichon volcano and on the area adjacent to the SV lake. 
The preliminary results of CO₂ fluxes indicate EC lake has a high CO₂ flux with a mean value of 3500 g m^-2 d^-1, with the highest values alignment across NW-SE and NE-SW faults and a high degassing by bubbling gases, especially near the strongest NE fumarolic field. While SV has a mean value of the CO₂ flux ~ 10 g m^-2 d^-1 and limited bubbling on the lake surface. High CO₂ flux was measured from the soil near the lake at the Mofeta place. A net mean diffusion flux (without bubbles) from EC lake is about 350 times higher than that from SV lake (3500 g m^-2 d^-1  vs 10 g m^-2 d^-1). SV has the total CO₂ flux by diffusion of ~3 ton d^-1 from an area of 0.3 km² and the total flux of 0.44 ton d^-1 by bubbling areas at SW and S zones. The EC lake has the total CO₂ flux of 840 ton d^-1 from an area of 0.24 km². The total CO₂ output from SV is nevertheless about two times higher taking into account the seepage from the lake (~ 8 kg s-1) of highly carbonated water.