Diurnal CO2-cycles and temperature regimes in a natural CO2 gas lake

Gaseous CO2 concentrations and concomitant air temperatures were measured within a CO2-emitting valley-mofette in Central Italy. 'Il Bossoleto' is a bowl-shaped, 5 m deep depression with several active CO2 vents that continuously emit high amounts of dry CO2 gas. Using a meteorological measurement mast, air temperatures and CO2 concentrations were recorded every 40 cm, from the bottom up to a vertical elevation of 6.4 m. The data reveal a transient, natural CO2 gas lake with three daily phases: a stratified, a homogeneous (up to 80% CO2) and an empty gas phase. The stratified gas lake builds up during the evening and night hours. After solar irradiation in the early morning, the lake evolves over a short time period into two distinct layers: at the bottom, a 2-2.6 m high homogenous gas lake with CO2 concentrations up to 80% and on top of it a gas layer with low CO2 concentrations. Due to infrared absorption by CO2 the temperatures inside the homogenous lake rises up to 60 °C (greenhouse effect). During the morning hours thermal heating leads to a collapse and complete emptying of the gas lake within tens of minutes, leaving Bossoleto free of CO2 during the day.