HELIOTHERMIC HEATING OF DENSITY STRATIFIED GROUNDWATER - AN ALTERNATIVE INTERPRETATION TO GEOTHERMAL CIRCULATION

The coastal carbonate aquifers of the Yucatan Peninsula (Mexico) and Andros Island (Bahamas) are density stratified, with fresh to brackish water at the surface, and water of marine salinity encountered at depth. Cenotes or inland blue holes are common in both areas, and these are ground-water filled dissolution and collapse shafts which can be used to obtain temperature and salinity profiles to depths in excess of 100 m below the water table. Previous studies in the Yucatan (Stoessell et al. 2002, Geology, 40:416-424) have reported the presence of elevated temperatures within the fresh-saline transition zone, which were interpreted as evidence of rising geothermal waters. We have observed such thermal anomalies at four sites in the Yucatan and six sites in the Bahamas, with the water within or immediately below the transition zone having temperatures from 0.1 to more than 10^oC greater than both the overlying fresh water and the underlying saline water. Sites with thermal anomalies have surface openings of several metres to tens of metres diameter, and the transition zone lies above the light extinction depth. Subsurface thermal anomalies are absent in many other large-diameter sites where the transition zone lies below the light extinction depth, or the site morphology limits direct insolation of the transition zone. We propose that the thermal anomalies arise from heliothermic heating, as commonly occurs in chemically stratified meromictic lakes and fjords (Kirkland et al. 1983, Arch. Hydrobiol. Suppl. 65:1-60). Saline water has a lower specific heat capacity than fresh water, such that the water in the transition and saline zones will warm to a greater degree for a given rate of insolation adsorption than the overlying fresher water. Despite the subsurface heating, the water column remains stably stratified as the salinity gradient is the dominant control on the density profile. Thermal convection therefore does not occur, and the heated water is trapped at the density interface. Thermal losses are limited to the slow processes of conduction and double diffuse convection. Although geothermal heating may provide a significant drive to saline circulation in the both the Bahamian and Yucatan platforms, we argue that the subsurface thermal anomalies considered here are best explained by heliothermic heating.