Interpretation of Geochemical Signatures from Modern Carbonate Springs to the Rock Record

Despite the importance of carbonate aquifers as reservoirs of oil, of water, and of carbon dioxide, questions remain about the diagenetic processes that enhance porosity and permeability. Some of the questions are: 1) what are the water-rock interactions that control the geochemistry of spring waters; 2) are the geochemical signatures of saline-fresh water mixing zones preserved in the carbonate rock record; and 3) how much carbon dioxide outgases along spring runs? To address these questions, two field sites, one in Oklahoma (the site of an active saline-fresh water mixing zone) and the other in Colorado, the location of a former mixing zone, were studied. The results from the site with the modern-day mixing zone indicate that mixing between saline water and meteoric water along with dissolution of anhydrite and dolomite and precipitation of calcite controls spring water chemistry. In addition, the flux of CO2 was calculated to be similar to that of estuaries and from other carbonate springs and greater than that of headwater streams and rivers. The results from the site with a former mixing zone indicated that changes in redox conditions during deposition, and after dolomitization of the carbonate were preserved; however a clear indication of mixing, as indicated by discernible differences in REE and trace elements, was not observed. Overall, these results indicate that despite clear indications of water-rock interaction at an active mixing zone of saline and fresh waters, the signature of mixing zones are not as clearly preserved in the rock