Geochemical evolution of hypersaline cave pools, Guadalupe Mountains, New Mexico

Abstract

Several saline and hypersaline pools have been identified in caves of the Guadalupe Mountains in New Mexico, U.S.A. Among the many pools found scattered throughout 207 km of Lechuguilla Cave, the Briny Pool contains the highest TDS (6%) of any pool identified. In this first detailed investigation of Briny Pool chemistry, a conceptual model was developed to describe the geochemical evolution of the Briny Pool by using existing principles of brine evolution, which incorporates the concept of a chemical divide. The net effect of dolomite (CaMg(CO 3) 2) dissolution followed by the first chemical divide of calcite and/or aragonite (CaCO 3) precipitation produces a low-Ca water containing Mg in excess of alkalinity. With continued evaporative concentration, a second chemical divide is reached when the water becomes saturated with respect to hydromagnesite (Mg 5(CO 3) 4(OH) 2·4H 2O), and continues to evolve into a Mg–SO 4 dominated brine. The results of inverse and forward geochemical modeling calculations support the conceptual model. This study demonstrates a unique application of Hardie–Eugster principles, with modification to include Mg-bearing hydromagnesite (rather than sepiolite), to identify and quantify the geochemical origin and evolution of a hypersaline cave pool. The principles used to describe the evolution of the Briny Pool should be applicable in studies of other dolomite-hosted hypersaline cave pools in the Guadalupe Mountains and throughout the world.