Evolution of Mg/Ca Ratios During Limestone Dissolution Under Epikarstic Conditions

The Mg/Ca ratios in karst water are generally believed to comprise information on climate, and, being encoded in speleothems, they are utilized as paleoenvironmental proxy. However, the mechanism and dynamic of Mg release from limestone during dissolution is not well understood. A theoretical evolution of the Mg/Ca ratios during limestone dissolution under epikarstic conditions ( T = 10 °C, log P CO 2 = −1.5) was studied via a dynamic model. The results were compared with (1) the dripwater data set collected in Punkva Caves (Moravian Karst, Czech Republic) during one-year period and (2) the published data from various locations worldwide. The modeling showed that the Mg/Ca ratios are governed by composition of Mg-calcite present in limestone. Two distinct stages in the dissolution dynamics were recognized: (1) an initial congruent dissolution with stoichiometric release of Ca and Mg and, subsequently, (2) an incongruent dissolution demonstrated by the gradual release of Mg with simultaneous Ca decrease via calcite precipitation. Additional identified factors influencing the reaction path and Mg/Ca ratio evolution were the dolomitic component of limestone and the ratio of limestone/solution boundary area to water volume. Finally, the water–rock interaction time controls the resulting Mg/Ca ratio in dripwater determining how far the dissolution proceeds along the reaction path. Thus, the study results indicate that Mg/Ca ratio depends on many factors in addition to climatic variables.