Causes and implications of the seasonal dissolution and precipitation of pedogenic carbonates in soils of karst regions – A thermodynamic model approach

Abstract

[EN] Red Mediterranean Soils developed over carbonate rocks can precipitate pedogenic carbonates at the base of their profiles, forming an evolutionary stage of calcretes. We studied a 0.6 m deep soil profile in Dalmatia (Croatia) where the lowest section of the soil is a calcic horizon formed by diffuse calcite particles and small nodules. These nodules record different events of dissolution and precipitation. Using a 3-month period of soil environmental monitoring, where soil temperature, soil water content, soil bulk electrical conductivity and soil air CO2 were measured, we implemented a thermodynamic model for dissolution and precipitation of calcite in the soil. The simulation shows a stage dominated by calcite dissolution during spring and early summer followed by a stage dominated by calcite precipitation. Soil air CO2 is the main control of the calcite reactions, with concentration of solutes also being of some importance during the stage of calcite precipitation. Soil water content and temperature, both affect soil air CO2. Precipitation events also have high-frequency impact on soil air CO2, but the response is complex. The model enables soil water solutions to be supersaturated in relation to calcite without precipitation of that mineral. Simulated soil water reached variable values of supersaturation in relation to calcite before its precipitation was triggered, suggesting that there is no fixed threshold value for the precipitation of calcite in a particular soil. During the 3-month simulated period, 83% of the calcite dissolved was reprecipitated as pedogenic carbonate. Most of the initially dissolved calcite, was expected to be diffuse pedogenic carbonate particles, although nodules and bedrock at the base of the soil should also have contributed to the solutes dissolved. Therefore, karst landscapes where soils have pedogenic carbonates are expected to record less denudation rates than those regions without calcretes.