ALBERT

Description: Carbonate dissolution in soil-groundwater systems depends dominantly on pH, temperature and the saturation state of the solution with respect to abundant minerals. The pH of the solution is, in general, controlled by partial pressure of CO2 (pCO2) produced by ecosystem respiration, which is controlled by temperature and water availability. In order to better understand the control of land temperature on carbonate weathering, a database of published spring water hydrogeochemistry was built and analysed. Assuming that spring water is in equilibrium with the soil-water-rock-atmosphere, the soil pCO2 can be back-calculated. Based on a database of spring water chemistry, the average soil-rock CO2 was calculated by an inverse model framework and a strong relationship with temperature was observed. The identified relationship suggests a temperature control on carbonate weathering as a result of variations in soil-rock pCO2, which is itself controlled by ecosystem respiration processes. The findings are relevant for global scale analysis of carbonate weathering and carbon fluxes to the ocean, because concentration of weathering products from the soil-rock-system into the river system in humid, high temperature regions, are suggested to be larger than in low temperature regions. Furthermore, results suggest that, in specific spring samples, the hydrochemical evolution of rain water percolating through the soil-rock complex can best be described by an open system with pCO2 controlled by the ecosystem. Abundance of evaporites and pyrite sources influence significantly the chemistry of spring water and corrections must be taken into account in order to implement the inverse model framework presented in this study. Annual surface temperature and soil water content were identified as suitable variables to develop the parameterization of soil-rock pCO2, mechanistically consistent with soil respiration rate findings.

Description: Carbonate weathering and transfer of carbon towards the coastal zone is one of the relevant sinks for atmospheric CO2, controlled by hydrology, ecosystem respiration, river water degassing, and further factors. Specifically, the connection between the soil-rock system to the river systems and instream processes affecting the weathering product fluxes remain under-researched. Based on constraints for soil-rock PCO2, river PCO2, and an identified dependence of river alkalinity on temperature, this work tested which controls should be considered at the global scale to accomplish a more holistic carbonate rock weathering model. Compiled river data suggests that with increasing land temperature, above approximately 11 °C, the amount of instream alkalinity in carbonate catchments decreases due to the temperature effect on the carbonate system, while the converse holds true at lower temperatures. Latter is in accordance with calcite dissolution controlled by soil-rock PCO2 estimates based on ecosystem respiration. In addition, the type of the weathering system (open, semi-closed to closed system with respect to CO2) was identified to be highly relevant for global weathering estimations. Open systems seem to be the most dominant boundary condition of calcite weathering in the soil profile. Tropical areas with thick soil layers, however, cause the carbonate weathering system to shift from open to semi-closed or closed system conditions. The findings support that calcite weathering fluxes in the soil profile are higher than the fluxes to the ocean transported by rivers. Furthermore, an increase in mean land temperature does not necessarily translate into an increase of lateral weathering fluxes because it might have an influence on soil development, discharge, CO2 degassing, soil respiration and calcite dissolution. All these named factors need to be addressed to be able to quantify global carbonate weathering fluxes and to assess the sensitivity of carbonate weathering fluxes on climate variability. Future works should focus on collecting more temporal river chemistry data, mainly in tropical regions, to understand the main mechanism causing the observed decrease of alkalinity concentration with temperature.