Carbon dioxide in scree slope deposits: A pathway from atmosphere to pedogenic carbonate

Abstract A continuum of carbon, from atmospheric CO 2 to secondary calcium carbonate, has been studied in a soil associated with scree slope deposits in the Jura Mountains of Switzerland. This approach is based on former studies conducted in other environments. This C continuum includes atmospheric CO 2 , soil organic matter (SOM), soil CO 2 , dissolved inorganic carbon (DIC) in soil solutions, and secondary pedogenic carbonate. Soil parameters (pCO 2 , temperature, pH, C min and C org contents), soil solution chemistry, and isotopic compositions of soil CO 2 , DIC, carbonate and soil organic matter (δ 13 C CO 2 , δ 13 C DIC , δ 13 C car and δ 13 C SOM values) have been monitored at different depths (from 20 to 140 cm) over one year. Results demonstrated that the carbon source in secondary carbonate (mainly needle fiber calcite) is related to the dissolved inorganic carbon, which is strongly dependent on soil respiration. The heterotrophic respiration, rather than the limestone parent material, seems to control the pedogenic carbon cycle. The correlation of δ 13 C org values with Rock-Eval HI and OI indices demonstrates that, in a soil associated to scree slope deposits, the main process responsible for 13 C-enrichment in SOM is related to bacterial oxidative decarboxylation. Finally, precipitation of secondary calcium carbonate is enhanced by changes in soil pCO 2 associated to the convective movement of air masses induced by temperature gradients (heat pump effect) in the highly porous scree slope deposits. The exportation of soil C-leachates from systems such as the one studied in this paper could partially explain the “gap in the European carbon budget” reported by recent studies.