Inorganic carbon cycle in soil-rock-groundwater system in karst and fissured aquifers

Abstract

The paper presents a systematic analysis of the isotopic composition of carbon (δ13CCaCO3) in carbonate rocks in central Slovenia, representing karst and fissured aquifers, and share of carbon contributions from carbonate dissolution and degradation of organic matter in aquifers, calculated from the mass balance equation. 59 samples of rocks (mainly dolomites) from Upper Permian to Upper Triassic age were analyzed. Samples of carbonate rocks were pulverized and ground to fraction of 45 µm and for determination of δ13CCaCO3 analyzed with mass spectrometer for analyses of stable isotopes of light elements-IRMS. The same method was used for determination of isotopic composition of dissolved inorganic carbon (δ13CDIC) in groundwater for 54 of 59 locations. Values of δ13CCaCO3 are in the range from -2.0 ‰ to +4.1 ‰, with an average δ13CCaCO3 value of +2.2 ‰. These values are typical for marine carbonates with δ13CCaCO3 around 0 ‰, although δ13CCaCO3 values differ between groups depending on the origin and age. Early diagenetic dolomites have relatively higher values of δ13CCaCO3 compared to other analyzed samples. The lowest values of δ13CCaCO3 were observed in Cordevolian and Bača dolomite, probably due to late diagenesis, during which meteoric water with lower isotopic carbon composition circulated in the process of Sedimentation. Values of δ13CDIC range from -14.6 ‰ to -8.2 ‰. Higher δ13CDIC values (-8.2 ‰) indicate a low Proportion of soil CO2 in the aquifer and rapid infiltration, while lower values (-14.6 ‰) indicate higher proportion of soil CO2 in the aquifer and slower infiltration. Calculated contributions of carbon from organic matter / dissolution of carbonates in the karstic and fissured aquifers show a similar proportion (50 % : 50 %).