13-C AND 14-C ISOTOPES OF DISSOLVED INORGANIC CARBON AS A TOOL TO EVALUATE BIOTIC AND ABIOTIC CONTROLS ON CARBON CYCLING IN TWO LIMESTONE AQUIFER ASSEMBLAGES

Based on analysis of ¹⁴C and ¹³C isotopes of dissolved inorganic carbon (DIC) we evaluated water flow and carbon turnover of two superimposed karstified limestone aquifer assemblages. We applied the graphical method proposed by Han et al. (2012) to identify the main biotic and abiotic processes affecting δ¹³C and ¹⁴C signatures of DIC in groundwater. In addition, we used the geochemical modelling program NETPATH to quantify fluxes of different carbon sources contributing to the DIC pool. These methods identify the relative contributions of different carbon sources and sinks and demonstrated that microbially metabolized carbon within the upper aquifer assemblage influenced ¹⁴C and δ¹³C of DIC.

Although water residence times in both aquifers are short, DIC isotopes in the upper aquifer were depleted in ¹⁴C compared to the deeper aquifer. In the lower, highly karstified aquifer fast flow of modern water is reflected by the presence of bomb-carbon influencing DIC radiocarbon signatures. In this aquifer, the major influence on DIC isotopes is carbonate equilibration.

Oxygen depletion and reduced radiocarbon values of DIC indicate dramatically different biogeochemical evolution of waters in the upper aquifer. Depletion of ¹⁴C and ¹³C in the upper aquifer over what is expected from establishing equilibrium with carbonates results from a number of biotic and abiotic processes, including oxidation of ¹⁴C depleted organic matter derived from recycled microbial carbon and sedimentary organic matter as well as water rock interactions. Support for the importance of microbial pathways inferred from DIC isotope shifts were compared with in situ microbial community structure based on 16S rRNA gene analyses.

Our findings demonstrate the large variation in the importance of biotic as well as abiotic controls on ¹³C and ¹⁴C of DIC in closely related aquifer assemblages. Further, they support the importance of rock-derived organic matter for supporting subsurface microbial communities and indicate that even shallow aquifers have microbial communities that use a variety of carbon sources.

References:

Han, L.-F., Plummer, L.N. and Aggarwal, P. (2012) A graphical method to evaluate predominant geochemical processes occurring in groundwater systems for radiocarbon dating. Chemical Geology 318–319, 88-112.