Open to closed system transition traced through the TDIC isotopic signature at the aquifer recharge stage, implications for groundwater 14C dating

Abstract

14C dating models are limited when considering recent groundwater for which the carbon isotopic signature of the total dissolved inorganic carbon (TDIC) is mainly acquired in the unsaturated zone. Reducing the uncertainties of dating thus implies a better identification of the processes controlling the carbon isotopic composition of the TDIC during groundwater recharge. Geochemical interactions between gas, water and carbonates in the unsaturated zone were investigated for two aquifers (the carbonate-free Fontainebleau sands and carbonate-bearing Astian sands, France) in order to identify the respective roles of CO 2 and carbonates on the carbon isotopic signatures of the TDIC; this analysis is usually approached using open or closed system terms. Under fully open system conditions, the seasonality of the 13C values in the soil CO 2 can lead to important uncertainties regarding the so-called “initial 14C activity” used in 14C correction models. In a carbonate-bearing unsaturated zone such as in the Astian aquifer, we show that an approach based on fully open or closed system conditions is not appropriate. Although the chemical saturation between water and calcite occurs rapidly within the first metre of the unsaturated zone, the carbon isotopic contents (δ 13C) of the CO 2 and the TDIC evolve downward, impacted by the dissolution–precipitation of the carbonates. In this study, we propose a numerical approach to describe this evolution. The δ 13C and the A 14C (radiocarbon activity) of the TDIC at the base of the carbonate-bearing unsaturated zone depends on (i) the δ 13C and the A 14C of the TDIC in the soil determined by the soil CO 2, (ii) the water’s residence time in the unsaturated zone and (iii) the carbonate precipitation–dissolution fluxes. In this type of situation, the carbonate δ 13C–A 14C evolutions indicate the presence of secondary calcite and permit the calculation of its accretion flux, equal to ∼ 4.5 ± 0.5 × 10 - 9 mol g rock - 1 yr - 1 . More generally, for other sites under temperate climate and with similar properties to the Astian sands site, this approach allows for a reliable determination of the carbon isotopic composition at the base of the unsaturated zone as the indispensable “input function” data of the carbon cycle into the aquifer.