Carbon isotope exchange during calcite interaction with brine: Implications for ¹⁴C dating of hypersaline groundwater

Due to its possible role in solid/water carbon isotope exchange, the effect of salinity on radiocarbon dating of groundwater was examined by batch interaction of alluvial sediment and calcite powder with freshwater (Cl^- = 100 mg L^-1) and Dead Sea (DS) brine (Cl^- = 225 g L^-1). These 2 water types were spiked with H¹³CO_3- tracer and kept under constant agitation for about 1 yr. Several bottles were respiked twice with the tracer. The uptake of the ¹³C by calcite was monitored through repeated isotopic measurements of the aqueous solutions, and the effect on ¹⁴C groundwater dating was evaluated using a simple transport reaction model. The results indicate that the kinetics of water/calcite isotope exchange start with a very fast initial step followed by a slower one, which was used here to simulate the long-term water/solid exchange in "real" aquifers. The exchange model that best fits the data was homogeneous recrystallization that formed just a very thin layer of newly formed calcite. The estimated recrystallization rates for calcite powder/solution interaction were much smaller for the DS brine than for freshwater: 3 × 10^-5 to 7 × 10^-6 and 9 × 10^-4 to 7 × 10^-5 mol m² yr^-1, respectively. The ¹³C experimental data imply a very small effect of the brine/calcite isotope exchange on the ¹⁴C age estimate for the brines within the DS coastal aquifer. However, when calcite recrystallization reaches ~1% of the solid, the ¹⁴C groundwater dating estimates will show aging by ~10%.