Carbon Isotope Exchange During Calcite Interaction With Brine: Implications for 14C Dating of Hypersaline Groundwater

Abstract

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 H13CO3– 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%.