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%.

Stable mineral recrystallization in low temperature aqueous systems: A critical review

“…Determining which of these models most accurately represents reality has proven to be a difficult task because of technical challenges, which has been discussed extensively in the literature (Criss et al, 1987;Dubinina and Lakshtanov, 1997;Cole and Chakraborty, 2001;Curti et al, 2005;Curti et al, 2010;Avrahamov et al, 2013;Handler et al, 2014). The most straightforward way to test the models would be to collect complementary data that spatially resolves the isotopic composition of the solid.…”

“…Due to the inability to determine what model is most appropriate based on complementary data, the interpretive model is most commonly selected using an indirect approach in which experimental data is fit with each model and the goodness-of-fit parameters (e.g., R 2 values) are compared using least squares regression analyses (Curti et al, 2010;Avrahamov et al, 2013). An additional challenge in applying these models to experimental data is that the models are often founded on the assumption that the recrystallization rate remains constant over time.…”

“…Redox-inactive studies Calcite (CaCO3) (Lahav and Bolt, 1964;McBride, 1980*;Mozeto et al, 1984;Davis et al, 1987;Zachara et al, 1991;Stipp et al, 1992*;Das et al, 1995;Carlsson and Aalto, 1997;Cheng et al, 1997*;Reeder et al, 2004*;Curti et al, 2005*;Elzinga et al, 2006*;Heberling et al, 2008*;Avrahamov et al, 2013;Heberling et al, 2014) Barite (BaSO4) (Bosbach et al, 2010;Curti et al, 2010;Vinograd et al, 2013;Klinkenberg et al, 2014*;Brandt et al, 2015*) Gypsum (CaSO4) (Lestini et al, 2013) Dolomite (CaMg(CO3)2) (Lahav and Bolt, 1964;Malone et al, 1996) Ferrihydrite † (Rea et al, 1994;Poulson, 2005) Mackinawite (FeS) † (Guilbaud et al, 2011) Redox-active studies Goethite (α-FeOOH) (Crosby et al, 2005;Pedersen et al, 2005;Handler et al, 2009;Beard et al, 2010; Si-stabilized Ferrihydrite † (Wu et al, 2012) Manganite (γ-MnOOH) (Frierdich et al, 2016) Vernadite (δ-MnO2) (Elzinga, 2016) time and approached a value indicating that the solid and solution reached complete isotopic mixing after 400 days (Figures 4A and 4B; the gray line indicates complete isotopic mixing between the solid and solution). In a third experiment containing a lower barite concentration (0.01 g/L) than in the other two experiments (Figure 4C), the dissolved concentration of 133 Ba 2+ also substantially decreased, but did not approach the complete isotopic mixing line over 300 days.…”

“…Evidence collected in laboratory studies have shown that a mineral's elemental and/or isotopic composition can be significantly altered in low-temperature aqueous environments at rapid rates in the absence of overt structural or morphological changes. Using isotopic or elemental tracers, researchers found that extensive isotopic exchange occurs between mineral and aqueous ions over experimental time scales -without any overt changes in the mineral structure or grain sizethat exceed rates expected for solid-state diffusion (Stipp et al, 1992;Handler et al, 2009;Curti et al, 2010;Frierdich and Catalano, 2012a;Avrahamov et al, 2013;Lestini et al, 2013;Handler et al, 2014). In many cases, investigations have concluded that the mineral has undergone complete exchange with the aqueous solution, as evidenced by the solid and solution having virtually identical isotopic compositions at the end of the experiment (Zachara et al, 1991;Handler et al, 2009;Bosbach et al, 2010;Avrahamov et al, 2013;Handler et al, 2014;Joshi and Gorski, 2016).…”

“…Translating the observations of this phenomenon from laboratory studies to natural systems is not straightforward. Analyses of minerals and their surrounding pore fluids collected from field sites indicate that materials such as deep sea sediments and carbonates undergo significant post-depositional isotopic and elemental exchange with pore fluids at low temperatures (Killingley, 1983;Mozeto et al, 1984;Richter and DePaolo, 1987;Spivack and Edmond, 1987;Schrag et al, 1992;Richter and Liang, 1993;Schrag et al, 1995;Fantle and DePaolo, 2007;Jones et al, 2009;Fantle et al, 2010;Wu et al, 2011;Avrahamov et al, 2013;Fantle, 2015). Determining the extent to which this post-depositional exchange is coupled to structural and/or morphological changes in the mineral is obviously challenging, however, since the system is open and the original specimen is not well characterized.…”