Kinetic carbon isotope effects during calcite precipitation: role of water–air exchange geometry and precipitation rate

Stable carbon and oxygen isotopes in calcite are widely used to reconstruct environmental and hydrological conditions, but kinetic isotope effects related to CO₂ degassing and carbonate precipitation are still poorly quantified [1]In particular, the role of water–air exchange geometry and water height on the δ¹³C (and δ¹⁸O) of precipitated calcite remains difficult to isolate from that of other controls in natural systems [2].Here we present a series of laboratory precipitation experiments to establish an empirical relationship between air–water exchange geometry surface and ¹³C fractionation between calcite and Dissolved Inorganic Carbon, providing a framework to quantify how changes in exchange surface area and water height modulate δ¹³C signatures.CaCO₃ precipitates from the same Ca2+–HCO₃- rich bottled water in containers with two exchange geometries: low vs high air–water exchange, expressed as S/h (air–water surface area divided by water height), with the low-exchange configuration having S = 130 cm² and h = 11.5 cm and the high-exchange configuration having S = 273 cm² and h = 5.5 cm. All experiments start from identical temperature, volume and initial chemistry. Conductivity, pH and temperature are measured every 24 h. Major ions concentrations, δ¹³C of DIC and δ¹³C of precipitated calcite are measured each day. Precipitation rates are quantified from the temporal decrease in dissolved Ca²⁺ concentration. They are higher when air–water exchanges increase: 1.0 × 10⁻³ mol L⁻¹ d⁻¹ for low air–water exchanges vs 1.6 × 10⁻³ mol L⁻¹ d⁻¹ for high air–water exchanges during the first day of the experiment. δ¹³C of calcite is higher for high air-water exchanges than for low air-water exchanges at a same time step (e.g., at second day: −7.1 ±0.2‰ vs −9.2 ±0.3‰).δ¹³C of DIC increases by +8.5‰ (mean) for high air–water exchanges compared to +5.7‰ (mean) for low air–water exchanges over 4 days. However, the evolution δ¹³C of DIC with DIC concentrations appears to depend little on the air–water exchanges and follows an apparent Rayleigh-type trend.The calcite–DIC enrichment factor ε becomes more negative with increasing precipitation rate, indicating stronger kinetic fractionation under conditions favouring rapid CO₂ degassing, consistent with the rate dependent trends observed in cave analogue precipitation experiment [3]. At low precipitation rates 4.3 × 10⁻⁴ mol L⁻¹ d⁻¹ , ε is close to equilibrium near to 0.2‰ compared to the equilibrium value of 0.5–0.8‰ at 17–23°C [4], whereas at higher precipitation rates 1.87 × 10⁻³ mol L⁻¹ d⁻¹, ε shows a much larger deviation from equilibrium, reaching −2.8‰.These experiments provide quantitative data on isotope effects linked to exchange geometry and precipitation kinetics, that could be used to interpretations of δ¹³C signatures in natural carbonate deposits such as speleothems and tufas.[1] Dreybrodt, W. & Fohlmeister, J. (2022) doi:10.1016/j.chemgeo.2021.120676[2] Fairchild et al. (2006) doi:10.1016/j.earscirev.2005.08.003[3] Hansen et al. (2019) doi:10.1016/j.chemgeo.2018.12.012[4] Salomons, W. & Mook, W. G. (1986) doi:10.1016/B978-0-444-42225-5.50011-5