Isotope exchange rates in dissolved inorganic carbon between 40 °C and 90 °C

Abstract

Carbonate minerals are normally closely related to the stable and clumped isotope composition of the parent solution and are often assumed to record the equilibrium conditions in these proxies. Variations in e.g., temperature, pH or salinity lead to changes in the dissolved inorganic carbon (DIC) composition and to temporary deviations from expected isotopic equilibrium values. The exchange rate, at which the new equilibrium of stable and clumped isotopes is reached, has only been assessed experimentally for clumped isotopes between 5 °C and 25 °C and for δ18O up to 40 °C.In this study the δ18O and Δ47 evolution in the DIC pool of 0.1 molar sodium bicarbonate solutions at pH 8 and various temperatures (40 °C, 55 °C, 70 °C, 90 °C) was examined. At time intervals corresponding to the evolution of the DIC, a quantitative and instantaneous precipitation of the DIC was accomplished by adding strontium chloride and sodium hydroxide solutions. As the δ13C of the SrCO3 precipitates was constant and no significant non-first order trend was observed in Δ47, δ18O and Δ47 values could be simply fitted with an exponential approach to isotopic equilibrium. The temperature dependence of the rate constants was evaluated with the Arrhenius equation. The resulting rate constant based on the combined data points from δ18O and Δ47 follows askT[min-1]=exp-1.00±0.15·104∗1T+(29±4)The inferred exchange rate constants for Δ47 are of the same order of magnitude as those of the oxygen isotopes and show a similar temperature dependence in the investigated temperature range.Our experimental results suggest that the isotopic equilibration of DIC in a solution with pH < 10 and elevated temperatures is rapid (at 70 °C: 4.5 min, at 90 °C: 36 s for 99% equilibrium) and that even in the case of short-term disturbances an equilibrated DIC composition is very likely for solutions in the diagenetic temperature range above 100 °C. In contrast, it is important to consider the generally long equilibration times at low temperatures (e.g., 2–6 days at pH 8.0–8.5 and 0 °C) and, in particular, at higher pH (e.g., >100 days at pH 11 and 0 °C).