BUSCADOR RECOLECTA

Raw data of a mansucript to be submitted to Geochimica et Cosmochimica Acta, Calcium carbonate isotopic compositions have been widely used to reconstruct past environments. However, if the isotopic composition is modified as a result of the mineral formation from an amorphous precursor, the reliability of carbonate minerals as paleo-geochemical proxies can be compromised. This study explores the changes in the oxygen isotope compositions during the transformation of amorphous calcium carbonate (ACC) into crystalline carbonate under different conditions of relative humidity (RH of 33 to 95%), temperature (T of 5 ºC and 20 ºC) and in the presence/absence of atmospheric CO2. The data shows that at low RH and_T (e.g., RH ≤ 45% and 5 ºC), a complete ACC–crystalline carbonate transformation does not take place and the primarily ACC δ18O values remained relatively constant (δ18OCaCO3 = −15.9 ± 1.0 ‰, VPDB) throughout our experimental runtime (up to 144 days). In contrast, in fully crystallized CaCO3 (e.g., at RH ≥ 60 %) the δ18OCaCO3 values increased rapidly over the first few days, followed by a slower consistent increase. By the end of the experiments (after 103 to 144 days) almost constant δ18OCaCO3 values were reached, ranging from −10.4 ‰ to −8.1 ‰ in the presence of atmospheric CO2 and from −12.6 ‰ to −9.5 ‰ in the CO2-free experiments. During the experiments the changes in oxygen isotope compositions of the CaCO3 reaction product were mainly driven by exchange with H2O from the hydrated ACC. Calcite and/or vaterite were found as final crystalline products. Considering the well-known oxygen isotope fractionation between calcite and fluid at equilibrium, in the presence of CO2, the herein obtained Δ18OCaCO3-H2O = 18OCaCO3 - δ18OH2O(ACC) tended to approach but did not reach isotopic equilibrium at the given temperature. The latter effect could be explained by a decrease in the pH of the aqueous fluid released from ACC dissolution during CO2 absorption, which increased the oxygen isotope exchange kinetics between H2O and dissolved inorganic carbon (DIC) via the hydration/hydroxylation of CO2(aq) and H2O in the fluid. However, at 60 to 95 % RH rapid deprotonation could have led to 18O-enriched CO32− due to the incorporation of heavier HCO3−; and resulting in fractionation factors that exceeded equilibrium values. In the CO2-free experiments, the isotopic equilibrium between the crystalline phase with the synthesis fluid was not reached. This oxygen isotope disequilibrium suggests that the CO32− could have been buffered by a lighter source, such as, the CO32− released from the ACC/calcite during the dissolution-reprecipitation process. The results suggest that the oxygen isotopic composition of natural carbonates formed from ACC transformation in air and at low water/solid ratio (e.g., biominerals or carbonates formed in caves) are complex and cannot be used as simple proxy if kinetics (RH/CO2/T) and H2O sources are not known and quantified., This project has received funding from the Junta de Andalucía through the EMERGIA research programme under the grant agreement EMERGIA20_38594, the European Research Council Advanced Grant 788752, the Institute of Earth Surface Dynamics (IDYST, University of Lausanne, Switzerland), the research groups of the Junta de Andalucía RNM 179 and 190, and the PID2021-125619OB-C21 funded by Ministerio Ciencia e Innovación/Agencia Estatal de Investigación/ 10.13039/501100011033/ and Fondo Europeo de Desarrollo Regional “Una manera de hacer Europa”.