Trace metal and carbon isotope composition of amorphous calcium carbonate-derived CaCO3 minerals as a function of temperature and time

Amorphous calcium carbonate (ACC) is a common precursor of crystalline CaCO 3 minerals whose chemical and isotopic composition are readily used as environmental archives. Yet the existing understanding on how chemical composition of ACC derived solids evolves during and after crystallization is rather limited. To address this question, in this study, impurity free ACC was introduced in a solution containing a high concentration of dissolved inorganic carbon (DIC), as well as B, Li, Na and Mg and let transform to crystalline CaCO 3 . The aqueous fluid was also enriched in 13 C in order to assess the progress of the transformation reaction. Experiments were conducted at 5, 25 and 40 °C and the total reaction time was 90 days. The transformation of ACC mainly proceeds via its dissolution and re-precipitation and the mineralogy of the crystalline product varies at the different reaction temperatures. The observed mineral transformations are mainly affected by temperature, whereas the reaction progress is not the same in all reaction temperatures and a complete mixing of C pools between solid and reactive fluid is only observed in 25 °C runs. Moreover, the distribution coefficients of Li and Na between the solids and the fluid reduce significantly over time and at the end of the runs appear similar to those observed in seeded growth experiments conducted at low degrees of supersaturation with respect to CaCO 3 . These observations are discussed in combination with the implications they have for the interpretation of the chemical compositions of biogenic and abiotic samples that are derived from ACC and are routinely used as environmental proxies.