Stable carbon isotope fractionation during the transformation of amorphous CaCO3 at low water–solid ratios: impact of humidity and temperature

The isotopic composition of carbonates has been widely used as a proxy for reconstructing Earth’s past environmental conditions, which requires the preservation of the isotope composition of the primary precipitate or knowledge about its transformation pathway. In this context, the formation of crystalline carbonates via amorphous precursors has been identified as a common formation process for natural carbonates, where the impact to the isotopic composition of the final CaCO₃ products is insufficiently known to date. This study focusses on the stable carbon isotope fractionation during the transformation of amorphous calcium carbonate (ACC) into crystalline carbonate in the presence/absence of atmospheric CO₂ induced by (i) humidity (from 33 to 95 % RH) at a temperature of 5 and 20 °C and (ii) solid-state transformation of ACC to calcite induced by heating up to 500 °C. During the crystallization of ACC at very low water ratios, induced by humidity, and in the presence of CO₂, δ¹³C values of the solids increased with reaction time, where the crystalline carbonate-ACC enrichment factors (¹³ε_cc−ACC) range between 2.0 and 8.4 ‰ at the final stage. This ¹³ε_cc−ACC evolution reflects the incorporation of ambient CO₂(g) alongside carbonate ions released during ACC dissolution. In contrast, without CO₂(g) the crystalline CaCO₃ almost matches the isotope composition of the ACC (¹³ε_cc−ACC = −0.4 ± 0.3 ‰), thus indicating non-significant CO₂ degassing and/or subsequent re-equilibration trend. The solid-state transformation of ACC to calcite during heat-induced dehydration yields a slightly negative enrichment factor (¹³ε_{calcite−ACC} = −0.8 ± 0.3 ‰) that may reflect carbon isotope fractionation due to rapid recrystallization-rate kinetics. Our results suggest that the carbon isotope compositions of carbonates formed via an amorphous precursor at low water/solid ratios (e.g., in caves, soils or biominerals) may not accurately reflect the original conditions of the carbonate formation, hence paleoenvironmental conditions should be interpreted with care.