Stable carbon isotope fractionation during the transformation of amorphous CaCO3: Impact of humidity and temperature

Abstract

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 both man-made and natural carbonates, where the impact to the isotopic composition of the final CaCO3 products is insufficiently known to date. This study focusses on the changes in stable carbon isotope fractionation during the transformation of 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 20ºC and (ii) solid-state transformation of ACC to calcite induced by tempering up to 500ºC. During the humidity induced crystallization of ACC and in the presence of CO2, δ13C of the solids increased with reaction time, where the crystalline carbonate-ACC enrichment factors (13c−ACC) are ranging between 2.0 and 8.4‰ at the final stage. This 13cc−ACC evolution reflects the uptake of CO2(g) besides carbonate gained from the ACC dissolution. Accordingly, ACC transformation occurs via dissolution-reprecipitation, which is induced by physiosorbed gaseous H2O that combines with liberated H2O from ACC dissolution. In this way, generated reactive solutions contain DIC from the dissolution of the ACC precursor, and subsequently from the hydration/hydroxylation of CO2(g) from the air, where carbon isotope re-equilibration kinetics between CO2(g) and dissolved inorganic (DIC) species have to be considered. In contrast, without CO2(g) the crystalline CaCO3 almost matches the isotope composition of the ACC (13εc−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 (13εc−ACC =−0.6 ± 0.2‰) that may reflect carbon isotope fractionation due to fast recrystallization-rate kinetics accompanied by variable amounts of CO2 degassing during transformation. Our results suggest that the carbon isotope compositions of carbonates formed via an amorphous precursor at low water/solid ratios (e.g., in caves or biominerals) may not accurately reflect the original conditions of the carbonate formation, hence paleoenvironmental should be interpreted with care.