Effect of pH and temperature on oxygen and carbon isotope fractionation during ACC transformation to crystalline carbonates.

Carbonates are widely used as paleoenvironmental archives because they record past environmental conditions through their chemical and isotopic signatures. However, primary crystallization processes and subsequent diagenetic alterations can modify these signatures, potentially affecting their reliability as paleoenvironmental proxies.

 

This study investigates isotopic changes during the precipitation of amorphous calcium carbonate (ACC) into crystalline CaCO₃ under variable pH and temperature (T) conditions, in order to better constrain the role of ACC in calcification processes and its influence on the final isotopic composition of the crystalline carbonate polymorphs. ACC was synthesized by automated titration of an equimolar CaCl₂ solution into NaHCO₃ (+NaOH) solutions. A first set of experiments was conducted over a pH range of 8–11 and at temperatures of 10, 20, and 30 °C. A second set was performed at pH 8 and T of 10, 20, and 30 °C in the presence of polyaspartic acid (pASP) to simulate biomineralization effects on ACC metastability and its transformation to crystalline CaCO₃ polymorphs. Precipitates were characterized using scanning electron microscopy, Fourier-transform infrared spectroscopy, X-ray diffraction and in-situ Raman; oxygen and carbon isotope ratios were measured by isotope-ratio mass spectrometry.

The onset of vaterite precipitation from ACC occurs rapidly at all investigated pH and T conditions, with transformation times less than 1 min. In the presence of pASP, ACC is stabilized and crystalline phase precipitation is delayed to 5 min. The transformation of ACC into calcite is strongly T dependent, with shorter transformation time periods at higher T for all pH conditions. Spherulitic ACC size is strongly controlled by pH and T, decreasing from ~0.25 µm at pH 8 and 10 °C to ~0.10 µm at pH 11 and 30 °C.

 

For all investigated temperatures and pH conditions, oxygen isotope values of the initial ACC (e.g. at 10 °C and pH 8: δ¹⁸O_VPDB = –4.94 ‰) decrease during CaCO₃ precipitation, reaching lower values in the resulting calcite (e.g. δ¹⁸O_VPDB = –6.10 ‰), with values systematically decreasing with increasing T and pH. In contrast, carbon isotope values are comparatively more constant, showing only limited differences between ACC and crystalline phases (e.g. at 10 °C and pH 8, δ¹³CVPDB= –3.99 ‰ for ACC and –4.95 ‰ for calcite). This relative stability reflects the weaker temperature dependence of carbon isotope fractionation and the dominant control exerted by pH on dissolved inorganic carbon (DIC) speciation, sensitive to pH variations.

Oxygen and carbon isotope equilibrium between carbonate phases and the initial reactive water is variably approached depending on pH, T, and mineral phase. At high pH (≥10) and elevated T, isotopic equilibrium is not reached for ACC and the resulting crystalline phases due to rapid precipitation and transformation kinetics that limit isotope exchange with the aqueous phase. Lower pH and moderate T favor closer approach to equilibrium, whereas low water/solid ratios and the presence of pASP promote isotopic disequilibrium by limiting recrystallization-driven exchange.

These results highlight the potential for kinetically controlled isotopic signatures in carbonates formed via amorphous precursors, with implications for paleoenvironmental interpretations.