Effects of ionic strength and pH on 18O enrichment kinetics in aqueous carbonate and bicarbonate solutions

Oxygen isotopic composition plays a crucial role in the study of Earth's past climate conditions as well as understanding fluid-mineral reactions. Geochemical models commonly assume oxygen isotopic equilibrium between oxyanions and water. However, this may not be valid if the fluid-mineral reactions proceed more rapidly than isotopic enrichment in solution, where the enrichment rate can be influenced by pH and ionic strength. In this study, we used in-situ Raman spectroscopy to investigate the 18O enrichment kinetics into carbonate and bicarbonate oxyanions using solutions with 18O-enriched water containing different concentrations of NaCl. We report species-specific rates of 18O enrichment as a function of pH and ionic strength. The observed trends reflect expected dissolved CO2 behavior under different chemical conditions indicating that the experiments are following an incorporation reaction during the formation of different dissolved inorganic carbon species. In the carbonate system, increasing ionic strength significantly slows isotope incorporation. In contrast, the bicarbonate system shows a stronger dependence on pH, with minimal ionic strength effect. These findings highlight the distinct roles of pH and ionic strength in governing isotope incorporation kinetics across different dissolved inorganic carbon species. Our results refine the understanding of oxygen isotope behavior in aqueous, inorganic carbon systems, particularly with regards to environmental conditions with fluctuating pH and salinity.