A review of dissolved inorganic carbon isotopic fractionations in carbonate-bearing areas: Processes, models and applications

Dissolved inorganic carbon (DIC) represents a dynamic system of carbon species in aqueous solutions, exhibiting significant variations in both concentrations and isotopic compositions. Over the past decades, researchers have extensively reported the chemical and isotopic compositions of DIC, significantly enhancing our understanding of the mechanisms governing DIC cycling. While significant advances have been made in applying DIC isotopes (δ¹³C_DIC and Δ¹⁴C_DIC) for groundwater dating and carbon sources discrimination, some studies still fail to select the appropriate models (i.e. neglecting necessary isotopic fractionation), leading to misunderstandings. This review synthesizes published theories and models of carbon isotopic fractionation to examine DIC cycling under the following conditions: (I) carbonate weathering in both closed and open systems; (II) the transition from open to closed system; (III) the DIC−carbonate exchange process; (IV) strong acid-driven weathering; (V) temperature variations; (VI) CO₂ degassing from rivers. The detailed analyses performed to DIC's chemical and isotopic compositions reveal substantial differences in its behavior under contrasting environmental conditions, and thus these findings would guide the applications of carbon isotopes in groundwater dating and carbon sources discrimination. Specifically, in groundwater dating, neglecting the processes influencing ¹³C-¹⁴C data can significantly limit the utility of radiocarbon dating, and we thus recommended incorporating carbon mixing between two carbon-bearing reservoirs (soil CO₂ and carbonate) along with DIC exchange with these reservoirs. Furthermore, the non-linear variations between δ¹³C_DIC and Δ¹⁴C_DIC in river systems suggest that Δ¹⁴C_DIC is more suitable for constraining DIC sources, while δ¹³C_DIC is more sensitive to CO₂ degassing. We propose that a comprehensive understanding of the fundamental mechanisms controlling DIC's chemical and isotopic compositions, coupled with extensive measurements across diverse climate zones, would substantially improve our comprehension of the dynamic interplay between carbon cycling and climate change.