Identifying Cave Carbonate Isotope Fractionation Mechanisms through Triple Oxygen Isotope Analysis

Recent studies have demonstrated the efficacy of high-precision ∆′¹⁷O analysis in carbonates, biogenic and abiotic origin, to deduce geological and environmental processes. The δ¹⁷O values can be influenced by processes such as kinetic fractionation during carbonate precipitation, which is associated with the hydroxylation of CO₂, thereby making it an emerging proxy crucial for interpreting the oxygen isotopic ratio in carbonates and improving the accuracy of palaeoclimate reconstruction efforts (Bajnai et al., 2024). Δ′¹⁷O in cave carbonates helps determine the various factors influencing speleothem formation, including evaporation, condensation, and cave kinetics, which have been inadequately captured by the conventional dual-isotope (δ¹⁸O and δ¹⁶O) systematics. We follow the framework developed by (Huth et al., 2022) wherein interpretations of speleothem formation are done by examining trends of data spread through the distribution of triple oxygen isotopes in Δ′¹⁷O versus δ′¹⁸O space, with the conventional excess of ¹⁷O expressed as Δ′¹⁷O = δ′¹⁷O – λ_RL * δ′¹⁸O. By comparing triple oxygen isotopic compositions across various speleothem samples from different caves in North East India, this study seeks to improve our understanding of the control mechanisms on Δ¹⁷O variability and its utility in reconstructing past environmental conditions. The analysis of samples involved the acid digestion (in ~105 % H₃PO₄) of carbonate powders (~10 mg) followed by the catalytic CO₂-O₂ exchange reaction method as followed in the triple oxygen isotope analysis (Fosu et al., 2020) using in-house equipment with a quartz reactor containing Pt sponge (99.98% trace metal purity). The results yielded Δʹ¹⁷O in the range of -83 to -129 per meg.  When plotted in the Δ′¹⁷O versus δ′¹⁸O space, the data expands across three dominant controlling factors, majorly indicating an interplay of cave kinetics, Rayleigh distillation and cave temperature. This study proves that Δ′¹⁷O in cave carbonates act as a potential proxy for identifying fractionation processes.

 

References

• Bajnai, D., et al. (2024). Correcting for vital effects in coral carbonate using triple oxygen isotopes. Geochemical Perspectives Letters, 31, 38–43.
• Huth, T. E., at al. (2022). A framework for triple oxygen isotopes in speleothem paleoclimatology. Geochimica et Cosmochimica Acta, 319, 191–219.
• Fosu, B. R., et al. (2020). Technical Note: Developments and Applications in Triple Oxygen Isotope Analysis of Carbonates. ACS Earth and Space Chemistry, 4(5), 702–710.