Δ₄₇-Δ₄₈ systematics of speleothem carbonate

Speleothem carbonate represents a key archive for the reconstruction of paleo-environmental conditions. Its oxygen isotope composition (δ18O) depends on temperature, the δ18O of cave water and kinetic isotope effects. No approach has been available yet that allows reliable reconstruction of both cave temperature and δ18O of cave water from the analysis of the stable isotopic composition of the carbonate. It has recently been postulated that dual clumped isotope thermometry (i.e. simultaneous Δ₄₇ and Δ₄₈ measurements) may overcome this limitation, making quasi-equilibrium speleothems identifiable for the purpose of robust paleoclimate reconstructions (Parvez et al., 2024).We have analyzed the dual clumped isotope compositions of a wide variety of speleothems considering full error propagation at the 95 % confidence interval level. Δ₄₇ and Δ₄₈ values of coarsely grained cryogenic and pool carbonates plot indistinguishably from equilibrium and measured Δ₄₇ values conform to independently constrained formation temperatures. These results imply that slow diffusion of CO₂ out of the water column becomes the rate-limiting process in the formation of these carbonates such that isotopic equilibrium in the H₂O-DIC-CaCO₃ system is likely attained during progressive precipitation.Δ₄₇ and Δ₄₈ values of stalagmites sampled closest to their growth axes also plot indistinguishably from equilibrium. However, most stalagmites exhibit a negative Δ₄₇ disequilibrium bias, i.e. measured Δ₄₇ values yield apparent formation temperatures that are higher than independently constrained cave temperatures. Observed disequilibrium bias agrees with theoretical predictions made by the isotope-enabled diffusion–reaction model IsoCave (Guo and Zhou, 2019a) according to which this bias predominantly originates from unidirectional Rayleigh removal of dissolved bicarbonate through dehydration and dehydroxylation reactions. Due to current shot noise limit constraints on Δ₄₈, isotopic disequilibrium along the stalagmite growth axis remains challenging to be resolved by Δ₄₇/Δ₄₈ analysis alone. Unless kinetic bias can be ruled out independently, Δ₄₇ values of stalagmites should, therefore, only be considered as upper limits of cave temperature.Correction of kinetic bias in carbonate formation temperatures seems possible yielding uncertainties of ±3–4 °C on the 68 % confidence interval level but requires further constraints on the accuracy of kinetic isotope effects associated with dehydration and dehydroxylation reactions.