Dual clumped isotope thermometry of speleothems

Speleothem carbonates are key archives for the reconstruction of terrestrial paleo-environmental conditions, but it is challenging to resolve between the temperature, hydrological and kinetic information that is recorded in their stable isotopic composition. Recently, it has been demonstrated that dual clumped isotope thermometry, i.e., analysis of Δ₄₈ alongside Δ₄₇ in CO₂ evolved from phosphoric acid digestion of carbonate [1], holds the potential to detect and quantify kinetic departures from isotopic equilibrium without having to know the oxygen isotope composition of the parent water, and, moreover, to correct for kinetic biases in carbonate formation temperatures [2].

We performed paired Δ₄₇-Δ₄₈ measurements on speleothems to investigate to which extent isotopic disequilibrium is recorded in these archives. No significant disequilibrium bias is recorded in slowly grown pool carbonates and cryogenic carbonates, and Δ₄₇ derived temperatures agree with expected formation temperatures. Most stalagmites, sampled closest to their growth axis where disequilibrium should be least pronounced, record a slight, but significant -Δ₄₇ disequilibrium bias. While the bulk set of investigated stalagmites exhibits a significant +Δ₄₈ offset from equilibrium, the extent of Δ₄₈ disequilibrium in individual samples remains below analytical resolution. Disequilibrium biases originate from rapid dehydration/dehydroxylation of bicarbonate occurring dissolved in the drip water. These results demonstrate that stalagmite Δ₄₇ values, without any additional evidence, should only be considered upper estimates of cave temperature, even if corresponding dual clumped data plots indistinguishably from the Δ₄₇-Δ₄₈ equilibrium line.

[1[ Fiebig et al. (2019), Chem. Geol. 522, 186-191

[2] Bajnai et al. (2020) Nat. Commun. 11:4005