Isotope fractionation mechanisms involved in carbonate formation revealed by high-precision triple oxygen isotope analyses

The oxygen (δ18O) and clumped (∆47) isotope composition of carbonates are widely used proxies for palaeotemperature. However, Earth surface carbonates are rarely formed in isotope equilibrium but often show oxygen and clumped isotope compositions that do not accurately reflect their crystallisation temperature. Isotopic disequilibrium commonly observed in biogenic carbonates and speleothems is inherited from the dissolved inorganic carbon pool of their parent solutions. A combination of isotope systems, e.g., ∆47–δ18O, ∆47–∆48, can uncover and correct for such kinetic effects [1].

The analysis of the 17O/16O ratio to the more commonly investigated 18O/16O ratio in carbonates — referred to as the triple oxygen isotope method — expands the traditional oxygen isotope scheme by another dimension and; thereby, allows the identification of fractionation processes involved in carbonate formation. In addition, the triple oxygen isotope method can give insight into the diagenetic history of ancient carbonates and offers the possibility of reconstructing the primary isotope compositions of altered samples [2].

Here, we report triple oxygen isotope ratios (Δ17O) of various altered and unaltered biogenic (e.g., brachiopods, belemnites) and abiogenic (e.g., speleothems, laboratory precipitates) carbonates. The high-precision measurements were performed on CO2 gas from acid digestion, using tunable infrared laser differential absorption spectrometry (TILDAS) [3]. Based on the results, we will discuss what combination of kinetic isotope fractionation and diagenetic processes could have played a part in the formation of the investigated carbonates.

[1] Bajnai et al., 2020, Nat Comms; [2] Wostbrock et al., 2020, GCA; [3] Pack et al., this conference.