Conference Agenda

Detection of small variations in triple oxygen isotope ratios (Δ’¹⁷O) of rocks, minerals, and water has opened new applications in the field of isotope geochemistry (1,2). A long-standing problem is extending the approach to CO₂ and carbonates because of analytical difficulties getting precise Δ’¹⁷O of CO₂. Direct measurement of Δ’¹⁷O of CO₂ by means of high-resolution gas source mass spectrometry has been successfully demonstrated but requires measurement times of days for reaching a precision <10 ppm (3). Here, we present data of high-precision analyses of carbonate-derived CO₂ and CO₂ from the conversion of air O₂ using a tunable infrared diode laser absorption spectroscopy (TILDAS) attached to a custom-built inlet system controlled by free software (PHP, Python, JavaScript, CSS, HTML) and low-cost electronic hardware, i.e., Raspberry Pi. With this device, we now achieve an external reproducibility as small as ±5 ppm. This opens new application fields, of which some will be presented.

1. A. Pack and D. Herwartz, The triple oxygen isotope composition of the Earth mantle and understanding Δ¹⁷O variations in terrestrial rocks and minerals. Earth Planet. Sci. Lett. 390, 138-145 (2014).

2. E. Barkan and B. Luz, High precision measurements of ¹⁷O/¹⁶O and ¹⁸O/¹⁶O ratios in H₂O. Rapid Commun. Mass Spectrom. 19, 3737-3742 (2005).

3. G. A. Adnew, et al., Determination of the triple oxygen and carbon isotopic composition of CO₂ from atomic ion fragments formed in the ion source of the 253 Ultra High‐Resolution Isotope Ratio Mass Spectrometer. Rapid Commun. Mass Spectrom. 33, 1363-1380 (2019).

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.

Carbonate classic and clumped isotope ratios (δ¹⁸O and Δ₄₇) are used to reconstruct paleotemperature. However, kinetic isotope effects (KIE) induce a bias on absolute temperature reconstructions. High precision δ¹⁷O measurements provide a refinement proxy to identify the direction and magnitude of KIEs in carbonates. Progress on this new proxy will be discussed.

The basic concept is similar to the combination of conventional oxygen and hydrogen isotope analyses in water. Combined δ¹⁸O and δ²H analyses are commonly used to approximate the magnitude of KIEs during evaporation. Triple oxygen isotope measurements in water are now used in a similar fashion. The simultaneous utilization of both trajectories provides a powerful approach to reconstruct quantitative paleoclimate information. We applied this approach to extracted water from ancient gypsum formed in the Atacama Desert and in Cyprus during the Messinian Salinity Crisis. Quantitative information on paleo-humidity and palaeohydrology are obtained. Effectively, this information is derived by quantifying KIE.

In this example, the KIE is related to diffusion of water molecules through air. Other examples of KIE include the breaking of chemical bonds, molecular mixing effects or steady states. Triple oxygen isotope trajectories allow to distinguish between such fundamental processes. Respective concepts will be explained using examples from chert and phosphate triple oxygen isotope systematics.