TRIPLE OXYGEN ISOTOPES IN THE STUDY OF TERRESTRIAL PALEOENVIRONMENTS: ADVANCES AND OPPORTUNITIES

The study of triple oxygen isotope variations in the hydrological cycle and atmosphere has progressed significantly in the last ten years, although largely under the radar of the community studying terrestrial paleoenvironments and paleoelevation. However, new analytical methods now permit high-precision triple oxygen isotope analysis of carbonates, hence extending triple oxygen isotope applications into the continental sedimentary record. We discuss the potential for triple oxygen isotopes to be used alongside traditional δ¹³C, δ¹⁸O, and Δ₄₇ (clumped isotope) data to help refine paleoelevation and paleoenvironmental interpretations.

The triple oxygen isotope deviation (or "anomaly"), Δ¹⁷O = ln(δ¹⁷O + 1) – λ*ln(Δ¹⁸O + 1), describes the departure of δ¹⁷O from an expected or arbitrary relationship with δ¹⁸O, represented by λ (here λ = 0.528, the slope of the global "triple oxygen isotope" meteoric water line). Evaporative processes have fractionation slopes λ that are lower than 0.528, leading to decreased Δ¹⁷O values of residual waters, while equilibrium fractionation between liquid water and water vapor carries a slope of 0.529, leading only to subtle variation in Δ¹⁷O. In addition, atmospheric O₂ carries a distinctly low Δ¹⁷O value that is related to stratospheric photochemistry and global carbon cycling.

In studies of paleoelevation and paleoclimate, the primary δ¹⁸O of meteoric precipitation is often sought because of its relationship with elevation and climate. However, the lake and soil waters from which carbonates grow may be evaporatively enriched in ¹⁸O relative to primary meteoric waters. Evaporated waters should be marked by low Δ¹⁷O values, and the combination of δ¹⁸O, Δ¹⁷O, and Δ₄₇ data will permit more accurate reconstruction of δ¹⁸O of meteoric precipitation.

In paleoecology and paleoclimate, the δ¹⁸O of vertebrate biogenic carbonate (e.g., mammalian tooth enamel) is often interpreted in terms of δ¹⁸O of meteoric water or aridity. Δ¹⁷O is sensitive to the effects of evaporation, and thus will help disentangle the effects of meteoric water δ¹⁸O and aridity on vertebrate oxygen isotope compositions. Finally, greenhouse carbon cycles should be marked by anomalously low Δ¹⁷O (atmospheric O₂) signals, and these will be reflected in vertebrate biogenic carbonate Δ¹⁷O compositions.