TRIPLE OXYGEN ISOTOPE COMPOSITIONS OF CARBONATES: NEW TOOLS FOR TERRESTRIAL PALEOENVIRONMENTAL RESEARCH

The potential of triple oxygen isotopes in carbonates as a record of past environments has been appreciated for many years, but the approach has remained underexplored because of a lack of analytical methods with sufficient precision to resolve small, environmentally-significant deviations of ¹⁷O/¹⁶O from values predicted by measurements of ¹⁸O/¹⁶O and mass fractionation relationships. New methods developed in the past two years change this situation, allowing measurements of ¹⁷O anomalies with the necessary precision of < 0.015 ‰ on CO₂ generated from < 10 mg samples of carbonate. We describe one such method developed in our laboratory, and then discuss potential and emerging applications. Relevant to terrestrial Earth-surface systems, our current understanding suggests that there are three dominant mechanisms for generating large ¹⁷O anomalies in waters (and hence recorded in carbonates forming in equilibrium with those waters): 1) ¹⁷O anomalies generated during the origin and transport of atmospheric water from oceanic source regions to the continents, 2) ¹⁷O anomalies generated during subsequent evaporation of waters, including evaporation from raindrops, lakes, soils, leaves, and animals, and 3) ¹⁷O anomalies reflecting metabolic water, which samples ¹⁷O-depleted atmospheric O₂ via the respiration reaction CH₂O + O₂ --> CO₂ + H₂O. The latter sets up a potential paleo-CO₂ barometer, as the ¹⁷O-depletion of atmospheric O₂ is predicted to increase with increasing atmospheric CO₂ levels, and this signal can be archived in materials like mammalian tooth enamel and dinosaurian eggshell that record the composition of body water. We present new data from meteoric waters, soil carbonates, marine carbonates, modern bird eggshell, and late Cretaceous dinosaur eggshell. We find that average ¹⁷O anomalies, or Δ¹⁷O values, of waters in equilibrium with these carbonates generally order as Δ¹⁷O(meteoric water) > Δ¹⁷O(soil water) > Δ¹⁷O(modern bird body water) > Δ¹⁷O(Cretaceous dinosaur body water). These results are consistent with a small amount of evaporative enrichment of soil water compared to meteoric water, a contribution of evaporated water sources and atmospheric O₂-sourced oxygen to the body water of modern birds, and higher levels of CO₂ in the late Cretaceous compared to the present day.