Paper No. 2
Presentation Time: 8:25 AM
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.