CONSTRAINTS ON LATE JURASSIC AND CRETACEOUS ATMOSPHERIC PCO2 AND PRIMARY PRODUCTIVITY FROM TRIPLE OXYGEN ISOTOPES IN DINOSAUR EGGSHELLS

The Δ¹⁷O value of atmospheric O₂ [Δ¹⁷O(O₂)] is related to pCO₂/pO₂ and gross primary productivity (GPP). Animals incorporate the Δ¹⁷O signal of atmospheric O₂ into body water through respiration, and biominerals forming in equilibrium with body water can preserve this signal over geological timescales. We have expanded the Kohn (1996) body water ¹⁸O/¹⁶O model to include ¹⁷O/¹⁶O, and we use the model to predict the degree of dilution of the Δ¹⁷O(O₂) signal by other sources of oxygen to the animal. Differences in animal physiology and climate can lead to a range in Δ¹⁷O(body water) of about 0.15 ‰ for contemporaneous animals (that is, equivalent to about 1000 ppm change in pCO₂ when pO₂ and GPP are held constant). We address this uncertainty by modeling maximum and minimum Δ¹⁷O(body water) endmembers under different Δ¹⁷O(O₂), and then by narrowing the range of concordant Δ¹⁷O(O₂) values that account for all observed Δ¹⁷O(body water) values of contemporaneous animals. We have applied this method to modern bird eggshells and to fossil dinosaurian eggshells (Late Jurassic and Cretaceous). We interpret Δ¹⁷O(body water) in terms of paleo-pCO₂ and GPP, as pO₂ variations during the Mesozoic likely had a minor influence on Δ¹⁷O(O₂). Mid- and Late Cretaceous fossil samples indicate slightly lower or similar Δ¹⁷O(O₂) values compared to modern samples, consistent with GPP/pCO₂ ratios that were similar to or slightly lower than present. The Late Jurassic samples indicate anomalously low Δ¹⁷O(O₂) values, pointing to pCO₂ several times higher than present, reduced gross primary productivity, or a combination of both. This points to a fundamentally different, "slow greenhouse" carbon cycle during the Late Jurassic. XRD and clumped isotope results suggest that most of the shells have been affected by diagenesis. However, diagenesis tends to elevate Δ¹⁷O of samples towards Δ¹⁷O of meteoric water, so the primary Δ¹⁷O values of the eggshells would have been lower than the observed values, indicating an even more anomalous carbon cycle in the Late Jurassic. The triple oxygen isotope approach, while unable to uniquely constrain pCO₂ or GPP, shows promise for identifying distinctive modes of the carbon cycle in the geological past.