Paper No. 335-9
Presentation Time: 3:25 PM
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.