Mesozoic atmospheric CO ₂ concentrations reconstructed from dinosaur tooth enamel

Air-breathing vertebrates incorporate a fraction of isotopically anomalous air O₂ in their body water. The ¹⁷O isotope anomaly of air O₂ (expressed as Δ'¹⁷O_air) is related to atmospheric CO₂ concentrations (<i>p</i>CO₂) and gross primary production (GPP). Tooth enamel records the Δ'¹⁷O of body water and can thus preserve such paleo-<i>p</i>CO₂ or paleo-GPP information over geological time periods. Here, we demonstrate the potential of respective reconstructions of atmospheric <i>p</i>CO₂ or GPP from the triple oxygen isotope composition of fossil dinosaur tooth enamel. The data from unaltered enamel samples, along with an assumed modern GPP_t/GPP₀ ratio of 1 for the Mesozoic, suggest a mean Late Jurassic <i>p</i>CO₂ = 1,200 ± 150 ppmv and Late Cretaceous <i>p</i>CO₂ = 750 ± 200 ppmv. These estimates are in good agreement with other <i>p</i>CO₂ proxy data for the same time intervals. When utilizing a <i>p</i>CO₂ inferred from other proxies, tooth enamel Δ'¹⁷O_PO4 may also serve as a proxy for GPP. Using published <i>p</i>CO₂ data, we reconstructed GPP_t/GPP₀ ratios with 1.20 ± 0.17 for the Late Jurassic and 2.24 ± 0.96 for the Late Cretaceous, which would imply a 20 to 120% higher GPP in the Mesozoic than today. Overall, triple oxygen isotope analysis of fossil teeth of terrestrial amniotes can provide insights into past atmospheric greenhouse gas content and global primary productivity.