DID ATMOSPHERIC CO2 FORCE THE MID-CRETACEOUS HOT GREENHOUSE? (Invited Presentation)

The mid Cretaceous Hot Greenhouse during the Turonian through early Campanian was characterized by high latitude ocean temperatures similar to those in the tropics. This exceptionally low meridional temperature gradient has been difficult to simulate with climate models. Atmospheric CO₂ is one factor that might have driven or helped support such warmth. Was atmospheric CO₂ elevated during this period compared with other times during the Cretaceous when the meridional temperature gradient was more similar to modern? Existing paleoatmospheric CO₂ estimates diverge during the Hot Greenhouse. Best estimates of paleoatmopsheric CO₂ levels based on several paleosol carbonate records increase during the Albian and Cenomanian whereas those based on stomata slightly decrease. The paleosol-based CO₂ concentrations are >2x higher than the stomata-based concentrations during the Hot Greenhouse. Such divergence is unusual for these two records, which largely overlap during the past 300 million years. The Hot Greenhouse stomata-based CO₂ levels (450 ppmV) are in the sensitive range of this proxy meaning the divergence between the two proxies cannot be attributed to the saturation of this proxy that occurs at higher CO₂ levels (~750 ppmV). The stomatal-based values are also supported by liverwort data, which result in similar CO₂ levels and a similar trend across this interval. The data density during the Hot Greenhouse is lower than at any other point during the Cretaceous. We conclude that atmospheric CO₂ levels during the Mid-Cretaceous Hot Greenhouse are poorly understood, perhaps more so than almost any other time during the past 300 million years.