Case studies on the role of orbital forcing in Cretaceous greenhouse climates

The Cretaceous was characterised by a greenhouse climate with high sea levels, resulting in its famous carbonate-rich strata which often present a remarkable rhythmicity. Orbitally-forced cyclic sedimentation patters have been used to obtain stage durations and to refine the Geologic Time Scale. Despite the common application of cyclostratigraphic techniques, the exact origin of rhythmic banding patterns often remains unknown, as well as the role of orbital forcing in climatic extremes such as the Oceanic Anoxic Events (OAEs).

This contribution will explore the role of orbital forcing in the nature of rhythmic sedimentation and in the timing of carbon cycle perturbations in selected intervals of the Cretaceous. The 405-kyr cycle of eccentricity exerts a strong influence on bulk carbonate carbon isotope records and likely controls the timing of climatic events immediately preceding and postdating the Cretaceous/Paleogene Boundary. In the mid-Cretaceous, the timing of some OAEs may be linked to the grand eccentricity cycle (~2.4 Myr). In both time intervals, the orbital configuration is likely not the cause of environmental change, but may rather exacerbate the effects of other forcing mechanisms, such as large scale volcanism, by temporarily enhancing seasonal extremes. Elucidating the role of orbital forcing in past greenhouse climates of the Cretaceous crucially depends on the integration of climate sensitive records with independent age control from radio-isotopic dating and integrated bio-, magneto- and chemostratigraphy.