Oceanic anoxic cycles? Orbital prelude to the Bonarelli Level (OAE 2)

The prelude to the second global oceanic–anoxic event of the Cretaceous (OAE 2), as recorded in the classic sections of the Scaglia Bianca Formation in the Umbria–Marches region of Italy, is characterized by an oscillation between black to gray, organic-rich cherts and white pelagic limestones. Spectral analysis of these lithologic alternations reveals a robust set of cycles whose periods are remarkably similar to the orbital cycles calculated by Laskar et al. [Laskar, J., Robutel, P., Joutel, F., Gastineau, M., Correia, A. C. M., Levrard, B., 2004, A long term numerical solution for the insolation quantities of the Earth. Astronomy and Astrophysics, 428, 261–285.]. Bandpass filtering shows that the 140–80 kyr signal approximates the envelope of the 26–16 kyr signal, just as expected if these components represent responses to orbital eccentricity and precession. Matching the presumed eccentricity signal to the theoretical eccentricity of Laskar et al. yields an age for the base of the Bonarelli Level (onset of OAE 2 in Italy) of 94.21 Ma (93.72 Ma provides an alternative), consistent with radiometric constraints from a correlative section in the western United States. Given the strong orbital control on the cyclic deposition of the organic-rich precursors of OAE 2 within the Scaglia Bianca Formation, we hypothesize that the timing of OAE 2 itself is similarly controlled by astronomical variations. Indeed, the occurrence of OAE 2 coincides with a protracted period of unusually low insolation variation, owing to simultaneous nodes in all orbital parameters. We speculate that other OAEs similarly coincided with insolation nodes and dampened seasonality. From this perspective, some (but perhaps not all) of the OAEs may be periodic events reflecting cycles of oceanic anoxia that predictably punctuated a ~ 50 Myr period of Cretaceous climate.