DRIVERS OF CYCLIC SEA LEVEL CHANGE DURING THE CRETACEOUS GREENHOUSE: A NEW PERSPECTIVE FROM THE LEVANT PLATFORM (JORDAN)

In this study, we evaluate potential cyclic sea level changes during the Cenomanian and Turonian, spanning ~6 million years of time, through a new analysis of sequence stratigraphic stacking patterns of the Levant Platform in Jordan. This Tethyan carbonate platform preserves a relatively complete stratigraphic record through the interval, as indicated by biostratigraphy and new carbon isotope data, and also provides a clear imprint of sea level fluctuation as evidenced in sedimentologic structures and geochemistry (e.g., wt. % CaCO₃). Application of the Average Spectral Misfit method for astrochronologic testing (Meyers and Sageman, 2007) to wt. % CaCO₃ data confirms orbital influence on sedimentation through the OAE 2 interval, where data resolution is sufficient to resolve precession, obliquity and eccentricity components. This analysis also reveals a strong 1.2 Ma rhythm in wt. % CaCO₃ and δ¹³C data, which demarcates the "3^rd-order" sequence boundaries immediately above and below the C/T boundary, and is consistent with a long-period obliquity influence on sea level and the carbon cycle. "Minimal tuning" exercises provide additional evidence to support the presence of a pervasive 1.2 Ma pacing for all five sequence boundaries present in the section, in each case coupled to a 1.2 Ma cycle in the δ¹³C data. In the Oligocene, a 1.2 Ma obliquity cycle has been shown to be related to carbon cycle events, 3^rd-order sea level fluctuations and glacial periodicity. We consider possible drivers for this long-period linkage between orbital cycles, sea level, and the carbon cycle during the Cretaceous greenhouse, a time during which continental ice sheets are expected to be either ephemeral or non-existent. Our mineralogic assemblage data from the Levant Platform indicates systematic changes in weathering patterns that are consistent with variable freshwater transport between the ocean and land, supporting the hypothesis that freshwater storage in aquifers played an important role in modulating eustacy.

Meyers, S. R., and Sageman, B. B., 2007, Quantification of deep-time orbital forcing by average spectral misfit: American Journal of Science, v. 307, no. 5, p. 773-792.