Quantification of Deep-time Orbital Forcing by Average Spectral Misfit

Quantification of Milankovitch orbital cyclicity within ancient strata has become a principal tool for refinement of the geologic time scale. However, accurate characterization of the orbital signal in deep time paleoclimate records is commonly challenged by inadequate radiometric time constraints for calibration of the spatial rhythms to temporal periods. This problem can potentially introduce large errors into derivative orbital timescales. In this study we develop a new method for the identification and calibration of orbital cyclicity in cyclostratigraphic records. The method (average spectral misfit, or ASM) yields an objective estimate of the optimal sedimentation rate for a stratigraphic interval that preserves a record of orbital forcing. The technique also provides a formal statistical test for rejecting the null hypothesis (no orbital signal). Application of the method to assess orbital cyclicity in the upper Bridge Creek Limestone Member (Turonian) of the Western Interior Basin highlights the utility of this new cyclostratigraphic tool, and provides a means to independently evaluate conflicting interpretations of the lithologic cycles. Importantly, ASM offers a new consistent standard by which orbital timescales may be compared. Hence, the quality of an orbital timescale can be formally qualified by reporting its average spectral misfit and null hypothesis significance level. This technique will permit improvement of Mesozoic/Cenozoic orbital timescales and extension of orbital time scale development into the Paleozoic, as the method is not dependent upon wellconstrained radiometric age data.