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 identifica- tion and calibration of orbital cyclicity in cyclostratigraphic records. The method (average spectral misfit, or ASM) yields an objective estimate of the optimal sedimenta- tion 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 well- constrained radiometric age data. introduction As originally observed by Gilbert (1895), many rhythmically bedded hemipelagic deposits appear to reflect environmental oscillations on a timescale consistent with the astronomical cycles. Subsequent studies have recognized that lithologic rhythms preserved in deep-sea settings, carbonate platforms, and even lacustrine environments may also reflect orbitally forced/paced variations in climate (Schwarzacher, 1947; Fischer, 1980; Fischer and others, 1985; Herbert and Fischer, 1986; Olsen, 1986; Goldhammer and others, 1990; and many others). To move from these general observations to the empirical identification of specific orbital periods, construction of an orbital time scale, and interpretation of how the orbital-insolation signal influenced climate and sedimentation, however, requires a series of specific measurements, as well as application of certain uniformitarian assumptions. Generally, confirmation of orbital influence requires reduction of the lithologic signal to a finely sampled data series (for example, geochemical, color or bed thickness), application of appropriate Fourier techniques to identify significant spatial frequencies, and calibration of the observed spatial frequencies to temporal periods. Although this approach has become commonplace, there are many aspects of sam- pling and data analysis that can result in the introduction of error. Among these aspects, temporal calibration of spatial rhythms in the absence of adequate radiometric time control requires important assumptions about sedimentation rate, which can result in substantial errors in the orbital interpretation and derivative orbital time- scales. Variability in sedimentation rate within a given stratigraphic interval (for example, Meyers and others, 2001), stratigraphic changes in the dominance of one