We have completed analyses of three sections of shallow marine sedimentary cycles that were deposited at about 80 Ma and between about 510 Ma and 530 Ma in different parts of North America. In spite of the antiquity of these strata, and the differences in the age, location and cycle types, power spectra in all of the records have distinct spectral peaks with ratios that are consistent with forcing by eccentricity, obliquity and climatic precession. For two examples, Utah (530 Ma) and the southern Canadian Rockies (510 Ma), the spectra reveal prominent peaks corresponding to obliquity with a period closer to that estimated for early Paleozoic time, than for the modern. These are the first results supporting recent calculations of changes in astronomical periods over geological time. We were able to extract these intriguing results from rocks as old and as poorly dated as these only after applying a new method, the gamma method, which we have developed for scaling time in cyclic strata. The results are evidence that astronomical forcing has left a robust signal in the geological record and that the astronomical periods observed in the geological data are consistent with recent calculations of the shifts in those periods over Phanerozoic times.
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