EVIDENCE OF ASTRONOMICAL FORCING OF THE EARTHS CLIMATE IN CRETACEOUS AND CAMBRIAN TIMES

We have completed analyses of three sections of shallow marine sedimen tary cycles that were deposited at about 80 Ma and between about 510 M a and 530 Ma in different parts of North America. In spite of the anti quity of these strata, and the differences in the age, location and cy cle types, power spectra in all of the records have distinct spectral peaks with ratios that are consistent with forcing by eccentricity, ob liquity and climatic precession. For two examples, Utah (530 Ma) and t he southern Canadian Rockies (510 Ma), the spectra reveal prominent pe aks corresponding to obliquity with a period closer to that estimated for early Paleozoic time, than for the modern. These are the first res ults supporting recent calculations of changes in astronomical periods over geological time. We were able to extract these intriguing result s 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 tim e in cyclic strata. The results are evidence that astronomical forcing has left a robust signal in the geological record and that the astron omical periods observed in the geological data are consistent with rec ent calculations of the shifts in those periods over Phanerozoic times .