A 40-million-year lake record of early mesozoic orbital climatic forcing.

Sediments of the early Mesozoic Newark Supergroup of eastern North America consist largely of sedimentary cycles produced by the rise and fall of very large lakes that responded to periodic climate changes controlled by variations in the earth's orbit. Fourier analysis of long sections of the Late Triassic Lockatong and Passaic formations of the Newark Basin show periods in thickness of 5.9, 10.5, 25.2, 32.0, and 96.0 meters corresponding to periodicities in time of roughly 25,000, 44,000, 100,0003,, 13000 and 400,000 years, as judged by radiometric time scales and varve-calibrated sedimentation rates. The ratios of the shortest cycle with longer cycles correspond closely to the ratios of the present periods of the main orbital terms that appear to influence climate. Similar long sequences of sedimentary cycles occur through most of the rest of the Newark Supergroup spanning a period of more than 40 million years. This is strong evidence of orbital forcing of climate in the ice-free early Mesozoic and indicates that the main periods of the orbital cycles were not very different 200 million years ago from those today.