Insolation changes, ice volumes, and the O18 record in deep‐sea cores

A detailed curve of ice volume versus time is needed in order to test the validity of the hypothesis that changes in the earth's orbital parameters are the cause of oscillations in Pleistocene climate. Although absolute ages available for glacial moraines and raised coral reefs provide a number of key points, they by no means allow a continuous curve to be drawn. Those points that exist, however, are entirely consistent with the hypothesis that the O 18 /O 16 curves from deep‐sea cores provide good approximations to the ice volume record. If so, then the primary glacial cycle must be sawtoothed in character; gradual glacial buildups over periods averaging 90,000 years in length are terminated by deglaciations completed in less than one tenth this time. Modulating this primary cycle are secondary oscillations. Those recognized during glacial growth phases average 20,000 years in length and those during the retreats about one thousand years in length. When the ice volume curve obtained in this way is compared with the summer insolation curve for the northern hemisphere, it is seen that the rapid deglaciations occur during times of unusually great seasonal contrast and that the secondary cycles modulating the glacial buildups closely parallel the insolation variations. Although these findings provide convincing evidence for the influence of orbital changes on climate, the cause of the primary sawtoothed cycle is still an open question. In conjunction with this study, we have determined the O 18 /O 16 record for Caribbean core V12‐122 and find it to be compatible with those given by Emiliani for cores P6304‐8 and P6304‐9. Our dating of this core by Pa 231 ‐Th 230 and by magnetic reversals, however, strongly suggests that the absolute time scale adopted by Emiliani for deep‐sea cores must be increased by 25%.