NONLINEAR RESPONSE OF PLIOCENE CLIMATE TO ORBITAL FORCING: EVIDENCE FROM THE EASTERN EQUATORIAL PACIFIC

A high-resolution radiolarian record from the eastern equatorial Pacific is used to examine paleoceanographic events during the Pliocene, from 1.8 to 4.9 Ma. These data provide a means to evaluate the response of the equatorial Pacific to the onset of northern hemisphere glaciation near 2.47 Ma and to the closure of the Isthmus of Panama near 3.2 Ma. Responses are recorded both as long-term trends and as variations within the Milankovitch band (10(4) to 10(5) years). Radiolarian-based sea surface temperature (SST) estimates indicate a mean cooling from lower to upper Pliocene, but no significant changes occur near the onset of northern hemisphere glaciation, and no abrupt changes are observed before or after the closure of the isthmus. Spectral and cross-spectral analyses of this SST time series indicate concentrations of variance within the precession (23-kyr) and obliquity (41-kyr) bands in several but not all time intervals. Variance is commonly concentrated at lower frequencies. Cross spectral analysis of SST time series and calculated orbital variations indicates that the SST record is not coherent with, and therefore not linearly related to, orbital variations. Detailed time series analysis of the SST and delta O-18 time series indicates the absence of a direct linear relationship but rather the presence of a strongly nonlinear relationship between Pliocene climatic variables and Earth's orbit. High coherence is seen between the amplitude of the 23-kyr component of SST and the amplitude modulation of precession. The amplitude modulation of the 41-kyr component of the SST record is dissimilar to the amplitude modulation of obliquity but is extremely similar to and coherent with the amplitude of the 23 kyr component of SST. The amplitude modulations of SST and delta O-18 at a range of frequency bands have the same features, features that roughly follow the modulation of precession. This suggests a strongly nonlinear response of equatorial Pacific SST and planktonic delta O-18 to orbital forcing in the Pliocene. Such a response, extending over a number of frequency bands, has not previously been seeen in a Paleoclimatic data set but may be consistent with proposed models describing paleoclimatic variability. Although the reasons for this response are still unclear, it is evident that strong nonlinearities in climate are present during this time.