Pleistocene climate evolution in the eastern Pacific and implications for the orbital theory of climate change

Oxygen isotope records show that the transition from 41-kyr glacial cycles to 100-kyr cycles occurred in the mid-Pleistocene; however, we know little about the glacial world before the mid-Pleistocene, especially the surface oceans. Here three sea surface temperature (SST) and marine productivity records, as well as ice volume records when they were not available, were reconstructed for the past 1.8 Myr (million years) from the California margin and the eastern equatorial Pacific (EEP). All the ice volume records showed that early Pleistocene ice volume was only two thirds of the late Pleistocene, whereas glacial-interglacial SST variations were 6--8°C off the California margin (ODP Sites 1020 and 1012) and 3--4°C in the EEP (ODP Site 846) throughout the entire Pleistocene. However, these two of the largest upwelling areas in the world, controlled by different circulation systems, displayed different glacial-interglacial responses. During the interglacial period when the SSTs were relatively high, more marine productivity occurred off the California margin and less in the EEP.; Early-Pleistocene tropical SSTs (ODP 846) were dominated by 41-kyr cycles and anti-phased to local annual insolation. Thus a high-latitude process must control tropical (EEP) SSTs, as well as marine productivity, in the early Pleistocene through the obliquity forcing. Predominant 41-kyr climatic cycles off the California margin in the early Pleistocene can be also explained in the framework of obliquity forcing which redistributes insolation between high latitudes and tropics.; Most records clearly show the transition from the early-Pleistocene 41-kyr cycles to the late-Pleistocene 100-kyr cycles. There appeared to be an apparent interplay between 41-kyr and 100-kyr power in the SST records and an intermediate period (∼80 kyr), subharmonic of the fundamental obliquity period, in the mid-Pleistocene. The amplitude of climatic variations also matched better with the amplitude of obliquity forcing than precession forcing. Thus the late-Pleistocene 100-kyr cycles may be nonlinearly connected to the obliquity forcing through the subharmonics of obliquity cycles. The evolution of Pleistocene glacial-interglacial changes is interpreted as the development of nonlinearity within the climate system in the mid- and late Pleistocene, which acts on the primary obliquity resonance.