Plio-Pleistocene megacycles: record of climate and tectonics

Abstract

Abstract Paleoclimatic records from the deep sea floor show that in the last 3 m.y. more than 100 oscillations of mild and cool global climate took place with a larger amplitude than any experienced during the Holocene. Variations of dry and wet climates in the Chinese Loess Plateau and in the loess belt of central Europe have similar frequency. It has been convincingly proved that the periodicity of the past gross climate changes agrees with that of the earth's orbital perturbations. However, while the amplitude of the orbital cycles and of the oceanic oxygen isotope variations is relatively uniform, several of the cold climate episodes recorded on land appear more severe than the remainder. They correspond to Chinese loess units L1, L2, L5, L6 and L9, correlating with central European loess of cycles B, C, F, H and J. These units correspond to oxygen isotope stages 2, 6, 12, 16 and 22 and to the Weichselian, Saalian and Elsterian ice advances. In addition, Chinese loess layers L13, L15, WL1, WL2, WL3 and WL4 appear to represent exceptionally dry and cold episodes, apparently correlative with the inter-Bavelian cooling and with Menapian, Eburonian and Praetiglian glacials of the Rhine Delta. Based on this observation, we propose to distinguish within the last 2.5 m.y. ten megacycles, describe them in the Chinese loess sequences, and name them in the order of increasing age MC1–MC10. Each megacycle starts with an unusually thick loess unit interpreted as a deposit of an exceptionally severe glaciation and terminates with a cluster of well developed soils, indicative of warm humid climates interrupted by relatively mild glacials. The megacycles of the last 2.5 m.y. lasted between 200 and 340 millennia each, with an average duration of 255 thousand years. When correlated with the stratigraphic record of the Chinese Loess Plateau, the Wurm terrace as well as the Saale glacier advances, the Eem marine transgression, the Weichsel ice advance and the Holocene correspond to a still incomplete megacycle MC1. The Riss terrace, the Elster ice advance, Holstein transgression and subsequent mild and cool intervals ending with the Schoningen interglacial (sensu Urban, 1992) belong to the megacycle MC2. Recognition of four megacycles within Brunhes resolves the long standing discrepancy between the limited number of classical continental glacial and interglacial units and the frequent climate oscillations recorded in the deep sea sediments and the loess sequences. Only in the Holsteinian and Eemian interglacials was the crustal rebound delayed enough to permit oceanic transgressions reaching deep inland. There is no apparent astronomic reason for the exceptional intensity and deep southward penetration of continental ice sheets during only some of the cold intervals. Since the differences are much more pronounced on land than in the ocean, and since the major ice advances are closely linked with terraces, it seems that the megacycles were preconditioned by tectonic movements. Intermittent pulses of rapid uplift of mountain ranges could have impacted climate worldwide by deflecting atmospheric circulation in the sense of Ruddiman and Raymo (1988). It is also probable that the tectonically changing shape of the sea floor sills reoriented the oceanic circulation, which then could have affected Plio-Pleistocene climates world-wide. The loess-covered terraces in the Lanzhou area demonstrate that the tectonic movements in the northern part of the Tibet Plateau correlate closely with the advances of continental ice in Europe, suggesting that the uplift of the Tibet Plateau significantly contributed to the gradual climate deterioration of the last million years.