PLIOPLEISTOCENE MEGACYCLES - RECORD OF CLIMATE AND TECTONICS

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 p lace with a larger amplitude than any experienced during the Holocene. Variations of dry and wet climates in the Chinese Loess Plateau and i n the loess belt of central Europe have similar frequency. It has been convincingly proved that the periodicity of the past gross climate ch anges agrees with that of the earth's orbital perturbations. However, while the amplitude of the orbital cycles and of the oceanic oxygen is otope variations is relatively uniform, several of the cold climate ep isodes recorded on land appear more severe than the remainder. They co rrespond to Chinese loess units L1, L2, L5, L6 and L9, correlating wit h central European loess of cycles B, C, F, H and J. These units corre spond to oxygen isotope stages 2, 6, 12, 16 and 22 and to the Weichsel ian, Saalian and Elsterian ice advances. In addition, Chinese loess la yers 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 with in the last 2.5 m.y. ten megacycles, describe them in the Chinese loes s sequences, and name them in the order of increasing age MC1-MC1O. Ea ch 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 int errupted by relatively mild glacials. The megacycles of the last 2.5 m .y. lasted between 200 and 340 millennia each, with an average duratio n of 255 thousand years. When correlated with the stratigraphic record of the Chinese Loess Plateau, the Wurm terrace as well as the Saale g lacier advances, the Eem marine transgression, the Weichsel ice advanc e and the Holocene correspond to a still incomplete megacycle MC1. The Riss terrace, the Elster ice advance, Holstein transgression and subs equent 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 betwe en the limited number of classical continental glacial and interglacia l 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 oceani c transgressions reaching deep inland. There is no apparent astronomic reason for the exceptional intensity and deep southward penetration o f continental ice sheets during only some of the cold intervals. Since the differences are much more pronounced on land than in the ocean, a nd since the major ice advances are closely linked with terraces, it s eems that the megacycles were preconditioned by tectonic movements. In termittent pulses of rapid uplift of mountain ranges could have impact ed climate worldwide by deflecting atmospheric circulation in the sens e of Ruddiman and Raymo (1988). It is also probable that the tectonica lly changing shape of the sea floor sills reoriented the oceanic circu lation, 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 corr elate closely with the advances of continental ice in Europe, suggesti ng that the uplift of the Tibet Plateau significantly contributed to t he gradual climate deterioration of the last million years.