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preceded the Mid-Pleistocene revolution at 0.9 myr when the boreal ice sheet increased in size and the prevailing glacial cyclicity changed from ~40 kyr to ~100kyr. Before the Quaternary, δ13Cmax usually occurred at times of minimal eccentricity forcing, and the long term-variations in δ13C and δ18O display the same 400 kyr cycles; but in the Quaternary, δ13Cmax-II and δ13Cmax-III are out of phase with this astronomical cycle (Fig.1). Therefore, prior to the formation of large ice sheets in the Northern Hemisphere, δ13C co-varied with δ18O in the ocean records but in Quaternary times the 400-kyr cyclicity waned in the δ18O records and apparently “stretched” into 500 kyr in δ13C. The origin of the δ13Cmax episodes in the Quaternary and the nature of the long-term cycles remain unclear. Our working hypothesis suggests that the long-term cycles in weathering intensity in tropical areas may give rise to increased fl ux of Si from land to ocean, which may change the diatom/coccolith ratio in phytoplankton and subsequently the organic/inorganic carbon ratio in sediments. There is strong evidence for the 400-kyr cyclicity in monsoon climate and in opal production in the ocean that is well-correlated with the δ13C record. However, more work is needed to fi nd out the mechanism behind the observed changes in carbon cycling. A common practice in Quaternary climate history studies is just to peruse ice-volume variations as exhibited by δ18O, by considering carbon system changes as the consequences of ice-volume changes. The new discovery shows that long-term changes in carbon reservoirs on the Earth have their own periodicity and their own history, and do not simply follow ice cap variations in the Northern Hemisphere. The Quaternary period has passed through three major stages defi ned by four δ13Cmax events, and each appears to represent a further step in icecap development. Now the Earth is passing through a new carbon isotope maximum, δ13Cmax-I. It is therefore crucial to understand the physical and climatic signifi cance of the long-term carbon cycles, if we are to predict the natural long-term changes of global climate.

A 1437-year Precipitation History From Qilian Juniper in the Northeastern Qinghai-Tibetan Plateau