MID-PLEISTOCENE CLIMATE SHIFT INCREASED THE AMPLITUDE OF GLACIAL/INTERGLACIAL WINTER AND SUMMER MONSOON VARIATIONS IN THE CHINESE LOESS PLATEAU

A Mid-Pleistocene shift recorded in marine sediments starting at around 0.9 Ma represents a significant change in the frequency and intensity of global glacial/interglacial fluctuations. However, our evidence shows that the terrestrial systems such as the Chinese Loess Plateau recorded these changes after 0.65 Ma. A multiproxy database we have assembled for a section near Jiaodao, 50 km N of Luochuan, has provided important evidence for the changes associated with and following the mid-Pleistocene climate rearrangement. The first evidence of changes in the duration and intensity of glacial/interglacial fluctuations is recorded by the most prominent pedocomplex S5 (correlated to marine isotope stages 13-15). In the interval of formation of S5-L1 (0.65-0.05 Ma), we see a significant increase in the amplitude of variation of magnetic properties (e.g., magnetic susceptibility) between loess and paleosol layers. Moreover, the depth of decalcified zones in paleosols increases compared to older paleosols. Both proxies imply a significant increase in summer monsoon precipitation levels during soil-forming (interglacial) episodes of the past 0.65 Ma compared to earlier soil forming episodes. In contrast, other proxies, such as particle size, suggest an increase in winter monsoon strength during depositional (glacial) episodes. This mid-Pleistocene increase in precipitation gradient between periods of intensified summer monsoon (interglacials) and periods of intensified winter monsoon (glacials) is superimposed on a gradual long-term cooling trend (evidence from rubification and %clay) accompanied with long term gradual increase in winter monsoon strength (evidence from median particle size) from 2.6 Ma to present.