SPATIAL AND TEMPORAL RECONSTRUCTIONS OF CHANGES IN THE ASIAN PALAEOMONSOON - A NEW MINERAL MAGNETIC APPROACH

A new quantitative approach is proposed for estimating palaeoprecipita tion across the Chinese Loess Plateau. At present, there is a strong r ainfall gradient across the plateau from approximately 300 mm/yr in th e near-desert conditions in the northwest to over approximately 750 mm /yr in the southeast, just 700 km distant. We find that the concentrat ion of ferrimagnetic iron oxide minerals in nine modem soil types (rep resented by 37 individual soil profiles) is strongly correlated with t his contemporary rainfall gradient. The ferrimagnetic concentration ri ses along this gradient, from 0.01% in the northwest to over 0.2% in t he southeast. The nine modern soil types have been used in the constru ction of a rainfall vs. magnetic susceptibility (least squares regress ion) climofunction. Past variations of loess-soil iron oxide content a re easily established through magnetic susceptibility measurements and so can be used to reconstruct the rainfall of former interglacial and glacial periods. The physical and pedological basis of the rainfall v s. susceptibility relationship is discussed and potential limitations of our rainfall reconstruction method are explored. Our palaeoclimate reconstructions indicate dramatic changes in rainfall due to variation s in the structure of the Asian monsoon. The rainfall variations are a bout four times greater than has been suggested for this region by atm ospheric general circulation modelling. Our data indicate increased ra infall throughout central China both in interglacial periods and in th e early Holocene. The increases in monsoonal rain were particularly pr onounced at our westernmost sites, adjacent to the northeastern edge o f the Tibetan plateau. In contrast, for glacial periods a reduction in rainfall is found across the whole loess area, with the greatest decr eases in the southeast.