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

A new quantitative approach is proposed for estimating palaeoprecipitation across the Chinese Loess Plateau. At present, there is a strong rainfall gradient across the plateau from approximately 300 mm/yr in the near-desert conditions in the northwest to over approximately 750 mm/yr in the southeast, just 700 km distant. We find that the concentration of ferrimagnetic iron oxide minerals in nine modem soil types (represented by 37 individual soil profiles) is strongly correlated with this contemporary rainfall gradient. The ferrimagnetic concentration rises along this gradient, from 0.01% in the northwest to over 0.2% in the southeast. The nine modern soil types have been used in the construction of a rainfall vs. magnetic susceptibility (least squares regression) climofunction. Past variations of loess-soil iron oxide content are 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 vs. susceptibility relationship is discussed and potential limitations of our rainfall reconstruction method are explored. Our palaeoclimate reconstructions indicate dramatic changes in rainfall due to variations in the structure of the Asian monsoon. The rainfall variations are about four times greater than has been suggested for this region by atmospheric general circulation modelling. Our data indicate increased rainfall throughout central China both in interglacial periods and in the early Holocene. The increases in monsoonal rain were particularly pronounced at our westernmost sites, adjacent to the northeastern edge of the Tibetan plateau. In contrast, for glacial periods a reduction in rainfall is found across the whole loess area, with the greatest decreases in the southeast.