QUANTITATIVE ESTIMATES OF PEDOGENIC FERROMAGNETIC MINERAL FORMATION IN CHINESE LOESS AND PALEOCLIMATIC IMPLICATIONS

The quasi-continuously deposited loess sediments of the central Chinese loess plateau constitute one of the most complete and long lasting records of Quaternary palaeoclimate on land. Much of this record has been uncovered by measurement and interpretation of the magnetic properties of the loess. The polarity of natural remanent magnetization has provided absolute age dates by correlation with the geomagnetic polarity time scale. Magnetic low field susceptibility has been used to reconstruct the cyclic changes of Pleistocene palaeoclimates, although the origin of the signal is still poorly understood. From comparison with the lobe concentration throughout the sequence we are able to determine the contributions to susceptibility from inherited and authigenic sources for the last 130 ky. Since the production of authigenic magnetite is directly related to regional precipitation, annual palaeoprecipitation rates can be calculated. Palaeoprecipitation rates similar to present day rates are derived for oxygen isotope stage 5 including the last interglacial. In contrast, cold periods corresponding to oxygen isotope stages 2 and 4 experienced negligible precipitation. The beginning and end of the time interval represented by oxygen isotope stage 3, however, was characterized by short episodes when precipitation exceeded the Holocene average.