Earth and Planetary Science Letters, 103 (1991) 301-310 301 Elsevier Science Publishers B.V., Amsterdam [vN Magnetic susceptibility of loess in China Friedrich Heller, Liu Xiuming, Liu Tungsheng and Xu Tongchun Institute of Geophysics, ETH-HOnggerberg, CH-8093 Ziirich, Switzerland Institute of Geology, Academia Sinica, Beijing 100029, People's Republic of China Institute of Geophysics, State Seismological Bureau, Beijing 100081, People's Republic of China Received August 22, 1990; accepted October 22, 1990 ABSTRACT The low-field susceptibility of the loess deposits in the Chinese Loess Plateau is due to both paramagnetic and ferromagnetic minerals. Paramagnetism can be seen in the less- to unweathered loess horizons where superparamagnetic contributions which arise from ultrafine magnetite particles, are not as strongly developed as in the heavily weathered palaeosols. Frequency-dependent susceptibility and minimum unblocking temperatures of low-temperature thermoremanent magnetization generally indicate finer grain sizes in the palaeosols as compared to the loess beds. This grain size difference and the fact that susceptibilities of the same stratigraphic palaeosol layer vary by an order of magnitude between the western and central regions of the plateau, are considered as strong evidence that the ultrafine magnetite is not of detrital (aeolian) origin but was formed in situ. The pedogenetic processes are clearly related to palaeoclimatic changes. Therefore the various susceptibility properties may serve for a detailed reconstruction of the continental palaeoclimatic conditions for the last 2.5 Ma since the palaeomagnetically dated beginning of loess sedimentation in China. 1. Introduction Extremely thick loess sediments of mainly aeolian origin cover large parts of northern China (Fig. 1). Towards the west near Lanzhou (site Baichaoyuan), the loess sediments have accu- mulated to thicknesses of > 300 m, whereas in the central part, between Xifeng and Luochuan, 130- 180 m has been measured. The loess stratigraphy in China consists of alternating loess and palaeo- sol layers which can be correlated over large dis- tances [2]. Loess horizons represent times of in- creased aeolian dust deposition under dry-cold climatic conditions when weathering is not very strong. Palaeosols are produced in a humid-warm environment involving often extensive weathering and severe pedogenesis [3]. Low-field susceptibil- ity as a palaeoclimate indicator was first put for- ward by Heller and Liu [4,5] during magnetic investigations of several loess profiles near Luochuan (Fig. 1). The very comprehensive and detailed study of Liu et al. [6] verified that highs and lows of the susceptibility curve are closely correlated with the occurrence of palaeosol and 0012-821X/91/$03.50 ~) 1991 - Elsevier Science Publishers B.V. loess layers, respectively, in the loess sections at Xifeng (Fig. 1). The highest susceptibility values are observed here in soil layer S 5 where pedogene- sis was strongest, while susceptibility minima oc- cur mainly in the silty loesses, L 9 and Lts, with minimum weathering (Fig. 2). The Chinese loess susceptibility curves from Xifeng and Luochuan which have been dated magnetostratigraphically, are now widely accepted by Quaternary geoscien- tists as reliable continental palaeoclimate re- corders during the whole Pleistocene. The suscept- ibility variations have been linked directly to the oxygen isotope record obtained from deep sea sediments, thus allowing for a refined age de- termination between the well established polarity boundaries and for a worldwide comparison of oceanic and continental Quaternary climate cycles [5 91. At present, however, the fundamental relation- ship between susceptibility and palaeoclimate has not yet been clarified. No satisfactory explanation has yet been given to the question of why high/low susceptibility magnitudes correspond to periods of warm/cold climate in the Chinese loess deposits.