MAGNETIC ENHANCEMENT OF CHINESE LOESS - THE ROLE OF GAMMA-FE2O3

Various mechanisms have been proposed for the observed link between palaeoclimate and magnetic susceptibility in the Chinese loess. Many authors have recently pointed to the controlling influence that ultrafine (stable single domain/ superparamagnetic) magnetite/maghemite has on the magnetic susceptibility signal. However, there is still no clear evidence as to the origin of this susceptibility-enhancing magnetic fraction. We have collected samples from two sites representing the range of magnetic susceptibilities to be found in the Chinese loess record. Magnetic measurements indicate that susceptibility enhancement, throughout the loess plateau, is associated with the concentration of a ferrimagnetic fine fraction. This magnetic fraction has a uniform, non-variable grain-size distribution which spans the stable single-domain/superparamagnetic boundary. High-field thermomagnetic analysis reveals a trend in behaviour with increasing susceptibility, towards linear, more reversible curves. We propose that both the particular grain size of the enhanced ferrimagnetic fraction and thermomagnetic reversibility, can be explained by size-induced phase transitions in Fe2O3, which results in maghemite being the energetically favoured phase for certain grain sizes. Thus, the production of Fe2O3, which spans this grain-size range, is the only requirement for magnetic enhancement in the loess.