Properties of magnetic mineralogy of Alaskan loess: Evidence for pedogenesis

The in situ pedogenic enhancement of ferrimagnetic content provides the well-established patterns of magnetic susceptibility variation within mid- to low-latitude loess deposits such as those of China and Central Europe. However, this pattern of high magnetic susceptibility in palaeosols, and lower values in unweathered loess, is not replicated in the higher-latitude loess deposits of Alaska and Siberia. In these localities the relationship is inverted, with high values in loess, and low values in palaeosols. This inverse relationship has been explained by the idea that magnetic susceptibility is reflecting the magnitude of an aeolian ferrimagnetic component of consistent mineralogy, the grain size of which is related to average wind velocity. However, the results of the magnetic study presented in this paper suggest that there are differences in magnetic properties between Alaskan loess and palaeosols, not only in magnetic grain size and concentration but also in magnetic mineralogy. This complicates the simple hypothesis of a 'wind velocity' signal by introducing an additional factor into the climatic signal. In contrast to the enhancement of susceptibility observed in palaeosols of the Loess Plateau, China, we suggest that the low magnetic susceptibility values in the Alaskan palaeosol units are a reflection, at least in part, of the alteration of the ferrimagnetic content by post-depositional processes associated with waterlogging (i.e. gleying) of the soils. (C) 2000 Elsevier Ltd and INQUA. All rights reserved.