Magnetic parameters reflecting pedogenesis in Pleistocene loess deposits of Argentina

Abstract

Quaternary loess sequences of Argentina, with interbedded loess and buried soils (palaeosols), provide terrestrial records of past climates and environmental conditions. Study of rock magnetic parameters measured over a large area of the Pampean loess seems to indicate that the existing magnetoclimatological models cannot adequately account for the complexities of the Pampean loess. The Chinese loess has been considered as typical, where magnetic properties are largely controlled by pedogenesis. On the other hand, the Siberian loess is an alternative magnetoclimatological model in which palaeosols appear as magnetic lows and the intercalated loess as magnetic highs. Argentine loess is apparently closer to the Siberian model. However, considering the data obtained in Argentina, the situation seems to be more complicated. The higest magnetic values (SIRM and susceptibiliy values) in silty and sandy loess indicate a more efficient entrainment of dense iron oxides particles during stormy dry (glacial) intervals. The parent material shows the highest susceptibility values (>100 × 10 −8 m 3/kg) while the waterlogged horizons show the lowest ones (below 20 × 10 −8 m 3/kg). Pedogenesis resulting in the development of BC and B soil horizons of palaeosols in the parent loess produced decreases in susceptibility values and increases in the F factor. This phenomenon occurs at some degree of humidity in which the process of gleying caused the total depletion of both susceptibility and frequency factor. The magnentic data allows consideration of the relevance of major cycles (arid/humid) separated by discontinuities as the main factor favoring one particular behavior of the magnetic parameters. The B horizons of palaeosols developed during an arid cycle will not show a notable difference in the magnetic records from the parent material. In contrast, during humid climate condition the pristine loess can be progresively obliterated by pedogensis through to the extreme situation of gleying.