Superparamagnetism of two modern soils from the northeastern Pampean region, Argentina and its paleoclimatic indications.

The magnetic susceptibility (χ) carried by pedogenic fine-grained ferrimagnets has been widely used as paleoclimatic proxy to elucidate long-term paleoclimatic variations for wind-blown terrestrial loess/paleosol sequences. However, the magnetic properties of the lithogenic parent material can mask the pedogenic signals. In this study, we systematically investigated the origin of the superparamagnetism of two modern soils from the northeastern humid Pampean region, Argentina, developed on loess materials of different mineralogical composition. The samples were treated with the citrate-bicarbonate-dithionite (CBD) reagent, which is known to dissolve the submicron, pedogenic ferrimagnets while leaving unaltered the coarse grained ones. The magnetic material accounting for the frequency-dependent magnetic susceptibility peak at about 50 K remained in the residuals and is independent of the pedogenic processes. In addition, pedogenic ferrimagnetic particles in the two soils have a magnetic signature comparable to that of the soils from the Chinese Loess Plateau. It is also suggested that the χ for the bulk samples does not seem to be a reliable paleoclimatic proxy for the Pampean soils investigated in this study. Instead, the CBD-soluble magnetic signals could be more useful to detect paleoenvironmental variations in this region. These new findings provide improved understanding of the magnetic assemblage in the Pampean loess soils and make it feasible to retrieve the paleoclimatic signals carried by the pedogenic, CBD-soluble, iron oxides after removing the effects of the lithogenic inputs. [ABSTRACT FROM AUTHOR]