Variability of magnetic character of S5-1 paleosol (age ∼470 Ka) along a rainfall transect explains why susceptibility decreased with high rainfall

- Both strongly magnetic maghemite and weakly magnetic hematite form simultaneously with development of pedogenesis in warm and wet interglacial times.
- Parts of the fine-grained oxidized magnetite and maghemite suffer reductive dissolution under periodic heavy rainfall when pH is lowered. Subsequent drying creates additional late generation weakly magnetic hematite and goethite.
- The variations in composition, size and concentration of both strongly magnetic magnetite and maghemite as well as the weakly magnetic hematite along the rainfall transect led to a more complete picture of iron mobility during pedogenesis.

This study was based mainly on environmental magnetic, soil science and geochemical methods to analyze the spatial variation of magnetic characteristics of the S5-1 paleosol (age ∼470 Ka) in a NW-SE transect from Xifeng, Linyou and Baoji sections from the Chinese Loess Plateau. The results show that (a) pedogenesis and chemical weathering of the coeval S5-1 paleosol layers increased with rainfall from Xifeng to Baoji, (b) the proportion of the pedogenic maghemite component decreased, while (c) the concentrations of weakly magnetic antiferromagnetic minerals (hematite and goethite) relatively increased. Maghemite and hematite appear to have formed simultaneously with development of pedogenesis, thus being similar in this aspect to soil in arid regions of SW Spain. However, amount of goethite did not show a consistent relationship as observed from magnetism in the three sections. Maghemite was found to be the main contributor to magnetic enhancement. Iron-manganese cutans, an indicator of diagnesis, increased with the rainfall in the S5-1 paleosol layer from Xifeng to Baoji. As expected, they show that pedogenesis occurred intermittently between wet and dry conditions causing iron mineral transformation. However, we suggest that parts of the fine-grained strongly magnetic minerals (mainly oxidized magnetite and maghemite) were converted into weakly magnetic minerals (mainly hematite and goethite) by pedogenesis, which resulted in a decline in the finest superparamagnetic (SP) and stable single-domain (SSD) ferrimagnetic minerals, thus decreasing the susceptibility of S5-1 paleosol from north to south. This behavior in a high rainfall region is different from the aerobic soil in SW Spain in which part of the initial hydro-maghemite transformed only to hematite rather than goethite and hematite.