Mechanism of magnetic enhancement and depletion of the loess from northern piedmont of Dabie Mountains, east-central China

Loess from the northern piedmont of the Dabie Mountains is exceptional, with classical loess to the north (i.e. the Chinese Loess Plateau), the Quaternary red soils to the south, and the Xiashu loess to the east. However, the mechanisms underlying the magnetic variation (enhancement and depletion) are not sufficiently understood. In this study, a multi-methodological approach (magnetic, geochemical, colorimetric, granulometric, and citrate-bicarbonate-dithionite (CBD)) was applied to improve our understanding of these issues. The following findings are highlighted. (1) The magnetic properties were remarkably different between the three light yellowish-brown silts and other units in the Hudian section. Specifically, minor fine ferrimagnetic minerals and more goethite were found in all three units. (2) The magnetic enhancement had a pedogenic origin, as suggested by the magnetic and CBD measurements. Specifically, ∼90 % of the magnetic mineralogy is affected by magnetic enhancement and ∼ 10 % is of detrital origin. (3) The fact that the three units underwent strong soil development was inconsistent with the magnetic records. Based on our records, combined with evidence from previous studies, our findings show that fine maghemite produced by strong pedogenic development were transformed into weakly magnetic goethite under humid environmental conditions. Dissolution of ferrimagnetic minerals can also contribute to magnetic depletion; however, the effect is limited. In summary, the magnetic enhancement of the loess from the northern piedmont of the Dabie Mountains was first induced by the development of pedogenesis, and then decreased by the conversion of fine pedogenic maghemite into weakly magnetic goethite and by dissolution due to wet pedogenic conditions. This provides critical regional evidence for studying the spatial evolution of the magnetic characteristics of loess in the north-south transect of China.