The role of maghemite in the enhancement of magnetic signal in the Chinese loess-paleosol sequence: An extensive rock magnetic study combined with citrate-bicarbonate-dithionite treatment

Abstract

We have conducted a rock magnetic study of selected Chinese loess samples before and after citrate-bicarbonate-dithionite (CBD) treatment. Our study is more extensive than previous ones and reveals new details of the magnetic carriers which are related to paleoclimate change. Three pairs of paleosol S1 (the top paleosol layer in the Lishi Formation) and loess L2 (underlying S1) from three localities (Xining, Xifeng, and Jixian) were investigated by using a variety of magnetic methods. After the paleosols were treated with CBD, which removes all maghemite grains and the finest grains of magnetite, magnetic susceptibility decreased from 77 × 10 −8 to 20 × 10 −8 m 3/kg in Xining, from 165 × 10 −8 to 30 × 10 −8 m 3/kg in Xifeng, and from 180 × 10 −8 to 25 × 10 −8 m 3/kg in Jixian, anhysteretic remanent magnetizations (ARM) were reduced to less than one tenth of the original values in the three sites, and saturation magnetization ( J s) decreased by about 60% in Xining and 70% in Xifeng and Jixian. These reduced values in the paleosols after CBD treatment are similar to those of the corresponding underlying loess samples, which, however, did not show significant decrease after CBD treatment. Room-temperature frequency dependence of susceptibility, high-temperature dependence of susceptibility, and low-temperature SIRM demagnetization of the paleosol and loess samples before and after CBD treatment confirm that in addition to superparamagnetic (SP) particles (magnetite and/or maghemite), the CBD removable magnetic carriers also include maghemite of grain sizes larger than SP.