Effects of CBD Treatment on Magnetic Minerals of Natural Samples

Abstract

AbstractLoess‐paleosol sequences are good archives for research on paleoclimate over the last 2.5 million years and even earlier. Magnetic susceptibility of loess‐paleosol sequences has been used extensively as a proxy for paleoclimate interpretations. Magnetic minerals in loess/paleosols are of lithogenic (allochthonous) origin or pedogenic (authigenic) origin. It is of great significance for loess magnetism to separate magnetic signals of them and investigate its magnetite or maghemite that mainly contributes to magnetic susceptibility enhancement in paleosol. Though the combination of CBD (Citrate‐Bicarbonate‐Dithionite) procedure with loess magnetism sheds light on these questions, the CBD method is still debated and its mechanism unclear. To better understand the effects of CBD method on magnetic minerals, CBD treatment were made to loess/paleosol samples from Western Sichuan plateau, Tianshan mountains and Kurtak, Siberia and non‐aeolian samples from Subtropics in this work. Magnetic parameters, including low‐field magnetic susceptibility, frequency dependent magnetic susceptibility, anhysteretic remanent magnetization, saturation isothermal remanent magnetization and thermomagnetic curves (J‐T curves), were measured before and after the CBD treatment. Changes in magnetic mineralogy, magnetic concentration and magnetic domain were analyzed. The results suggest that the CBD method is not apparently selective in dissolving various magnetic minerals, and the dissolving quantity is controlled by grain size distribution if the temperature and duration are given. The CBD method can efficiently remove fine grains which have higher ration of surface to volume, and dissolve a rim from the outside of the coarse grains resulting in finer grain size. There are multiple possibilities of changes in magnetic susceptibility after CBD treatment. The CBD method can be used to obtain magnetic signal of pedogenic original. It is not advisable to use the method to separate lithogenic and pedogenic magnetic minerals in loess from arid or over‐wet areas.