Effects of CBD Treatment on Magnetic Minerals of Natural Samples

Abstract

Loess-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.

DCB dissolution of iron oxides in aeolian dust deposits controlled by particle size rather than mineral species

“…All samples treated with DCB have an obvious small peak at 450–550 °C. This phenomenon has also been reported in previous studies 27 , 29 , 47 . There are differences in the interpretation of this peak, which was called the Hopkinson peak by Deng et al 29 .…”

“…Sun et al 27 considered that DCB chemicals affect the structure of magnetite or remove the outermost stressed layers of magnetite grains, and then, magnetite is more susceptible to the Hopkinson effect. Lü et al 47 proposed that it is due to the formation of new magnetic minerals during the heating process. We believe that the appearance of this small peak is affected not only by the Hopkinson effect in single domain (SD) just below its Curie point 61 , 62 but also by the production of magnetite or other higher susceptibility phases during heating 62 .…”

“…There are two possible dissolution mechanisms. The first is that the finest iron oxide particles are completely dissolved by DCB, and the second is that the coarse-grained iron oxide particles are partly dissolved by DCB, e.g., the outer layer of the particles is dissolved 47 . Both dissolution mechanisms led to a decrease in the content of the corresponding grain size fractions.…”

“…The relative change in each fraction between 1 μm and 6 μm (i.e., 1–2 μm, 2–3 μm, 3–4 μm, 4–5 μm, and 5–6 μm) after the DCB treatment decreases with increasing grain size, indicating that micron-sized iron oxides are partly dissolved and that the dissolution efficiency decreases with increasing particle size. The main reason may be that the dissolution efficiency decreases with decreasing specific surface area (i.e., mass surface area) as the iron oxide grain size increases; thus, for coarse grains, only the outer layer can be dissolved by the DCB treatment 43 , 47 . For coarse-grained particles with sizes greater than 6 μm, dissolution by DCB is negligible.…”

“…However, the DCB dissolution effect on pseudosingle domain (PSD) grains between these two components is not clear. Currently, the dissolution effect of DCB on coarse iron oxides is controversial 16 , 29 , 37 , 42 – 47 . Regarding the selectivity of DCB dissolution of iron oxides with different origins and mineral species, one study shows that the DCB can remove maghemite produced by pedogenesis but has no effect on protogenic magnetite; therefore, it is proposed that the magnetic susceptibility signals in the loess-paleosol sequences in China are mainly derived from pedogenic maghemite 37 .…”