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

Abstract

Dithionite-citrate-bicarbonate (DCB) treatment is a classical method for removing iron oxides from soil. The DCB-induced dissolution effects on iron oxides are controversial. In this paper, samples from a typical loess-paleosol sequence in the Chinese Loess Plateau (CLP) and from other aeolian dust deposits in southern China were collected, and changes in the grain size composition and magnetic properties of the samples after DCB treatment were analyzed. The results show that the dissolution of iron oxides in loess-paleosol samples from the CLP is highly grain size dependent. In addition to completely dissolving nanometer-sized pedogenic iron oxides (< 0.2 μm), the standard DCB procedure can also dissolve submicron- and micron-sized aeolian iron oxides (0.2–6 μm). For these aeolian iron oxides, the submicron-sized (0.2–1 μm) iron oxides are sufficiently dissolved, and the solubility of the micron-sized (1–6 μm) iron oxides decreases with increasing particle size. The dissolution of > 6 μm aeolian iron oxides is negligible. DCB can neither separate pedogenic iron oxides from aeolian iron oxides nor selectively dissolve magnetite or maghemite. Although the total amount of dissolved iron oxides in the profiles from southern China is higher than that in the LC profile from northern China, the submicron- and micron-sized aeolian iron oxides in the latter are more easily dissolved.

Loess deposit reveals variations of 100-kyr cycle of temperature and 20-kyr cycle of precipitation at the low latitude of East Asia

“…Similar to MS, FeD in loess also comprises lithogenic and pedogenic components and DCB treatment will dissolve few lithogenic components (Yang et al, 2022). In this sense, FeD is an approximate proxy indicator of chemical weathering intensity.…”

Loess deposits in the low latitudes of East Asia reveal the ~20-kyr precipitation cycle

“…The MS and FeD/FeT records in the CLP show similar cycles, suggesting that smoothing has a similar influence on both proxies. In contrast, the MS and FeD/FeT records in Madang show different cycles, which would require precisely controlled differential smoothing to preserve the 20-kyr cycle of FeD/FeT but erase the 20-kyr cycle of MS. One possible interpretation is that the climatic conditions at the Madang site favor the production of FeD compared to the CLP 48 . Consequently, the FeD content at the Madang site significantly exceeds the concentration of ultrafine ferrimagnetic grains, which determines the degree of magnetic enhancement.…”