Two pedogenic models for paleoclimatic records of magnetic susceptibility from Chinese and Siberian loess

Abstract

The magnetic susceptibility (MS) of Chinese loess showing a general proportional relationship to pedogenic grade has been widely recognized and used for reconstruction of paleoclimate by Quaternary scientists. The in-situ pedogenic enhancement of ferrimagnetic content is normally believed to be the main reason for the increase of susceptibility in soil units. However, this pattern of high magnetic susceptibility in palaeosols, and low values in loess, are not replicated in some loess deposits. Siberian loess deposits display a completely opposite susceptibility behavior: high values in loess and low values in palaeosols. This inverse relationship has been explained by the idea that magnetic susceptibility is reflecting the magnitude of an aeolian ferrimagnetic component of consistent mineralogy, the grain size of which is related to average wind velocity. Our magnetic study of Siberian samples in this paper suggests that there are notable differences in magnetic properties between Siberian loess and developed palaeosols, not only in magnetic grain-size and concentration but also in magnetic mineralogy. This evidence is difficult to explain fully through variation in wind strength alone, but implies that the low magnetic susceptibility values in the Siberian paleosol units are a reflection, at least in part, of the alteration of the ferrimagnetic content by post-depositional processes. The Loess Plateau is a very arid area where potential evaporation is always higher than precipitation; pedogenesis occurs under dry oxidising conditions. The Siberian Kurtak region is located on the edge of the tundra where it is always wet and saturation during interglacials will lead to a reducing pedogenic environment. Ferrimagnetic minerals under this condition will be destroyed, resulting in lower magnetic susceptibility. Therefore, great care should be taken when using susceptibility values for paleoclimatic reconstruction.

Susceptibility variations of multiple origins of loess from the Ily Basin (NW China)

“…Intermittent reduction is required for the formation of the ferrimagnets. Constant oxidation or reduction favors formation and accumulation of non-ferrimagnets [1, [17][18][19][20]41,73]. The wet reducing conditions in NLK resulted in destruction of ferrimagnets and the formation of non-ferrimagnets.…”

“…In south Siberia, all paleosols, including S0, usually display low susceptibility. This can be explained by the local climate, which causes very moist conditions in the soils [20]. However, during glacial periods, the climate was conjectured to be drier, resulting in relatively oxidizing conditions and preservation of ferrimagnets.…”

“…It argues that for loess from Alaska and Siberia, allochthonous input plays an important role in magnetic concentration and intensified wind vigor is responsible for increases in susceptibility [9][10][11][12][13][14][15]. However, pedogenesis influences not only the magnetic concentration but also the magnetic mineralogy and contributes at least partly to susceptibility variations in loess from Alaska and Siberia [16][17][18][19][20].…”

Magnetic properties of surface soils across the southern Tarim Basin and their relationship with climate and source materials

“…The Ms (T) curves show that all samples have a rapid loss of magnetization near 580°C and that the magnetization of samples taken from the desert zone drops to zero (Figure 4), indicating magnetite is the dominant mineral in these samples. According to the different characteristics of the Ms (T) curves (Figure 4), samples can be divided into three categories: (1) desert samples, for which the magnetization in the cooling curve is slightly greater than that in the heating curve, implying that weakly magnetic materials, such as iron sulfides or other paramagnetic minerals, are converted to strongly magnetic magnetite as a result of oxidation during heating [19,20], (2) loessic sand and loess samples between 2100 and 3300 m, for which the magnetization in the cooling curve is less than that in the heating curve, which is probably due to thermally unstable maghemite minerals converting to hematite, resulting in a loss of magnetization [19,20], and (3) loess samples above 3300 m, for which the magnetization in the cooling curve is much greater (more than double) than that in the heating curve and all heating curves have a clear "hump" at around 500°C. These observations indicate that the types and contents of weakly magnetic minerals differ greatly from those of desert samples [20].…”

“…For example, higher χ during glacial periods and *Corresponding author (email: fangxm@itpcas.ac.cn) lower χ during interglacial periods have been observed in the cases of Alaska loess and Siberia loess. The wind-vigor model [15][16][17][18] and pedogenic model [19,20] have been proposed to explain the complicated correlation. In addition, many studies have shown that coarse aeolian magnetic minerals also affect χ, especially in the loess-paleosol sequence adjacent to the desert edge [21,22].…”