Magnetic mineralogy of soils across the Russian Steppe: climatic dependence of pedogenic magnetite formation

Abstract

Formation of ferrimagnets in well-drained, buffered, unpolluted soils appears to be related to climate, and especially rainfall. If robust, this magnetism/rainfall couple can be used to estimate past rainfall from buried soils, particularly the multiple soils of the Quaternary loess/soil sequences of Central Asia. However, dispute exists regarding the role of climate vs. dust flux for the magnetic properties of modern loessic soils. Here, we examine the mineralogical basis of the magnetism/rainfall link for a climate transect across the loess-mantled Russian steppe, where, critically, dust accumulation is minimal at the present day. Magnetic and independent mineralogical analyses identify in situ formation of ferrimagnets in these grassland soils; increased ferrimagnetic concentrations are associated with higher annual rainfall. XRD and electron microscopy show the soil-formed ferrimagnets are ultrafinegrained ( 6 V50 nm) and pure. Ferrimagnetic contributions to Mo « ssbauer spectra range from 17% in the parent loess to 42% for a subsoil sample from the highest rainfall area. Total iron content varies little but the systematic magnetic increases are accompanied by decreased Fe 2þ content, reflecting increased silicate weathering. For this region, parent materials are loessial deposits, topography is rolling to flat and duration of soil formation effectively constant. The variations in soil magnetic properties thus predominantly reflect climate (and its co-variant, organic activity)ŝ tatistical analysis identifies strongest relationships between rainfall and magnetic susceptibility and anhysteretic remanence. This magnetic response correlates with that of the modern soils across the Chinese Loess Plateau. Such correlation suggests that the rainfall component of the climate system, not dust flux, is a key influence on soil magnetic properties in both these regions. ß

Ordered ferrimagnetic form of ferrihydrite reveals links among structure, composition, and magnetism

“…This observation suggests a common origin of nano-sized soil ferrimagnets over a wide range of climatic conditions and initial input of natural magnetic phases. Although magnetic extracts of soils are commonly dominated by coarse-grained natural ferrimagnets that make characterization of nano-sized ferrimagnets in soils difficult by conventional methods (45), transmission electron microscope images revealed the presence of 10-50 nm magnetic particles in soils from the Russian Steppes (41). Fine-grained ferrimagnets in soils thus encompass the size range of ferrifh found in the present study.…”

El potencial del magnetismo en la clasificación de suelos: una revisión

“…Los minerales de hierro de manera natural presentan transformaciones debido a las condiciones oxidantes o reductoras en las que se encuentran, lo que da lugar a aumento o disminución de la señal magnética (Figura 1). Bajo condiciones oxidantes y con falta de oxígeno como las que se provocan con el fuego agrícola o por el uso de combustibles fósiles se induce la transformación de los minerales de hierro II a III del tipo de la maghemita (γFe 2 O 3 ) y magnetita (Fe 3 O 4 ), lo cual da lugar al aumento de la señal magnética (Maher et al, 2003). En condiciones naturales en sitios no contaminados se ha reportado la transformación de los precursores de la ferrihidrita (5Fe 2 O 3 •9H 2 O) en minerales ferrihidrita ferrimagnética (Michel et al, 2010).…”

“…Transformaciones de oxido-reducción de los minerales de hierro, χ valor de susceptibilidad magnética másica en µm 3 kg -1 (Modificado de Maher et al, 2003) ambiente, ya que los encontramos en las rocas, suelos, cuerpos de agua, atmósfera y biota.…”

Quantifying paleoprecipitation of the Luochuan and Sanmenxia Loess on the Chinese Loess Plateau

“…Balsam et al (2011) investigated the relationship between χ and precipitation from different climate zone (temperate, subtropical and tropical) and stated that χ rises with MAP from 200 to 1000-1200 mm and then drops as MAP exceeds 1200 mm. Besides χ, other magnetic parameters or ratios have also been used as precipitation proxies, such as χ ARM (susceptibility of anhysteretic remanent magnetization) (Maher et al, 2003), the ratio of χ ARM to IRM (isothermal remanent magnetization) (Geiss et al, 2008), χ FD % (Wang et al, 2015), and ratio of pedogenic goethite and hematite (Hyland et al, 2015).…”

“…During pedogenesis, the concentrations of magnetite and hematite change as the MAP increases and their behaviors are different (Balsam et al, 2004;Liu et al, 2007;Maher et al, 2003;. The concentration ratio of magnetite and hematite has great potential as an indicator of precipitation .…”