Department of Earth Sciences, University of Oxford, OX1 3AN Oxford, United Kingdom;
Centre for Environmental Magnetism and Palaeomagnetism, Lancaster Environment Centre, University of Lancaster, LA1 4YQ Lancaster, United Kingdom
Proc Natl Acad Sci U S A. 2018 Feb 20;115(8):1736-1741. doi: 10.1073/pnas.1719186115. Epub 2018 Feb 5.
In the world-famous sediments of the Chinese Loess Plateau, fossil soils alternate with windblown dust layers to record monsoonal variations over the last ∼3 My. The less-weathered, weakly magnetic dust layers reflect drier, colder glaciations. The fossil soils (paleosols) contain variable concentrations of nanoscale, strongly magnetic iron oxides, formed in situ during the wetter, warmer interglaciations. Mineralogical identification of the magnetic soil oxides is essential for deciphering these key paleoclimatic records. Formation of magnetite, a mixed Fe/Fe ferrimagnet, has been linked to soil redox oscillations, and thence to paleorainfall. An opposite hypothesis states that magnetite can only form if the soil is water saturated for significant periods in order for Fe to be reduced to Fe, and suggests instead the temperature-dependent formation of maghemite, an Fe-oxide, much of which ages subsequently into hematite, typically aluminum substituted. This latter, oxidizing pathway would have been temperature, but not rainfall dependent. Here, through structural fingerprinting and scanning transmission electron microscopy and electron energy loss spectroscopy analysis, we prove that magnetite is the dominant soil-formed ferrite. Maghemite is present in lower concentrations, and shows no evidence of aluminum substitution, negating its proposed precursor role for the aluminum-substituted hematite prevalent in the paleosols. Magnetite dominance demonstrates that magnetite formation occurs in well-drained, generally oxidizing soils, and that soil wetting/drying oscillations drive the degree of soil magnetic enhancement. The magnetic variations of the Chinese Loess Plateau paleosols thus record changes in monsoonal rainfall, over timescales of millions of years.
在中国黄土高原举世闻名的沉积物中，化石土壤与风成粉尘层交替出现，记录了过去约 300 万年季风的变化。风化程度较低、磁性较弱的粉尘层反映了较干燥、较寒冷的冰川作用。化石土壤（古土壤）含有可变浓度的纳米级强磁性氧化铁，这些氧化铁是在较湿润、较温暖的间冰期就地形成的。为了解读这些关键的古气候记录，对磁性土壤氧化物的矿物学鉴定至关重要。磁铁矿的形成，一种混合的 Fe/Fe 亚铁磁体，与土壤氧化还原振荡有关，进而与古降雨量有关。相反的假设是，只有在土壤长时间处于饱和状态的情况下，Fe 才能被还原为 Fe，磁铁矿才能形成，这表明在依赖温度的情况下形成了磁赤铁矿，一种 Fe 氧化物，其中大部分随后会老化为赤铁矿，通常是铝取代的。这种氧化途径将取决于温度，而不是降雨量。在这里，通过结构指纹分析、扫描透射电子显微镜和电子能量损失光谱分析，我们证明了磁铁矿是主要的土壤形成的亚铁磁体。磁赤铁矿的浓度较低，且没有铝取代的证据，否定了其作为在古土壤中普遍存在的铝取代赤铁矿的前体的作用。磁铁矿的优势表明，磁铁矿的形成发生在排水良好、通常为氧化的土壤中，土壤的干湿振荡驱动了土壤磁性增强的程度。因此，中国黄土高原古土壤的磁性变化记录了季风降雨的变化，时间尺度可达数百万年。
