黄土高原全新世风成沉积的岩石磁学性质及其古气候意义

ROCK MAGNETISM OF THE HOLOCENE EOLIAN DEPOSITS IN THE LOESS PLATEAU: EVIDENCE FOR PEDOGENESIS

对黄土高原NW-SE断面上的现代沙丘砂、原生黄土、全新世黄土和黑垆土的κ-T曲线及其磁滞性质和X射线衍射特征进行了系统分析,结果表明,不论是在源区还是沉积区,磁铁矿都是对磁化率贡献最大的磁性矿物,源区风化作用没有使磁赤铁矿含量显著增加,黄土中的赤铁矿至少有一部分是原生的。黄土中的磁赤铁矿以成土成因为主。土壤化程度越高,磁赤铁矿含量越高,由于磁赤铁矿分解导致的磁化率下降越明显,κ-T曲线上510℃附近的磁化率峰越不显著。κ-T分析能非常灵敏地反映热处理过程中磁性矿物的转化情况,而且这种转化与样品所经历的土壤化作用强度密切相关。因此,κ-T分析可作为研究成土作用强度和古气候变化的一种新方法。

Using the temperaturedependence of susceptibility (TDS), hysteresis measurement as well as Xray diffraction (XRD) analysis,we conducted rock magnetic investigations of samples of modern dune sand, pristine loess, Holocene loess and black loam along a NWSE transect from the northern to the central loess plateau. The XRD spectra demonstrate that magnetite and hematite both exist in the Chinese loesspaleosol sequence and its modern source area and that they are the primary carriers of the natural remanent magnetization, but the TDS measurements show that magnetite is the predominant contributor to magnetic susceptibility. Maghaemite is present in the pristine loess, Holocene loess and black loam and mainly forms due to lowtemperature oxidation of magnetite during pedogenesis. Systematic analysis of the κT characteristics shows that the higher the pedogenetic level,the higher the magnetite content and that the lower the susceptibility caused by decomposition of maghaemite,the less evident the susceptibility peak at about 510 ℃. κT analysis can sensitively reflect magnetic transformation during heat treatment,and this transformation is closely related with strength of pedogenesis of the samples.In summary, this study shows that thermomagnetic analyses, hysteresis measurements, and petrographic methods can be combined to obtain a better understanding of the pedogenetic mechanisms responsible for the enhancement of the magnetic susceptibility in Chinese loess and paleosols.