黄土高原黄土沉积伊利石结晶度的时空变化特征

THE SPATIAL-TEMPORAL VARIATION OF ILLITE CRYSTALLINITY OF LOESS DEPOSITS IN THE CHINESE LOESS PLATEAU

在黄土高原地区，第四纪黄土沉积的伊利石结晶度是研究东亚夏季风的替代性指标，但以前对该指标有效性的研究主要集中在末次间冰期以来伊利石结晶度的时空对比分析。文章选取黄土高原地区位于南北横断面上的4个黄土-古土壤序列，分析了L6以来的代表性层位粘粒样品（〈2 μm）伊利石结晶度KI值的时空变化特征。结果表明，在每个剖面中，古土壤的伊利石结晶度KI值均高于其下覆黄土；在空间上，黄土KI值自西北向东南增加趋势较小，古土壤KI值自西北向东南呈现显著增加趋势。所有样品的KI值与反映风化成壤强度的指标，如磁化率和伊利石化学指数I （002）/I （001）值均存在显著的正相关关系。上述特征充分说明黄土沉积中伊利石结晶度KI值的变化主要受风化成壤作用所致。由于黄土沉积的KI值空间变化趋势与黄土高原现代和过去气候格局变化相一致，因此伊利石结晶度指标KI值可以作为东亚夏季风变化的有效指标。与冰期相比，间冰期时该指标南北变化梯度大，表明间冰期时黄土高原夏季风南北气候变化梯度大于冰期。

Illite crystallinity（Kübler index, KI）of loess deposits is a useful proxy for the East Asian summer monsoon study in the Chinese Loess Plateau. However, a few studies have only mainly concentrated on temporal-spatial analysis of the record since the last interglacial periods. In this paper, the temporal-spatial changes of the KI were investigated on the 〈2 μm of samples（S0, L1, S1, L2, S3, L4, S5-1, and L6）from 4 loess-paleosol profiles in the Chinese Loess Plateau spanning the last 600 ka.The 4 studied sections, Xifeng（XF：35°53＇N, 107°58＇E）, Changwu（CW：35°12＇N, 107°48＇E）, Yongshou（YS：34°41＇N, 108°9＇E） and Weinan（WN：34°18＇N, 109°30＇E） lie along a NW-SE transect across the Chinese Loess Plateau. The mean annual temperature（MAT）and mean annual precipitation（MAP）of these sections are 8.3℃ and 555 mm, 9.1℃ and 584 mm, 10.8℃ and 619 mm, and 13.6℃ and 691 mm, respectively. Clay minerals were measured by X-ray diffraction（XRD）on oriented mounts of clay-sized particles（〈2 μm）. All samples analyzed for clay minerals were disaggregated in deionized water and treated with 10% H2O2 and 1 mol/L HAC to remove organic materials and carbonate. The particles（〈2 μm）were then subject to pipetting based on Stokes＇ law. Oriented specimens were prepared on glass slides by the pipette method at room temperature. Illite crystallinity（KI）was obtained from the full width half maximum（FWHM）of the 1.0 nm peak（Δ°2θ）. Lower KI values represent higher crystallinity, characteristic of weak hydrolysis in continental sources and dry and cold climate conditions. Illite chemistry index I（002）/I （001） was determined by the ratio of the illite （002） and illite （001） peak areas. Higher I（002）/I （001） values represent stronger chemical weathering under wet and warm climate conditions.Results show that the KI values in each section are higher in the palesols than in the underlying loess units. Spatially, the KI values of each of the stratigraphic palesol units, S0, S1, S3 and S5-1, increased significantly from northwest to the southeast, while the increasing trend was weak in the loess units. There is a positive correlation between the KI values and magnetic susceptibility, illite chemical index values I（002）/I（001） in all samples from the four sections. It indicates that the higher KI values in the palesols are primarily caused by in situ pedogenesis. Furthermore, the spatial variations of KI values along the four sections are highly consistent with the present-day climate pattern of the Chinese Loess Plateau. Therefore, the illite crystallinity（KI）can be used as a more quantitative, sensitive proxy of the East Asian summer monsoon. The higher KI values and the steeper S-N gradient in the palesols indicate a significantly strengthened summer monsoon and the stronger S-N gradients on the Chinese Loess Plateau during interglacial times. The lower KI values and weaker S-N gradients in loess unites indicate a negligible effect of the summer monsoon and weak S-N climate gradients during glacial times.