【摘要】 已有研究表明,黄土中的石英组分在黄土区的气候条件下基本不受风化作用的影响。文章对西峰剖面新近纪风成三趾马红土和第四纪黄土-古土壤序列样品进行全岩和石英粒度测试,并计算了不同风化成壤强度样品的全岩-石英粒度中值粒径差值。结果显示,与石英粒度相比,全岩样品含有更多的细粒组分,表明全岩粒度是风尘原始粒度与后期风化成壤作用共同的产物。根据此原理,用二者中值粒径差值定义一个风化指数(Gw),风化强度愈强,Gw愈大,可作为反映风化成壤强度的有效指标。该指标在西峰剖面6.2Ma以来风成堆积序列的应用显示,晚中新世-上新世期间风化强度较强,第四纪以来风化强度呈总体减弱趋势,其趋势与不断增大的全球冰量和粉尘沉积速率相吻合。我们认为,全球变冷和亚洲内陆干旱化程度的加剧是导致风化成壤强度降低的主要原因。更多还原

【Abstract】 Eolian deposits in North China are among the most important archives of paleoclimatic evolution. Generally,the eolian deposits are composed of loess and soil units. An apparent difference of loess and soil is that the loess units appear to be non-weathered or slightly weathered,whereas the soils have been subjected to relatively strong weathering process. Grain size of eolian deposits has been modified by the post-depositional weathering processes,thus bulk samples of loess and soil includes not only the primary eolian dust but also a pedogenic component. Because quartz is resident to weathering in the Loess Plateau region,quartz of the eolian deposits can represent the original eolian particles. The difference of grain-size between the bulk samples and quartz fraction can therefore reflect the weathering intensity of the eolian deposits. In this paper,Quaternary loess and soil as well as Tertiary Red Clay samples from the Xifeng section are studied by using grain size analytical method. Our objective is to determine the grain size differences of bulk samples and the quartz fraction and reveal the relationship between these differences and the weathering intensity.The Xifeng section includes the Quaternary loess-paleosol sequence and the underlying Red Clay Formation. The Quaternary loess-paleosol sequence is 172.5m in thickness. The Red Clay has a thickness of 56m,with a basal age of about 7.6Ma. Various proxies show that the lowest part (older than ca.6.2Ma) is a reworked deposit due to alluvial and slope processes. In this study,quartz and bulk grain-size analyses were made on 229 samples (180 from loess-paleosol sequence and 49 from Red Clay) in this section. The quartz fraction was extracted using the sodium pyrosulfate fusion-hydrofluorosilicic acid method. Grain-sizes of quartz and bulk samples were determined using a Malvern Mastersizer-2000 grain-size analyzer with an analytical precision <1%. The results indicate that the particle range of the bulk samples and quartz fraction are between 0.3μm and 120.0μm. The grain-size distributions are positively skewed with an asymmetric bimodal patterns. Contents of different fractions (<2μm,2～4μm,4～10μm,10～20μm,20～32μm,32～63μm and >63μm) of bulk samples are compared with those of the isolated quartz for typical Quaternary loess and soil and Red Clay samples. The results indicate that the bulk samples contain much more fine grains than the extracted quartz fraction. This change is attributed to the post-depositional processes,which is mainly controlled by climate. The medium grain-size difference of bulk samples and quartz fraction is determined as an index of weathering intensity. Therefore,a grain-size weathering index (Gw) can be defined using the difference between the quartz fraction and the bulk samples. This index is used to reflect the weathering intensity,with higher values for more weathered samples. Application of this index in the eolian deposits of the last 6.2Ma at Xifeng shows stronger weathering for the Late Miocene-Pliocene period,and weaker weathering for the Quaternary. This overall trend is consistent with the ongoing global cooling trends,which leads to cooler and drier climate. The increase in loess accumulation rate also attests the decrease of weathering. Global cooling and intensification of Asian inland aridification may be the main causes of the long-term decline in weathering and pedogenesis intensity.更多还原