黄土剖面中赤铁矿和针铁矿的定量分析与气候干湿变化研究

为了了解黄土剖面中赤铁矿和针铁矿的分布特征,文章采用对铁氧化物矿物灵敏的漫反射光谱法(DRS),开展了赤铁矿和针铁矿的鉴定和测定研究,提出了定量分析赤铁矿和针铁矿含量的DRS新方法.选择天然典型黄土和古土壤样品,首先采用柠檬酸盐-重碳酸盐-连二亚硫酸盐(CBD)方法去除其中成壤成因的铁氧化物矿物,以其为基体配制含不同赤铁矿和针铁矿的系列标样,然后进行DRS测试和多元逐步回归分析,分别建立测定赤铁矿和针铁矿含量的校准方程并加以检验.利用DRS方法,分析了多个黄土剖面的赤铁矿和针铁矿含量,发现黄土-古土壤剖面的赤铁矿/针铁矿比值可作为东亚季风干/湿变化的敏感指标.该比值较高时反映了干燥土壤环境,而较低时指示了潮湿土壤环境.对灵台和洛川剖面中赤铁矿/针铁矿比值的分析,初步揭示了2.6Ma年以来黄土高原东亚季风阶段性变强的特征.

QUANTITATIVE ANALYSIS OF HEMATITE AND GOETHITE IN THE CHINESE LOESS-PALEOSOL SEQUENCES AND ITS IMPLICATION FOR DRY AND HUMID VARIABILITY

The long, continuous deposition of dust in the Chinese Loess Plateau offers opportunity to study the nature of iron oxide formation in a wide range of climatic conditions. In order to understand the distribution and behavior of hematite and goethite in loess-paleosol profiles, we conducted diffuse reflectance spectroscopy (DRS) study for iron oxide and oxyhydroxide in Chinese loess. Here we propose a new method that can quantitatively measure hematite and goethite concentrations. First, typical loess and paleosol samples were deferrated using the CBD (Citrate-Bicarbonate-Dithionite) procedure to produce a natural matrix material to which synthetic hematite and goethite in known weight percentage were added to produce a set of calibration standards. Then spectral violet, blue, green, yellow, orange, and red of standards were calculated from the reflectance data and served as independent variables for a multiple linear regression analysis. The hematite and goethite concentrations can be obtained through the established regression equations respectively. Using the DRS method, we determined hematite and goethite concentrations in multiple loess sections, and found that the hematite/goethite (Hm/Gt) ratio in loess-paleosol sequences can be used as a sensitive indicator of dry/humid alternations of the East Asia Monsoon climate. A high Hm/Gt ratio indicates a dry soil environment, whereas a low Hm/Gt ratio reflects humid soil conditions. The Hm/Gt records from the Lingtai and Luochuan sections reveal a step-wise increase of the East Asian monsoon during the last 2.6Ma.