柴达木盆地早始新世大暖期古土壤中赤铁矿和针铁矿的定量分析及其气候意义

THE QUANTIFICATION OF THE HEMATITE AND GOETHITE AND ITS CLIMATE SIGNIFICANCE IN THE PALEOSOLS DURING EARLY EOCENE CLIMATIC OPTIMUM, QAIDAM BASIN

赤铁矿和针铁矿是古土壤中常见的两种风化产物，定量研究赤铁矿和针铁矿有利于了解成壤时期古气候特征。文章利用可见光波段的漫反射光谱法对早始新世大暖期期间柴达木盆地大红沟剖面路乐河组古土壤中的铁氧化物矿物进行了定量分析。测试结果清晰显示了赤铁矿和针铁矿的存在，其中赤铁矿含量约在0．09～17．16g／kg之间，针铁矿的含量大致为0．13—24．6g／kg；针铁矿（Gt）相对赤铁矿（Hm）平均含量较高（Gt／Hm变化范围0．04—34．34，平均值约为2．18），且数据存在较大波动。古土壤在早始新世大暖期时经历了3个阶段的古气候变化，早期古降雨量偏高，针铁矿／赤铁矿平均值为3．06，数据波动明显，指示相对温暖湿润且干湿波动大的气候条件；中期时古降雨量稍低，针铁矿／赤铁矿平均值降低至1．75，指示相对干旱的气候条件；晚期时古降雨量增加，针铁矿／赤铁矿平均值升高至1．80，指示相对温暖湿润的气候条件。其中早期的路乐河组古土壤对应了全球早始新世气候大暖期的峰值，路乐河组古土壤整体的古气候特征是对全球早始新世气候大暖期的区域响应。

Qaidam Basin is the largest intermont basin in the northeast of Qinghai-Tibet Plateau. It＇s continuous sedimentary sequence provides a good example for studying the global climate evolution since the Cenozoic. Our research mainly focus on the paleosols developed at the bottom of Luluhe Formation, Dahonggou section, Qaidam Basin. The stratigraphic tendencies are about 200°- 220°, and dip angles are about 25° -40°. The total thickness of the paleosol layers were calculated to be about 126 m（the length of Lulehe Formation is about 460 m, and the paleosols were sampled from ca. 50 m to 176 m）. The lithology is dominated by conglomerate, gravel, sandstone, siltstone and mudstone, with some thin laminated mudstone and paste. And the deposits are alluvial fan facies. According to the previous studying, we speculate the time span of paleosols of Luluhe Formation is during 52 -50 Ma. It is during the period of Early Eocene Climatic Opitmum（EECO）. Hematite（Hm） and goethite（Gt） are two common weathering products in paleosols. Their sensitivity to the environment can help us to understand the humidity of soil and pedogenesis as the paleo-climate changing. We use the diffuse reflectance spectroscopy（DRS） to quantify the iron-oxide in paleosols of Lulehe Formation. To collecte fresh samples, 19 channels were digged manually. Each sample weighs about 300-500 g, and the total number of samples is 228. The results showed the existence of hematite and goethite clearly in the paleosols. The content of hematite was about 0.09- 17.16 g/kg, and the content of the goethite was approximately 0. 13-24.6 g/kg. The goethite is higher than hematite in many samples（ Gt/Hm fluctuated between 0.04 to 34. 34, with an average of 2.18） , and the data fluctuated sometimes. During the early stage of EECO, the mean annual precipitation （MAP）was high with the average of Gt/Hm is 3.06. The data fluctuated sometimes, which indicates a warmer and wetter, with seasonally humid and dry climate. The MAP goes down between middle stage of EECO, meanwhile the average of Gt/Hm come down to 1.75, indicating a dryer climate. And during the late stage of EECO, the MAP is higher than before and the average of Gt/Hm is 1.80, indicating a warmer and wetter climate again. We also correlated the early stage of the paleosols to the peak stage of EECO. The climatic features of paleosols is the response to the global climatic warming events.