兰州第四纪黄土/古土壤序列物源演变: 来自于碳酸盐矿物的证据*

黄土/古土壤的物源研究对于揭示第四纪气候变化和青藏高原隆升历史具有重要意义。本研究以位于黄土高原西部1.4 Ma以来的兰州黄土/古土壤沉积序列为研究对象,基于X射线衍射技术分析了黄土/古土壤中的主要矿物组成,侧重于碳酸盐矿物含量,追溯了兰州黄土/古土壤的直接物源。结果显示: (1)1.4 Ma以来兰州地区黄土/古土壤沉积物的主要直接源区为柴达木盆地沙漠区和阿拉善干旱区。(2)基于二元混合模型计算的潜在原始源区对兰州黄土白云石和总碳酸盐矿物的相对贡献率以及长石与石英比值结果一致支持1.4 Ma以来兰州黄土原始物源发生了多次变化。1.4~1.1 Ma和0.9~0.3 Ma青藏高原东北缘造山带(昆仑山、祁连山)和中亚造山带对兰州黄土的贡献相当,而1.1~0.9 Ma和0.3 Ma以来,中亚造山带对兰州黄土的物源贡献增加,这可能分别是对中更新世气候转型和0.3 Ma以来青藏高原及邻近地区干冷气候增强的响应。1.15 Ma和0.8 Ma兰州黄土/古土壤中高的白云石含量、碳酸盐矿物总含量以及0.8 Ma长石与石英比值的快速升高可能是对“昆黄运动”的响应,进而造成了昆仑山、祁连山对黄土高原物源贡献的增加。

Provenance evolution of Quaternary loess/paleosol sequence in Lanzhou,China: evidence from carbonate minerals

The provenance of loess is of great significance to reveal the climate change and the uplift history of the Qinghai-Tibet Plateau in the Quaternary. In this paper,the main mineral compositions of the loess/paleosol sediments since 1.4 Ma in Lanzhou region,located in the western part of the Chinese Loess Plateau,are analyzed based on X-ray diffraction. Focusing on carbonate mineral content,this paper traces the main immediate provenance region of the loess/paleosol sediments in Lanzhou. The results indicate that: (1)the Qaidam Desert and the Alxa arid area have been the main immediate provenance region of the loess/paleosol sediments in Lanzhou since 1.4 Ma. (2)The relative contribution of potential provenance to dolomite and total carbonate minerals based on the binary mixed model and the feldspar/quartz ratio of the Lanzhou loess/paleosol sediments support that their provenance has changed frequently since 1.4 Ma. The northeastern Tibetan Plateau orogenic belt(Kunlun and Qilian Mountains)and Central Asian Orogenic Belt contributed equally to the loess in Lanzhou in 1.4~1.1 Ma and 0.9~0.3 Ma,while the contribution of the Central Asian Orogenic Belt to the loess in Lanzhou increased in 1.1~0.9 Ma and after 0.3 Ma,which may be a response to the Mid-Pleistocene transition and the enhancement of dry and cold climate in the Tibetan Plateau and its adjacent areas since 0.3 Ma. The high content of dolomite and total carbonate minerals in loess/paleosol sediments of Lanzhou at 1.15 Ma and 0.8 Ma and the rapid increase of feldspar/quartz ratio at 0.8 Ma may be a response to the Kunlun-Huanghe Movement,thus resulting in the increase of the contribution of the Kunlun and Qilian Mountains to the sediments of the Chinese Loess Plateau.