Characteristics and paleoclimatic significance of the soil bulk density for the loess deposit from Ganzi region, western Sichuan Plateau
CHENG Ting;YANG Shengli;LIU Weiming;LI Shuai;LIANG Minhao;LIU Nannan;CHEN Hui;Ministry of Education Key Laboratory of Western China's Environmental Systems, College of Earth Environmental Sciences,Lanzhou University;Institute of Mountain Hazards and Environment, Chinese Academy of Sciences;
Background, aim, and scope Loess deposits are widely distributed in the western Sichuan Plateau. Previous studies have focused on its origin, age, geochemistry, mineralogy, and its paleo-environmental significance. The bulk density characteristics of loess-paleosol sequences in this area has important significance for understanding the aeolian process of loess deposition and exploring the eastern Tibetan Plateau environmental evolution. However, few studies have been shown on the bulk density of the loess in this region. In this paper, a typical loess-paleosol sequence from Ganzi County was investigated in detailed to explore the characteristics and its paleoclimate significance of the soil bulk density of the loess-paleosol sequence on the Tibetan Plateau. Materials and methods Sampling site(31°37′13.02″N, 99°58′52.95″E; 3400 m) is located in northwest of Xinshiqu Village, Ganzi County, Sichuan Province(XS), and on the third terrace of the Yalong River. Measurements of bulk density, magnetic susceptibility and grain size are all carried out in the Ministry of Education Key Laboratory of Western China's Environmental Systems, Lanzhou University. The bulk density was measured by the oil-soaked method after samples were air dried. Each sample was measured twice to achieve the average value and 200 samples were analyzed for the entire profile. The magnetic susceptibility analysis was performed using British magnetization meter(Bartington MS2). Each sample was weighed, then stored in a nonmagnetic plastic box and measured twice to achieve the average value. Grain-size samples were analyzed using a Malvern Master Sizer 2000, with a size detection range of 0.02 — 2000 μm. Samples were pre-treated with 10% H_2O_2 and 10% HCl to remove organic matter and carbonates respectively, then washed by distilled water. After addition of 10 mL of 3.6% Na_4P_2O_7?10H_2O, the samples were dispersed using an ultrasonic bath for 10 min before the measurement. AMS-~(14)C dating was completed in Peking University AMS laboratory after sample pretreatment in the Lanzhou University laboratory. Optically stimulated luminescence dating used quartz mineral with the single aliquot regenerative dose protocol and measurements were completed in the luminescence dating laboratory, Institute of Mountain Hazards and Environment, CAS. Results Our results have shown that the soil bulk density of the XS section varies frequently with the depth since the last interglaciation, with a trend of high value in loess units and low value in paleosol units. It varies from 1.63 g ? cm~(-3) to 2.53 g ? cm~(-3) along the whole profile. The bulk density in loess units ranges between 1.67 — 2.48 g ? cm~(-3), with an average of 2.01 g ? cm~(-3). While the bulk density of the paleosol units ranges between 1.63 — 2.53 g ? cm~(-3), with an average of 1.94 g ? cm~(-3). The average bulk density of the XS loess-paleosol sequence is heavier than that of Luochuan section(1.25 — 1.86 g ? cm~(-3))and Baicaoyuan section(1.05 — 2.18 g ? cm~(-3)) from the Chinese Loess Plateau. It is also much heavier than the loess deposit from the north slope of the western Kunlun Mountian(1.25 — 1.60 g ? cm~(-3)). However, the bulk density of the Ganzi loess is quite similar to the Lingtai section(1.85 — 2.18 g ? cm~(-3)) and Mangshan section(1.50 — 2.50 g ? cm~(-3)) from the Loess Plateau. The variations of the bulk density are correlated with the magnetic susceptibility and the grain size records along the depth. Discussion Correlation analysis show that the soil bulk density has a negative correlation with the magnetic susceptibility and the grain size fraction of 2 μm. The magnetic susceptibility can be related to the soil formation and the 2 μm fraction may contain the most of the new self-generated magnetic particles produced during the soil formation process, which indicate that the bulk density can be affected by the pedogenic process. The soil bulk density is positively correlated with the mean grain size and the fraction above 63 μm, which are less susceptible to the post-depositional transformation and related with the variations of the wind field, the strength of the air circulation and the distance of the aeolian source. That means that the bulk density can be affected by the sedimentary process. Further analysis by scatter plots of the bulk density versus the magnetic susceptibility and the grain size of the XS sequence, shows that the aeolian process has much more impacts on the soil bulk density in Ganzi region, which is mainly controlled by westerly and the plateau winter monsoon. Thus, the bulk density of the loess can be used as a good proxy for variations of the westerly and the plateau winter monsoon, and the aridity of the inner Plateau. Conclusions We can conclude that the bulk density of the loess-paleosol sequences from Ganzi region can be affected by both the pedogenic process and the aeolian process, while aeolian process has much more impacts than the pedogenesis. The bulk density of the loess deposit from Ganzi region can be a good indicator for the westerly and the plateau winter monsoon, which are related to the aridity in the inner Tibetan Plateau. Recommendations and perspectives This work is of great importance for understanding the paleoclimatic significance of soil bulk density under different climatic and environmental conditions. Further detailed work on the records of the bulk density of the loess from the western Sichuan Plateau will be much helpful for better understanding the environment changes of the Tibetan Plateau.
【Fund】： 国家自然科学基金项目(41472147);; 兰州大学中央高校基本科研业务费专项资金(lzujbky-2015-k10 lzujbky-2015-bt01);兰州大学西部环境教育部重点实验室开放基金~~
【CateGory Index】： S152
【CateGory Index】： S152