吕镔2,3,
刘秀铭2,3,4,
叶玮1,
赵国永5
1. 浙江师范大学地理与环境科学学院, 浙江 金华 321004
2. 福建师范大学地理研究所, 福建 福州 350007
3. 福建省湿润亚热带山地生态重点实验室——省部共建国家重点实验室培育基地, 福建 福州 350007
4. Department of Earth and Environmental Sciences, Macquarie University, Sydney NSW 2109, Australia
5. 信阳师范学院地理科学学院, 河南 信阳 464000
基金项目: 国家自然科学基金项目（批准号：41402155、41772180和41877435）资助
详细信息
作者简介: 陈渠, 男, 40岁, 助理教授/博士, 环境磁学与环境演变研究, E-mail: chenqu@zjnu.cn
中图分类号: P595,P318 收稿日期:2021-04-30
修回日期:2021-08-23
刊出日期:2021-11-30
Rock magnetism and geochemical characteristics of major elements of typical loesss in the Ily Basin and their paleoclimatic significance
CHEN Qu1,,Lü Bin2,3,
LIU Xiuming2,3,4,
YE Wei1,
ZHAO Guoyong5
1. College of Geography and Environmental Sciences, Zhejiang Normal University, Jinhua 321004, Zhejiang
2. School of Geographical Sciences, Fujian Normal University, Fuzhou 350007, Fujian
3. State Key Laboratory for Subtropical Mountain Ecology of the Ministry of Science and Technology and Fujian Province, Fujian Normal University, Fuzhou 350007, Fujian
4. Department of Earth and Environmental Sciences, Macquarie University, Sydney NSW 2109, Australia
5. School of Geographical Sciences, Xinyang Normal University, Xinyang 464000, Henan
MSC: P595,P318
--> Received Date: 30 April 2021
Revised Date: 23 August 2021
Publish Date: 30 November 2021
摘要
摘要:选取有多层古土壤发育的新疆伊犁尼勒克黄土剖面，分析其地球化学特征与粒度分布，并做详细的岩石磁学研究，检测磁性矿物组成和亚铁磁性矿物粒度在各黄土-古土壤层的变化，探讨其记录的古气候演变过程。常量元素地球化学元素测试结果显示：尼勒克剖面MgO、Na2O与CaO在剖面下部变幅较大，在最底部的S3古土壤层含量低；各常量元素含量与伊犁的波马剖面和塔勒德剖面一致，普遍低于天山北麓黄土；元素含量比值风化指标显示古土壤层化学风化程度较高，其中又以S3最高；Fe2O3/Al2O3在古土壤层较高，反映化学风化过程中铁相对富集。剖面磁性强弱的垂向变化主要受控于风成输入的磁铁矿。就整个剖面而言，成壤生成的超细颗粒亚铁磁性矿物相对含量随深度减少呈降低的趋势，在古土壤层比在黄土层更高；亚铁磁性矿物的粒度在S3最细，较多分布在超顺磁至稳定单畴的粒径范围之内。各黄土-古土壤层样品热磁曲线变化都受磁铁矿主导，L3热磁曲线有别于L1和L2，而与S2相似。 < 4 μm粒度组分的磁性矿物组成并不像较粗组分一样以磁铁矿为主，而是含有较多赤铁矿，反映过湿条件下成壤产生的磁赤铁矿转化为赤铁矿，不同于黄土高原的成壤过程生成大量磁赤铁矿的磁性成壤增强模式。尼勒克黄土地球化学、粒度与环境磁学指标一致揭示自S3发育以来至全新世古气候干旱化的趋势。综合各指标变化与热磁曲线差异，研究区古气候演变可以分为两个阶段，即S3发育至L2堆积之前阶段与L2堆积以来至全新世阶段，在S2/L2过渡时期干旱化突然加剧。虽然伊犁黄土不同剖面的磁学性质不同，但都反映了干旱化的趋势。
关键词: 黄土/
古土壤/
伊犁/
古气候/
环境磁学/
地球化学/
粒度
Abstract:The Ily basin is located in Central Asia. Due to the geographic situation, the climate in the Ily basin is dominated by the westerlies and varies between low lands and mountain areas. The average annual precipitation within basin can be as high as 850 mm. Loess is widely distributed, extending from the slopes of the Tianshan Mountains to the edge of the deserts. There are outcrop loess profiles on the river terraces of the upper and middle reaches of the Ili River and its tributaries. It is generally believed that the loess in the Ily basin origins from deserts in Central Asia. However, recent studies suggest that local sources can contribute significantly to loess forming. In an arid climate at low altitude, the paleosols are generally not developed and show little difference from loess units, while in the eastern Ily basin with higher precipitation, loess deposits show relatively strong pedogenesis and clear distinction between loess-paleosol units. Since it is located to the east of the deserts and sensitive to the changes in the westerlies, its paleoclimatic significance has drawn much attention in the past several decades. Although much research work has been done using approaches such as grain size, geochemistry, chroma characteristics and rock magnetism, there have been few detailed studies using multiple approaches. Apart from the report on the Talede section at Xinyuan County, there have been little studies on the paleoclimatic reconstruction by loess records that spans more than one glacial-interglacial cycle. A 29.7 m-thick typical loess section at Nileke County(43°39.4'N, 82°44.8'E; 1237 m a.s.l.) is selected for the preset study. The topmost 0.5 m are Holocene soils(S0). In correlation to the Chinese Loess Plateau, the paleosol units of S1, S2 and S3 were designated to the depth intervals at 3~6 m, 17.0~19.3 m and 24.0~29.7 m respectively, and the loess units of L1, L2 and L3 to the interbeded layers. The paleosols distinguish from the loess units by red color and fine texture, with S3 noticeably more developed.
594 samples were collected in total at an interval of 5 cm, and subjected to granulometric analysis and rock magnetic measurement. Besides routine magnetic measurement, hysteresis loop measurement and thermomagnetic(J-T) analyses using a Variable Field Translational Balance(VFTB), temperature dependant susceptibility(κ-T) measurement using a Kappabridge MFK 1-FA(AGICO), and low temperature demagnetization measurement was performed on selected samples. The pipette method was used on selected samples to yield three particle-sized fractions(< 4 μm, 4~16 μm, 16~63 μm and >63 μm). The CBD method was used to dissolve the fine-grained minerals and collect a coarse fraction. The fractions attained by using both methods were also subjected to rock magnetic measurement for comparison. Typical samples from the paleosol-loess sequence were selected for geochemical analysis. Approximately 4 g of ground loess was mixed with 6 g of Li2B4O7 and tested for major element concentrations using an ARL PERFORM'X X-ray fluorescence spectrometer. Multiple proxies show consistent variations with depth at the Neleke section. The grain size distribution suggests that the loess is of aeolian origin. The >63 μm fraction is generally < 5% even in the loess units, which is indicative of the insignificance of proximal sources. The grain size parameters demonstrate conspicuous variations between the loess and paleosols and distinction between the lower part(17.0~29.7 m, S2 and below) and the upper part(0~17 m, L2 and above). The geochemical composition of the Nileke section is similar to that of the Boma section(at Zhaosu county) and Talede section(at Xinyuan county) in the Ily basin. Major element(Si, Al, Fe, K, Na, Ca, and Mg) concentrations are relatively constant in L1 and S1, but fluctuate below S1. Geochemical proxies show higher weathering intensity in S3 than in the other layers. The ratios of Fe2O3/Al2O3, Na2O/K2O and Na2O/Al2O3 are sensitive to the pedogenetic intensity at the Nileke section. The higher ratio Fe2O3/Al2O3 in the paleosol layers indicates the enrichment of iron during weathering, which is consistent with variations of color and magnetic mineralogy. It is worth pointing out that the major element concentrations of Nileke loess is more similar to Boma loess, which is further away and located at higher altitude, than to Talede loess. This can be attributed to the less contribution of proximal source at Nileke and Boma.
Intt the loess-paleosol sequence experienced different stages in terms of the relation between pedogenetic intensity and magnetic susceptibility, responding to paleoclimatic changes. The paleosols units show higher pedogenetic intensity but lower magnetic susceptibility in humid paleoclimates, while the imbedded loess units exhibits the dominance of wind strength in magnetic enhancement, indicating that the paleoclimtes were arid. The temperature-dependant susceptibility(κ-T) shows contrasting behavior between Nileke loess and the Chinese Loess Plateau. Furthermore, the κ-T curves show differentiation between L2 and S2, but similarity between S2 and L3, which is consist with the variations in grain size.
The magnetic parameters XARM/X and SIRM/X can be linked to paleoclimate through the relative concentration of stable single domain ferrimagnets like magnetites or maghemites, which result from decrease in wind strength and increase in pedogenic intensity. It is suggested that the ferrimagnetics in S3 are relatively fine, mainly in the SP-SD range, while those in the overlying units are relatively coarse, mainly in the SD-MD range. The low temperature demagnetization measurement provides evidence for the presence of coarse magnetite in loess units, which is of aeolian origin. Thermomagnetic measurement indicates the dominance of magnetites. However, the rock magnetic properties of particle-sized fractions of S3 paleosol samples suggest that they contain hematites and the hematities are of pedogenetic origin. In the Nikele section, hematite production was favored as opposed to ferrimagnet production and preservation, probably as a result of strong weathering intensity.
The strong weather intensity was caused by the humid paleoclimates. With a humid paleoclimate, the pedogenetic process led to formation of magnetically weak minerals, as indicated by the generally low X and Xfd% at Nileke. This might be attributed to the hydrothermal conditions and seasonal distribution of precipitation in the study area. However, the variable geographic exposure to polygenetic sources may complicate the correlation between different loess sections. The Nileke section can be correlated to Talede section in L1, S1, L2 and S2 layers, but not in L3 and below. The significant difference in grain size in L3 and S3 between Nileke and Talede implies that the provenance of the two sections might be different. The Talede section shows an up-section coarsen trend in grain size and an up-section increasing trend in X. This is most likely due to proximal source and significant contribution of wind intensity to magnetic concentration. In contrast, in a relatively humid paleoclimate, pedogenetic enhancement of magneitc susceptibility can be observed. The newly reported ZD17D section, which is adjacent to the Talede setion, shows higher X and Xfd% in paleosols than in loess units, dominated by susceptibility pedogenetic enhancement. The ultra-fine SP ferrimagnetics produced in pedogenetic process were preserved at ZD17D, while those at Nileke, where there was higher degree of precipitation and soil moisture, were destructed or transformed into magnetically weak minerals. The difference between the three sections strongly suggests that the loess mechanism varies in different localities, but all sections are indicative of a drying trend since S3 formation.
Given the consistent multidisciplinary results, it is suggested that the variations in weathing intensity and pedogenic intensity in the Nileke section revealed the regional paleoclimatic changes, which was dominated by the westerlies. The paleoclimate experienced a drying trend since S3 paleosols developed. During the forming of L3 and S2, the paleoclimate was relatively stable. The transition from S2 to L2 responded to significant aridification, which was followed by a continuous drying trend.
Key words:loess/
paleosol/
Ily Basin/
paleoclimate/
environmental magnetism/
geochemistry/
grain size
PDF全文下载地址:
http://www.dsjyj.com.cn/data/article/export-pdf?id=5bcf3a42-d18a-450f-8ae3-f1b04dd0d086
