研究论文 正式出版 版本 3 Vol 9 (3) : 238-244 2018
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元素和矿物组成揭示的金沙江干热河谷黄土状物质的物源研究
Elements and mineral composition indicating the provenance of loess-liked sediment in the Dry-Hot Valleys of Jinsha River
: 2018 - 05 - 16
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摘要&关键词
摘要:利用沉积物解读古环境信息的前提是必须清楚它的物源。金沙江干热河谷中存在的黄土状物质为解读金沙江河谷环境演化提供了一种物质载体。论文从物质组成相似性方面来研究金沙江干热河谷中黄土状物质的主要来源。通过测试分析黄土状物质以及其可能的物源,包括金沙江河漫滩、古堰塞湖沉积物以及金沙江流域山原红壤的元素和矿物组成后发现:金沙江古堰塞湖沉积物在常量元素、微量元素和矿物组成上与黄土状物质最为接近,说明古堰塞湖沉积可能是金沙江干热河谷黄土状物质的主要物源,这一结论与作者在野外观察到的凡有黄土状物质分布的地区,在附近就能找到古堰塞湖沉积的现象是一致的。
关键词:黄土状物质;物源;元素和矿物;金沙江干热河谷
Abstract & Keywords
Abstract: Background, aim, and scope The dry-hot valley is a unique and important natural landscape in south-west China, however, the evolutionary history of this landscape has not been clear for a long time. The loess-like sediment widely covering the bottom of the dry-hot valley can be used as stratigraphic records for solving this problem. To do this its provenance must be understood. In this article, based on the systematical analysis of the elemental and mineral composition between the loess-like sediments and its likely source such as ancient dammed lake sediments, overbank sediments and mountain red soil, the provenance of loess-like sediment has been determined in terms of composition similarity. Materials and methodsThe samples of loess-like sediments and their possible provenance (including paleo-dammed lake sediments, overbank sediments and mountain red soil) are collected in the reach of Jinjiangjie, Yuanmou and Qiaojia in Jinsha River. The same type samples from different river reach are mixed uniformly and sieved over 2 mm screen for measure. The major elements and trace elements of samples are measured by Magix PW2403 X-ray fluorescence spectrometer (XRF) and X’Pert3 Powder x-ray diffractometer (XRD) is used for the determination of mineral composition of samples. ResultsThe analyze test results of major elements and trace elements indicate that among overbank sediments, mountain red soil and paleo-dammed lake sediments, which are possible provenance, the mountain red soil is the biggest difference in composition of elements from loess-like sediments in dry-hot valley of Jingsha River, while the paleo-dammed lake sediments is smallest. The X-ray diffraction pattern also shows that not only in terms of signal strength, but at the peak position, the paleo-dammed lake sediments are most similar to the loess-like sediments among the three potential sources.Discussion The results of comparative analysis of elements and minerals suggests that paleo-dammed lake sediments are major provenance of loess-like sediment in dry-hot valley of Jinsha River and the local red soil contribute little to the provenance. In fact, the similarity of element composition between ancient dammed lake deposits and loess like materials has been reported as early as a few years ago, but at that time the ancient dammed lake deposits were not been regarded as major provenance due to lack of data from other area or other possible provenance, such as red soil covering the mountains on both sides of dry-hot valley, to support. In addition, the phenomena that as long as there is loess-like sediment, the paleo-dammed lake sediments can always be found nearby, also confirms the inherent relation between the paleo-dammed sediments and the loess-like materials.Conclusions Among the three-possible provenance, the dammed lake sediments are most similar to the loess-like sediments in the Jinsha River Dry-Hot Valleys in terms of elemental and mineral composition, which shows that dammed lake sediments may be the main source of the loess sediments in the Jinsha River Dry-Hot Valleys.Recommendations and perspectives Previous study show that the loess-like sediments in Jinsha River dry-hot valley has been deposited at last during the late Pleistocene. Determination of their provenance would benefit to reconstruct the evolutional history of dry-hot valley landscape, which is a major scientific issue in south-west China and is difficult to solve by stalagmites, tree ring records in this area.
Keywords: Loess-like sediments; Provenance; Elements and Mineral composition; Dry-hot Valleys of the Jinsha River
金沙江干热河谷是我国西南地区独特而重要的自然景观,关于这种自然景观的形成历史目前学术界存在较大争论(明庆忠和史正涛,2007;张建平等,2000;马国军和李红香,2012;许再富等,1985)。寻找可靠的地质记录载体是解决这些争论的有效途径。所幸的是早在上个世纪80年代,有学者就在金沙江干热河谷中找到了一定厚度的黄土状物质沉积(罗光照和沈林,1984),随后的年代学研究表明这些沉积物至少在晚更新世就已广泛堆积(蒋复初等,1999),完全可以成为解读干热河谷形成历史的重要载体(苏怀等,2013)。众所周知,利用沉积物解读古环境信息的前提是必须清楚它的物源(刘东生,1985),然而,目前这方面的深入研究不多(Pan et al,2014)。论文从元素和矿物组成对比分析入手,通过系统测定金沙江干热河谷黄土状物质及其可能物源的元素和矿物组成,从物质组成相似性方面来判断其主要来源,进而为利用这些黄土状物质沉积物指示研究区域环境演化历史奠定基础。
1   样品的采集与分析
金沙江干热河谷黄土状物质主要出现在金江街、元谋和巧家等几个段落(图1),通常堆积在海拔1600 m以下的河流阶地上,在少数露头上可以看到垂直节理和多层古土壤结构。以往的研究表明这些黄土状物质具有一定湿陷性,因而被认为是中国纬度最低的黄土(罗光照和沈林,1984)。传统理论认为,要发育黄土除了必须具备半干旱的气候条件外(干热河谷为稀树灌草丛环境,具备此气候条件),还必须有充足的碎屑砂源。从地理位置来看,在金沙江干热河谷,能为本地黄土状物质提供物源的无非是金沙江河漫滩粉砂、金沙江古堰塞湖粉砂以及金沙江流域内本地基岩风化的残积物(山原红壤)这3类物质。通过对比分析这些可能的物源与黄土状物质在元素和矿物组成上的差异,我们就可以大致判断出它们对黄土状物质堆积的相对贡献情况。


图1   金沙江干热河谷黄土状物质及古堰塞湖沉积的分布(根据徐则民(2011);张信宝等(2013);李朝柱等(2015)研究结果结合本文调查结果绘制)
Fig. 1 Distribution of loess-like sediments and dammed lake sediments in the Dry-Hot Valleys of Jinsha River(The figure is drawn based on the our field research and the result of Xu(2011);Zhang(2013);Li(2015).)
在金江街、元谋和巧家三个段落分别采集黄土状物质样品、河漫滩粉砂样品、古堰塞湖沉积物样品以及本地岩石风化土壤(山原红壤)样品,每段每类样品各随机采5个。之后,将同类样品等比例均匀混合,过2 mm筛,制成混合分析试样。
样品常量元素和微量元素的测定在兰州大学西部环境教育部重点实验室进行,使用的仪器是Magix PW2403 X荧光光谱仪(XRF),该仪器测量相对误差在2%以内。矿物测定在云南师范大学利用帕纳科X’Pert3 Powder多功能粉末X-射线衍射仪(XRD)完成,该仪器测角准确度优于0.02°。元素对比分析采用简单图表对比法(Guo et al,2002),设横坐标为黄土状物质的元素含量,纵坐标为被对比物质相应元素的含量,过原点斜线斜率为1(图2),如果某元素数据点越接近斜线,表明被对比物质在该元素组成上与金沙江干热河谷黄土状物质越相似。论文采用元素点与斜线间的距离(无单位)来定量描述这种相似性(表1)。矿物分析采用全样分析,为了能够从衍射峰强度上推断各矿物组份的相对含量,仪器测量参数的设定在每个样品测量时都保持一致。




 


图2   金沙江干热河谷黄土状物质与可能源区沉积物的常量元素含量比较
Fig. 2 The comparison of major element contents (%) of the loess-like sediments in the Jinsha River dry-hot valleys and the possible source region sediments
2   结果
2.1   元素组成特征对比分析
2.1.1   常量元素分析
测试结果表明,不论是金沙江干热河谷黄土状物质,还是河漫滩粉砂以及古堰塞湖沉积等几个可能的物源,Si是它们最主要的常量元素(图2),一般含量(以氧化物形式表示)都在55.48%以上,其次是Al,含量也不小于10.25%。但在Ca、Mg、Fe、Na、K等其它常量元素含量方面3类可能物源与金沙江干热河谷黄土状物质存在一定差异。研究表明,在3类可能物源中与金沙江干热河谷黄土状物质在常量元素上差距最大的是山原红壤,图表统计显示山原红壤的各常量元素与斜率为1的斜线间距离累计和达到44.90(表1),差距最小是古堰塞湖沉积物,距离累积和仅有4.86(表1),其中差距最大的是Ca元素,但其元素点与斜线间的距离也仅为1.39(表1)。
表1   金沙江干热河谷黄土状物质与可能源区常量元素含量距离
元素(%)
Element
金沙江河漫滩相砂
Fluvial sediments in the Jinsha River
山原红壤
Red soil in Jinsha River drainage basin
金沙江古堰塞湖沉积物
Dammed lake sediments in the Jinsha River
Fe2O31.4011.680.56
SiO23.2119.351.29
Al2O32.197.611.19
MgO2.180.900.18
CaO3.513.161.39
Na2O0.060.860.09
K2O0.571.340.16
累积和 Sum13.1244.904.86
2.1.2   微量元素分析
在金沙江干热河谷黄土状物质与金沙江河漫滩粉砂、古堰塞湖沉积等几个可能物源中,能检测到的微量元素主要有Mn、Ba、Zr、W、P、Sr、Cr、V、Co、Zn、Rb、Cu、Bi、Ni、Nb、Hf等16种,其中Mn、Ba含量普遍较高,多在100µg/g以上。其中,与金沙江干热河谷黄土状物质微量元素含量相差最大的还是金沙江流域当地基岩风化的山原红壤(图3),图表测算显示各元素点与斜率为1直线距离的累积和达到3091.66(表2)。其次是河漫滩相沉积物,距离累积和在1085.26(表2)。与常量元素分析结果相似,在微量元素含量方面最接近金沙江干热河谷黄土状物质的物源物质还是古堰塞湖沉积物,距离累积和仅有403.49(表2)。




图3   金沙江干热河谷黄土状物质与可能源区沉积物的微量元素含量比较
Fig. 3 The comparison of trace element contents (µg/g) of the loess-like sediments in the Dry-Hot Valleys of Jinsha River and the possible source region sediments


 
表2   金沙江干热河谷黄土状物质与可能源区沉积物微量元素距离Tab. 2 The table of trace element contents (µg/g) of the loess-like sediments in the Dry-Hot Valleys of Jinsha River and the possible source region sediments
表1   金沙江干热河谷黄土状物质与可能源区沉积物微量元素距离
元素(µg/g)
Element
金沙江河漫滩相砂
Fluvial sediments in the Jinsha River
山原红壤
Red soil in Jinsha River drainage basin
金沙江古堰塞湖沉积物
Dammed lake sediments in the Jinsha River
P175.02349.1688.30
Ba51.88308.1241.59
Bi30.8417.7814.52
Co45.0919.4812.86
Cr80.2921.6533.93
Cu37.85146.962.89
Hf2.890.450.57
Mn177.481497.1588.78
Nb1.1027.780.98
Ni11.7528.410.67
Rb38.5450.515.36
Sr135.0652.965.25
V51.46255.729.86
W160.88102.5782.65
Zn2.7436.955.95
Zr82.39176.019.33
累积和 Sum1085.263091.66403.49
2.2   矿物组成特征对比分析
从矿物的衍射分析图谱(图4)中可以看出,无论是在信号强弱方面还是在波峰位置上,与金沙江干热河谷黄土状物质最接近的是金沙江古堰塞湖沉积,它们不仅包含的主要矿物完全一致,而且各矿物含量也相差不大(表现在波峰信号强度上,各样品在对比测试时仪器测量参数设定完全一致)。另外,与金沙江干热河谷黄土状物质在矿物成分上比较接近的还有河漫滩相粉砂,但它含有一定芒硝(Th),而此矿物在黄土状物质中并未检测出。与元素分析的结果类似,山原红壤样品的矿物类型与金沙江干热河谷黄土状物质物质相差最大,长石(Fsp)、黑云母(Bt)的信号缺失明显。


图4   金沙江干热河谷黄土状物质与可能源区沉积物的X-射线衍射峰值曲线比较
Fig. 4 The X - ray diffraction peak curve contrast of the loess-like sediments in the Dry-Hot Valleys of Jinsha River and the possible source region sediments
3   讨论与结论
常量元素、微量元素和矿物的对比分析结果显示,与金沙江干热河谷黄土状物质在元素和矿物组成上最为接近的是金沙江古堰塞湖沉积物,相差最大的是同流域内的山原红壤,说明古堰塞湖沉积物是金沙江干热河谷黄土状物质的主要物源,而当地红壤对物源贡献不大。其实,关于古堰塞湖沉积与黄土状物质在元素组成上的相似性早在前些年就有报到(Pan et al,2014),只不过当时只在金沙江华弹段做过工作,缺少元谋、金江街等典型黄土状物质分布段落的数据支持,尤其是先前的研究没有同区域内覆盖面积最广的山原红壤进行对比,因此尚不能得出此结论。近些年通过野外追溯我们发现,在金沙江干热河谷内,只要有黄土状物质分布的地区总能找到一定数量的古堰塞湖沉积,甚至在一些地段还能出现古堰塞湖沉积阶地上堆积黄土状物质的现象,相反在缺少堰塞湖沉积物的河段,黄土状物质沉积比较少见(图1),这进一步证实了金沙江流域古堰塞湖沉积与黄土状物质的内在联系。
除了堰塞湖沉积外,金沙江河谷中的河漫滩粉砂与金沙江干热河谷黄土状物质在元素和矿物组成上也较为接近。河漫滩相粉砂对金沙江干热河谷黄土状物质的物源有贡献是可以理解的,因为在隆冬季节,我们经常可以在金沙江干热河谷内观察到河滩扬尘天气。从现有的资料来看,金沙江干热河谷的黄土状物质多数形成于晚更新世。因此,如果对这套黄土状物质进行年代学和环境代用指标的深入研究,人们不仅可以恢复出干热河谷晚更新世以来的区域环境变化历史,也可以将其作为晚更新世全球气候干旱化在西南地区的表现实例。另外,值得注意的是,在金沙江干热河谷内黄土状物质地层是区域环境演化研究中的不可多得的重要材料,因为在这一地区普遍缺乏石笋、树轮、冰芯等高分辨率环境记录载体,湖相地层也主要以堰塞湖沉积为主,堆积速率快,记录时间短,连续性差。唯有黄土状物质可以记录晚更新世以来,甚至更早时间(苏怀等,2013)以来的干热河谷环境连续变化的历史。
总之,从常量元素、微量元素和矿物组成测试分析结果来看,在山原红壤、金沙江河漫滩沉积物、古堰塞湖沉积物等3类可能的物源中,金沙江干热河谷黄土状物质在元素组成和矿物组成上与金沙江古堰塞湖沉积物最为接近,相差最大的是山原红壤。金沙江古堰塞湖沉积物可能是金沙江干热河谷黄土状物质的主要物源。
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稿件与作者信息
叶玉林1,苏怀1*,董铭1,明庆忠2,史正涛1
YE Yulin1, SU Huai1*, DONG Ming1, MING Qingzhong2, SHI Zhengtao1
基金项目∶ 国家自然科学基金项目(41362010, 41462007);云南省应用基础研究计划项目(2013FZ047)
Foundation item: National Natural Science Foundation of China (41362010,41462007);Yunnan Applied Basic Research Projects(2013FZ047)
出版历史
出版时间: 2018年5月16日 (版本3
参考文献列表中查看
地球环境学报
Journal of Earth Environment