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黄土高原上新世红粘土记录的古气候变化—来自碳酸盐校正的HIRM证据
Alternative TitleCarbonate-free hard isothermal remanent magnetization record of the Pliocene Red Clay sequence on the Chinese Loess Plateau
路彩晨
Thesis Advisor夏敦胜 ; 贾佳
2019-05-30
Degree Grantor兰州大学
Place of Conferral兰州
Degree Name硕士
Degree Discipline自然地理学
Keyword赤铁矿 风成 红粘土 上新世 黄土高原
Abstract黄土高原堆积有厚达几百米的风成沉积物序列,由于其沉积连续、年代序列长、分辨率高等特点而成为古气候研究的重要介质。这一完整的沉积序列能够较好的记录亚洲古季风的演化历史,反演青藏高原的隆升历史,同时记录亚洲内陆干旱化和沙漠化的过程。对于第四纪黄土-古土壤沉积序列的古气候研究已经相当成熟,通过环境磁学指标反演此时期古气候演变历史已无争议,但第三纪红粘土的研究还相对薄弱。硬等温剩磁(HIRM)不同于指示低矫顽力的强磁性矿物(磁铁矿/赤铁矿)含量变化的磁化率(χlf),而是指示高矫顽力的弱磁性矿物(赤铁矿/针铁矿)的含量变化,且能记录地球轨道要素偏心率的周期信号,在第三纪红粘土沉积序列中蕴含着丰盛的古气候演变信息,有待去进一步挖掘和加强研究。
本文选择黄土高原中部的西峰赵家川红粘土沉积序列为研究对象,对上新世时期的红粘土样品展开一系列环境参数的系统测量。包括环境磁学(χlf、χfd、HIRM和S-ratio)和非磁学手段(气量法测定碳酸盐含量、Atterberg法粒度分选、X-射线衍射、漫反射光谱分析和土壤微形态观测),综合分析红粘土的磁学性质,主要是进一步明确HIRM在红粘土中的指示意义,并结合漫反射光谱分析比较定量出HIRM在红粘土中指示的赤铁矿含量的比例以及确定红粘土序列中的赤铁矿的来源。
通过对黄土高原中部的西峰赵家川红粘土沉积序列的系统调查分析,本研究主要获得以下三点认识:
1、红粘土序列的碳酸盐含量介于3.15%-45.97% (X=22%, σ=6.74%) ,离散程度很高,因此,碳酸盐淋溶作用对环境参数波动的影响不容忽视,故使用高含量以及高振幅变化的碳酸盐校正各环境参数使其具有更准确的指示意义,使HIRM更确切的反映红粘土中赤铁矿的变化情况。
2、比较环境磁学指标硬等温剩磁(HIRM)和非磁学手段漫反射光谱分析结果发现,HIRM指示的赤铁矿含量占红粘土中所含总赤铁矿的62%-95%,即HIRM可指示红粘土中的大部分赤铁矿含量变化。
3、红粘土的HIRM值在不同粒级组分(<2 μm、 2-8 μm、8-31 μm 和31-63 μm)很接近且波动幅度很小,最大值出现在最粗(31-63 μm)粒级组分中,而第四纪黄土的HIRM值随着粒径组分变细而逐渐增大,最大值出现在成壤粘土(<2 μm)的粒级组分中;结合红粘土的HIRM变化与其他指示成壤作用的磁学参数有显著差异性,不同于第四纪黄土-古土壤序列中各磁学参数的同相位变化模式;由此得出HIRM在红粘土沉积序列中主要反映来自粉尘源区的风成碎屑成因赤铁矿的含量变化。
Other AbstractChinese Loess Plateau (CLP) is composed of hundreds-meters-thick windblown sedimentary sequences, which is famous for its continuous sedimentation, long chronological sequences and high resolution. Such a whole deposition sequences can completely document the evolution history of Asian paleomonsoon, the uplift history of the Tibet Plateau, and the process of aridification and desertification in Asian interior. The research on paleoclimate of Quaternary loess-paleosol has been quite mature, and there is no dispute about the paleoclimatic evolution information during this period by environmental magnetic parameters, but the research on tertiary red clay overlying quaternary sedimentary sequences is relatively weak. The hard isothermal remanence magnetization (HIRM) indicates the content of weakly magnetic minerals (hematite/goethite), which is different from magnetic susceptibility (χlf) indicating the content of strongly magnetic minerals (magnetite/maghemite). What's more, HIRM records orbital period (eccentricity) signal and contains abundant paleoclimatic evolution information of tertiary red clay, which deserves to be further explored and studied.
The zhaojiachuan(ZJC) red clay sequence of CLP was selected as the research profile in this paper, and a series of environmental magnetic parameters (χlf, χfd, HIRM and S-ratio) and the non-magnetism method (carbonate content measuring, size sorting, diffuse reflection spectrum(DRS), X-ray diffraction (XRD), and soil micromorphology) were measured systematically. A comprehensive analysis of the magnetic properties of red clay were conducted to mainly make further clear the indication of HIRM in red clay, quantity the proportion of hematite content reflected by HIRM and identify the source of hematite in red clay.
The systematic investigation and study of the ZJC red clay sequence obtain the following three understandings:
1. The carbonate content of red clay sequence is 3.15%-45.97% (X=22%, σ=6.74%), showing the high discrete degree. Therefore, it’s not allowed to ignore the effect of carbonate leaching on the fluctuation of environmental parameters. It’s significant to use the high carbonate content with high amplitude change to correct environmental parameters to reveal their accurate indication, making HIRM truly recover the hematite variation in red clay.
2. The comparison between the hematite content obtained by HIRM and the DRS shows that the hematite content indicated by HIRM accounts for 62-95% of the total hematite contained in the red clay. HIRM predominately reflects the hematite content of red clay sequences.
3. The HIRM values of red clay sequence on different grain size fractions(<2 μm、2-8 μm、8-31 μm and 31-63 μm) are very close, the maximum is on the coarsest fraction (31-63 μm), and the fluctuation range is very limited. While the HIRM values of Quaternary loess increases as the particle size becomes finer, and the maximum value appeared in pedogenic grain size fraction (<2 μm). The HIRM of red clay shows significant differences from other magnetic parameters indicating pedogenesis, which differs from the in-phase change patterns in Quaternary loess. To sum up, we suggest that the hematite reflected by HIRM in Pliocene Red Clay sequences was wind-blown and derived from the source area.
Pages52
URL查看原文
Language中文
Document Type学位论文
Identifierhttp://ir.lzu.edu.cn/handle/262010/344349
Collection资源环境学院
Affiliation资源环境学院
First Author AffilicationCollege of Earth Environmental Sciences
Recommended Citation
GB/T 7714
路彩晨. 黄土高原上新世红粘土记录的古气候变化—来自碳酸盐校正的HIRM证据[D]. 兰州. 兰州大学,2019.
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