Magnetic characteristics of surface samples from NE Iran and its paleoenvironmental implications
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摘要:
全球变化研究中的现代过程研究越来越成为定量重建工作的先决前提, 而由于如黄土等气候记录载体沉积分布的全球性与沉积环境的多样性, 更需要就具体的研究区进行有针对性的现代过程工作以最大程度地保证重建工作的可靠性。西起地中海、东至帕米尔的广大区域是丝路文明演化的关键地区, 其过去气候变化也是了解北半球中纬度气候演化与机制的重要拼图。本研究即采集位于该区之里海东南缘的伊朗黄土沉积区域的现代表土, 并联系气候要素考察其磁性特征的空间变化。结果显示主要分布于冬春季节的年均降水其变化控制了亚铁磁性矿物的空间分异, 包括次生超细颗粒含量变化与磁赤铁矿化程度, 典型的磁学参数如百分频率磁化率值的变化可以定量记录年均降水值的变化。结合相关关系函数, 可以为该区过去降水变化的定量重建提供准备并为进一步理解区域气候系统演化与机制提供支持。
Abstract:As globally distributed loess deposits distinctively from site to site, numerous researches have been conducted to empirically correlate modern climatic factors with the variations in magnetic parameters of topsoil and underlying loess at typical loess sites, in order to discover the correlations between magnetic properties and climatic factors by identifying the transferation and transformation of ferrimagnetic carriers for reliable quantitative paleo-reconstruction. In this work, systematic magnetic analysis has been performed aiming to obtain functionalized paleoclimatic proxies, on surface samples collected from the so-called Iranian Loess Plateau region in northeast of Iran, southeast of the Caspian Sea, which is climatically characterized by Mediterranean climate with hot, dry summers and mild, wet winters. What's more, regional annual average temperature is almost spatially unchanging, while the annual average precipitation increases gradually from 200 mm up to more than 800 mm southeastwards, providing an excellent natural site for modern processes research, and 20 topsoil samples were collected.
According to hysteresis measurement, apparent hysteresis phenomenon and loop-closing at magnetic field of ca.350 mT indicate the magnetic dominance of ferrimagnetism, such as magnetite and/or maghemite. Regional χlf values of surface samples vary between 28.40×10-8~56.05×10-8 m3/kg, with average value of 37.22×10-8 m3/kg, which is significantly lower than that of surface samples from Chinese Loess Plateau. As to frequency-dependent magnetic susceptibility χfd%, which is between 0.44%~5.43%, the average value is 2.82%, is lower than that of surface samples from Chinese Loess Plateau, both of which imply roughly low concentration of magnetic carriers, especially for ferrimagnetic assemblages, and possible less weathering and pedogenic processes. Granulometric state of magnetic assemblages are estimated via Day plot showing that data set falls in PSD region, similar to data of typical loess samples from central and south Chinese Loess Plateau, but even coarser. Moreover, domain state shown by Day plot for research region should be closer to its actual state considering its low concentration of SP particle. Warming curves of low temperature saturated isothermal remanent magnetization(SIRM) confirms the existence and dominance of magnetite and maghemite, derivative curve of which also shows that precipitation variations determine the post-alteration of detrital magnetic minerals(mostly magnetite) significantly and progressively. Regionally speaking, such kind of alternations on magnetic minerals are demonstrated in the way of maghemitization and fine particles generated in situ; the more annual average precipitation, the higher degree of post-alteration.
Overall, magnetic properties of surface samples from NE Iran are characterized via kinds of magnetic measurement and systematic analysis within this work. Correlations between spatial magnetic heterogeneity and climatic factor are thoroughly discussed subsequently. Speaking of paleoclimatic indicators, χfd% values are not background signal, but reflect spatial variation of ferrimagnetic fine particle generated in situ, which are determined significantly by annual average precipitation, making series of these parameters that are verified by well correlated functions are appropriate for quantitatively paleoclimatic proxies for regional environmental reconstruction.
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图 1
伊朗黄土高原区表土样点示意图
Figure 1.
Map showing sampling sites in the Iranian Loess Plateau and adjacent region
图 3
表土样品磁性特征的相关性变化图
Figure 3.
Correlations between magnetic properties for regional surface samples
图 4
表土样品低温饱和剩磁的热退磁曲线
Figure 4.
Thermal demagnetization of low temperature saturation magnetization for representative samples
图 5
表土磁化率、百分频率磁化率与区域年均降水量相关变化图
Figure 5.
Climatic dependence of magnetic susceptibility and frequency-dependent magnetic susceptibility for regional surface samples
表 1
伊朗黄土高原表土磁性特征
Table 1.
Magnetic properties of surface samples in the Iranian Loess Plateau
Lat. (°N) Long. (°E) MAP (mm) MAT (℃) Alt. (m) χ(×10-8 m3/kg) χfd%(%) Magnetic Properties χlf χhf χ1 χ2 χ3 χfd12% χfd23% χfd13% Bc (mT) Bcr (mT) Ms (×10-3Am2/kg) Mrs (×10-3 Am2/kg) Mrs/Ms Bcr/Bc SS01 37°44.208′ 54°56.404′ 274 18.1 54 28.40 27.88 31.10 30.72 29.89 1.23 2.70 3.89 9.84 36.97 32.16 3.49 0.11 3.76 SS02 37°38.311′ 54°58.212′ 278 18.1 86 33.78 33.51 37.21 36.67 35.70 1.45 2.62 4.04 9.74 36.08 36.99 4.11 0.11 3.71 SS03 37°37.298′ 55°01.991′ 305 18.1 118 32.05 31.31 34.84 34.37 33.43 1.35 2.74 4.06 9.52 34.85 34.95 3.91 0.11 3.66 SS04 37°34.998′ 55°04.662′ 328 18.2 81 31.36 30.87 33.66 33.21 32.38 1.33 2.50 3.79 9.43 36.53 38.66 4.20 0.11 3.87 SS05 37°31.801′ 55°04.304′ 336 18.2 68 32.28 31.52 34.57 34.13 33.16 1.28 2.85 4.09 9.80 36.98 34.85 4.01 0.12 3.77 SS06 37°30.342′ 55°07.431′ 369 18.2 95 32.91 32.29 36.02 35.37 34.37 1.81 2.83 4.59 9.94 37.18 38.73 4.39 0.11 3.74 SS07 37°30.122′ 55°09.914′ 392 18.2 96 29.89 28.85 32.46 32.03 31.08 1.32 2.96 4.24 10.01 36.95 35.16 4.16 0.12 3.69 SS08 37°27.757′ 55°11.822′ 447 18.2 126 33.95 33.19 37.43 36.81 35.61 1.66 3.25 4.86 9.33 36.17 38.95 4.46 0.11 3.88 SS09 37°27.076′ 55°12.847′ 462 18.2 106 30.32 30.19 33.67 33.06 32.00 1.79 3.23 4.96 9.38 36.54 34.82 4.06 0.12 3.90 SS10 37°24.733′ 55°13.887′ 487 18.1 76 45.01 43.05 48.59 47.05 44.75 3.18 4.88 7.90 9.30 31.41 41.61 5.31 0.13 3.38 SS11 37°23.826′ 55°14.040′ 492 18.1 76 50.07 48.22 54.66 52.82 50.13 3.36 5.10 8.29 9.05 30.27 43.72 5.78 0.13 3.34 SS12 37°23.250′ 55°16.959′ 541 18.3 72 56.05 53.00 61.19 58.77 55.24 3.96 6.00 9.72 8.91 28.08 39.35 5.72 0.15 3.15 SS13 37°22.464′ 55°18.593′ 577 18.2 74 41.62 39.73 45.16 43.85 41.83 2.91 4.60 7.38 9.01 31.87 38.80 4.69 0.12 3.54 SS14 37°19.919′ 55°20.818′ 652 18.0 83 52.61 50.00 57.28 55.06 52.09 3.88 5.40 9.07 9.17 29.53 39.94 5.54 0.14 3.22 SS15 37°18.140′ 55°21.189′ 691 17.9 105 40.75 38.77 43.86 42.40 40.32 3.33 4.90 8.07 9.85 33.73 33.91 4.78 0.14 3.43 SS16 37°18.419′ 55°21.762′ 694 17.9 102 36.62 35.96 40.66 39.38 37.46 3.15 4.88 7.88 9.94 34.33 30.33 4.44 0.15 3.45 SS17 37°17.659′ 55°22.618′ 735 17.8 126 35.44 34.22 38.75 37.31 35.40 3.71 5.12 8.64 10.26 34.91 26.63 4.13 0.15 3.40 SS18 37°17.371′ 55°23.011′ 752 17.7 135 29.57 29.05 32.37 31.44 30.14 2.89 4.14 6.91 11.01 36.41 22.14 3.35 0.15 3.31 SS19 37°15.188′ 55°28.055′ 807 17.1 300 31.92 31.14 34.75 33.82 32.41 2.68 4.17 6.74 10.65 38.54 30.40 4.19 0.14 3.62 SS20 37°15.835′ 55°24.890′ 816 17.5 186 39.79 38.92 44.02 42.85 40.99 2.65 4.33 6.87 9.88 35.32 37.36 4.81 0.13 3.58 
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