Magnetic properties of eolian sand sediments in the Mu Us Desert and their paleoenvironmental significance
Magnetic properties of eolian sand sediments in the Mu Us Desert and their paleoenvironmental significance
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DOI：
10.6038/cjg20151022
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发表时间：
2015-10
期刊：
影响因子：
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通讯作者：
Zhao Shuang;Gao Fu-yuan;J. Jia;Liu Guan-hua;X. Dun-sheng;Jin He-ling
中科院分区：
文献类型：
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作者：
Zhao Shuang;Gao Fu-yuan;J. Jia;Liu Guan-hua;X. Dun-sheng;Jin He-ling
在干旱沙漠中，由成土作用引起的磁化率增强是有限的，而且常常受到成岩磁信号的干扰。这两种磁分量的分离对于选择气候和尘埃变化的精确代理至关重要。本文对毛乌素沙漠东缘金界剖面风成沙沉积物进行了详细的磁学调查，探讨了其磁学参数与沉积环境的关系。通过测量磁滞回线和磁化温度依赖性（J-T曲线）来识别磁性矿物类型。通过低频磁化率（chi(lf)）、频率相关磁化率（chi(fd)）、非滞回剩磁磁化率（chi(ARM)）、饱和等温剩磁磁化率（SIRM）和“硬”等温剩磁磁化率（HIRM）等环境磁测量，确定了不同类型和域的磁性矿物含量。计算磁化率参数，包括频率相关磁化率百分比（chi(fd) %） chi(ARM)/SIRM和chi(ARM)/chi(lf)，以估计磁性矿物的粒度。采用多变量一元线性回归中的“均值概念”分离成岩磁化率分量chi(0)和chi（pedo）。结果表明：金街剖面磁性矿物以铁磁性矿物为主，少量反铁磁性矿物混杂；与邻近黄土矿床相比，磁性粒度较细，主要为粗粒稳定单畴（SSD）和超顺磁性（SP）颗粒。等效磁性粒度为0。1近似于1 μ m，古土壤层中出现更多的细/超细颗粒。金街剖面的成岩和成土磁性矿物含量均低于黄土高原的风成沉积。Chi （lf）取值范围为(5)。21类似于38。50) × 10(-8)m(3)。Kg(-1)，平均chi(0)值为7。663 × 10(-8)m(3)。公斤(1)。chi(0)和Zpedo在剖面质量磁化率中所占的比例波动较大，磁化率的古环境意义复杂，在将磁化率作为古沉淀指标时应引起高度重视。磁参数的比值chi(pedo)/chi(0)、chi(fd)/HIRM、chi(fd) %和chi(ARM)/SIRM之间具有显著的线性或对数/指数相关关系。这些比值都可以减小或抑制造岩磁信号的影响，从而准确地指示成土强度和降水。我们认为HIRM主要反映碎屑赤铁矿含量，这些赤铁矿来自尘源区。HIRM与chi(0)呈显著正相关，表明碎屑赤铁矿在块状风成磁性矿物中所占比例大致恒定，因此HIRM可以作为毛乌素沙漠尘埃通量的代表。
In arid deserts, magnetic susceptibility enhancement caused by pedogenesis is limited and usually disturbed by lithogenic magnetic signals. Separation of these two magnetic components is essential for selecting the precise proxy for climate and dust variation. This paper presents detailed magnetic investigation of eolian sand sediments from the Jinjie section at the east edge of Mu Us Desert to explore the relationship between the magnetic parameters and sedimentary environment. Hysteresis loops and temperature dependence of magnetization (J-T curves) were measured to identify the types of magnetic minerals. Environmental magnetic measurements, such as lowfrequency magnetic susceptibility (chi(lf)), frequency-dependent magnetic susceptibility (chi(fd)), susceptibility of anhysteretic remanent magnetization (chi(ARM)), saturation isothermal remanent magnetization (SIRM) and "hard" isothermal remanent magnetization (HIRM) were carried out to determine the content of magnetic minerals in different types and domains. Magnetic ratio parameters, including percentage of frequency-dependent magnetic susceptibility (chi(fd) %) chi(ARM)/SIRM and chi(ARM)/chi(lf), were calculated to estimate the grain-size of magnetic minerals. The "mean value concept" in a mono-linear regression of multi-variables was used to separate lithogenic and pedogenic magnetic susceptibility components (chi(0) and chi(pedo)). The results show that magnetic minerals of the Jinjie section are dominated by ferromagnetic minerals with a few antiferromagnetic minerals mixed. Magnetic grain-size is quite fine compared with the adjacent loess deposits, mainly coarse stable single domain (SSD) and superparamagnetic (SP) particles. The equivalent magnetic grain-size is 0. 1 similar to 1 mu m with more fine/ultrafine particles appearing in paleosol layers. Both lithogenic and pedogenic magnetic mineral contents of the Jinjie section are less than eolian depositions on the Chinese Loess Plateau. chi(lf) values range (5. 21 similar to 38. 50) X 10(-8)m(3) . kg(-1), with the average chi(0) value 7. 663 X 10(-8)m(3) . kg(-1). The proportion of chi(0) and Zpedo occupied in the mass magnetic susceptibility is widely fluctuant in the section, thus paleoenvironmental significance of magnetic susceptibility is complicated, and great attention should be paid when magnetic susceptibility is used as a paleoprecipitation indicator. The ratios of magnetic parameters, i. e. chi(pedo)/chi(0), chi(fd)/HIRM, chi(fd) % and chi(ARM)/SIRM, possess significant linear or logarithmic/exponential correlation between each other. All these ratios can reduce or refrain the effect from lithogenic magnetic signals, and thus can accurately indicate pedogenesis intensity and precipitation. We consider that HIRM mainly reflects detrital hematite content, which is derived from the dust source region. The prominent pOsitive correlation between HIRM and chi(0) indicates that detrital hematite possesses a roughly constant proportion in the mass eolian magnetic minerals, thus HIRM can be used as a proxy for dust flux in the Mu Us Desert.