毛乌素沙地风沙沉积物磁学特征及其古环境意义

在沙漠沉积环境中,成土作用对磁化率的贡献较小,往往被原生磁信号掩盖,因此分离两种磁组分对气候和粉尘代用指标的提取至关重要.本研究选取位于毛乌素沙地东缘的锦界风沙沉积剖面为研究对象,利用多变量一元线性回归中的"平均值概念"进行磁化率的原生碎屑组分0和次生成土组分Xpedo的分离,并探讨磁学参数所承载的气候和环境意义.结果显示,磁学比值参数(如Xpedo/X0、Xfd/HIRM、Xfd%和XARM/SIRM)彼此之间存在显著线性或指数/对数相关关系,它们对成土强度指示明确,可以在一定程度上减小或避免磁性矿物背景值差异所产生的误差,与磁化率相比更适宜用于该区的古降水量重建.HIRM主要由碎屑赤铁矿含量控制,HIRM与X0存在明显正相关关系,表明碎屑赤铁矿随原生磁性矿物总体含量的增加(减少)而增加(减少),在粉尘成因磁性矿物中所占比例大致稳定,从而HIRM可以指示源区粉尘通量的变化.锦界剖面的原生和次生磁性矿物浓度均明显低于黄土高原黄土,不同地层X0和Xpedo在磁化率中所占的比例存在较大差异,因此磁化率的环境意义比较复杂,在使用其恢复古气候古环境时需慎重.

Magnetic properties of eolian sand sediments in the Mu Us Desert and their paleoenvironmental significance

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 low-frequency magnetic susceptibility (X_lf), frequency-dependent magnetic susceptibility (X_fd), susceptibility of anhysteretic remanent magnetization (X_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 (X_fd%), X_ARM/SIRM and X_ARM/X_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 (X₀ and X_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 0.1~1 μm 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. X_lf values range (5.21~38.50)×10^-8m³·kg^-1, with the average X₀ value 7.663×10^-8m³·kg^-1. The proportion of X₀ and X_pedo 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. X_pedo/X₀, X_fd/HIRM,X_fd% and X_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 X₀ 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.