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

Abstract

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.