Magnetic Properties Of Loess In Bahawalpur,Pakistan And Its Environmental Significances

Aeolian loess,widely distributed over the world,is a good carrier for studying paleoclimate and paleoenvironment change.The mechanism of recording paleoclimate change is affected by different environments and different climates,and there may be great differences.Currently,the studies of loess in Pakistan are rare,and the studies on the mechanism of magnetic susceptibility variation are still unavailable.In this study,the the rock magnetic parameters of the newly discovered loess-paleosol section(29°19?56?N,71°42?65?E)in the Bahawalpur area of the Indus Plain of Pakistan were systematiclly measured.Combined with the grain size characteristics,the diffuse reflectance spectrum(DRS),geochemistry and scanning electronic microscopy(SEM),the mechanism of magnetic properties and its environmental significances was discussed.Based on the results of the samples,the following main conclusions are drawn:(1)The BH section is composed of three layers of light yellow loess and three layers of brown-yellow paleosol.The boundary between two strata is transitional and with out horizontal bedding.The results show that the characteristics of particle size and the distribution patterns of major elements and rare earths elements(REE)are very similar to Chinese loess.What's more,the surface morphology of quartz particles show obvious conchoidal fractures,dash-shaped concavities.These are typical characteristics of aeolian particles.Based on this,the wind formation of loess sediments in BH section can be indicated.(2)The grain size of loess is silt and extremely fine sand,the particle size composition is relatively coarse compared with that in the China Loess Plateau(CLP),probably owing to the sampling site is on the edge of the Great India Desert.And the grain size of paleosol is silt,the clay content is slightly more than that in the loess-paleosol of China.The geochemical parameters show a high degree of chemical weathering in BH section.These results show that loess sediments undergo obvious pedogenic process.(3)The magnetic susceptibility of the BH section is lower than that of the CLP,which is equivalent to the least developed loess in the northwest of CLP.The results show that the magnetic characteristics of loess and paleosol are quite different,the main magnetic minerals in loess are magnetite,coupled with goethite and maghemite.And the magnetic domain particles in the magnetic minerals are Pseudo-single domain(PSD)and multi-domain(MD).Compared with the loess,the main magnetic minerals in paleosol are goethite,coupled with magnetite and paramagnetic minerals.This changing mechanism of the susceptibility is different from that of CLP.The mechanism of magnetic susceptibility change may be different from the existing model.There is a possible threshold of magnetic susceptibility 12.8×10^-8m³/kg in the study area,above which the main magnetic minerals are strongly magnetic minerals(magnetite and maghemite);below this value,the main magnetic minerals are weakly magnetic minerals(such as goethite).This transition of magnetic minerals may lead to a decrease in magnetic susceptibility.At this point,the magnetic susceptibility indicates the change of wind force and source region above the threshold,and reflects the soil action under the threshold.(4)The environmental indicators of BH loess-paleosol sequence,such as median particle size,magnetic susceptibility,CIA and Na/K ratio,may reflect the trend of climate change from wet to dry in the region since the late Pleistocene and the expansion of the Great India Desert in the late late Pleistocene.