西昆仑山黄土的岩石磁学特征及其磁化率增强机制

ROCK-MAGNETIC CHARACTERISTICS AND THE ENHANCING MECHANISM OF MAGNETIC SUSCEPTIBILITY FOR WEST KUNLUN MOUNTAINS LOESS

成壤过程中形成的细颗粒的软磁性矿物被认为是导致古土壤磁化率增加的主要原因。但近来的研究表明,在一些地区,尤其是靠近沙漠边缘的黄土-古土壤序列,源区对黄土磁化率的影响要远大于成壤作用。因此,有必要对不同地区、不同环境条件下的典型黄土堆积进行详细的岩石磁学研究。日前,我们在西昆仑山北侧钻取了一根长达671m的岩芯,这为研究极端干旱区黄土的岩石磁学性质提供了难得的契机。本文对第一期黄土钻探得到的207m岩芯进行了详细的岩石磁学研究,结果表明:昆仑山黄土的主要载磁矿物为磁铁矿和磁赤铁矿,同时还含有少量的针铁矿、赤铁矿;该地区磁化率的变化主要受源区粗颗粒的软磁性矿物含量的影响,成壤作用形成的细颗粒磁性矿物对磁化率的贡献极小;磁化率、粒度在0.5Ma左右急剧升高和变粗,主要与气候干旱化加剧有关。

Variations in magnetic susceptibility（MS）of Chinese loess-paleosol layers show a high correlation with marine oxygen isotope records,and have been widely used as a proxy of East Asian summer wind intensity over the past two decades. At present, increasing concentration of SP （ superparamagnetic particles） and SD （single-domain） grains during the pedogenesis is considered as the main reason for enhancement of magnetic susceptibility （MS） of paleosol. Recently,however,many researches have shown the influence of coarse lithogenic magnetic minerals to be also very important,especially in the loess-paleosol sequence adjacent to the desert edge. Loess in the Kunlun Mountains is so far the thickest loess found in the extreme arid region of China＇s inland, recording continuously histories of the desert and dust-carrying winds and circulation. Lately,we obtained a 671m long loess core through drilling on the northern slope of the West Kunlun Mountains （36°12＇N,81°20＇E; elevation 3300m）, which provides a good opportunity to study the mechanisms of MS enhancement in the arid Asian heartland for us. In this study ,we investigated its 207m long loess core by integrating both rock-magnetic and grain size proxies. The study results suggest that the low coercivity minerals,i, e. ,magnetite and maghemite are dominated in the loess. The hard magnetic minerals including goethite and hematite were also found but made a minor contribution to the signal of susceptibility. The characteristics of typical hysteresis loops probably imply the presence of paramagnetic component to a certain extent. In addition,the results also show that MS and grain size both increase sharply at depth 111 m with an age of ca. 0.5Ma, and meanwhile,magnetic grain size became much coarser（from PSD to MD-like）and magnetic minerals change toward a higher concentration of soft magnetic components （i.e. magnetite and maghemite）. All these suggest a drying event might occur at that time. The event made a larger number of lithogenic materials relatively rich in coarse-grained（ MD-like grains）ferrimagnetic minerals magnetite and maghemite to be carried and deposited on the north slope of the Kunlun Mountains more easily, which resulted in the enhancement of susceptibility after depth 111 m sharply. These results improved our understanding of the enhancing mechanism of susceptibility at different sites with different paleoenvironmental conditions： in comparison with the uhrafine pedogenic minerals, coarse lithogenic magnetic minerals can also play an important role in enhancement of MS at some time.