中国北方上新世晚期-更新世早期风成沉积物磁化率各向异性初探

A PRELIMINARY STUDY ON ANISOTROPY OF MAGNETIC SUSCEPTIBILITY OF THE LATE PLIOCENE-EARLY PLEISTOCENE AEOLIAN DEPOSITS IN NORTHERN CHINA

前人对中国黄土高原晚更新世黄土沉积物的磁化率各向异性的研究表明,磁化率椭球体最大轴(Kmax)偏角的方向能够指示黄土沉积时期的搬运动力方向,即古风向,并可以通过这种途径进行古季风的重建。本研究试图通过对中国黄土高原晚上新世-早更新世转折时期的风成沉积磁化率各向异性研究进一步检验其在古风向重建方面的应用潜力。选择分别地处黄土高原中部、南部和东部地区的灵台、渭南和保德3个剖面,对红粘土向黄土过渡时期4个不同地层单元古土壤层(S32)、黄土层(L33)、过渡层(TU)以及红粘土沉积(RC)的样品进行了磁化率各向异性测量,结果表明磁线理L值很小,而磁面理F较为发育,这与前人对晚更新世黄土沉积磁化率各向异性的研究结果一致;同时,4个不同地层单元的磁面理F、各向异性度P和形状因子T在3个剖面上均表现出随土壤发育程度增强而减小的变化趋势,说明后期的成壤作用能够不同程度地降低磁面理、磁化率各向异性度以及形状因子的大小。与前人对于晚更新世黄土沉积磁化率各向异性的研究结果不同的是3个剖面的S32,L33,TU以及RC的Kmax的偏角似乎并不能反映出当时的搬运动力方向,即古风向,而长期的压实作用以及不同程度的后期土壤化作用是造成这一时期黄土和红粘土沉积物磁化率各向异性最大轴偏角方向未能反映出当时搬运动力方向(古风向)的原因之一。

Anisotropy of magnetic susceptibility （AMS） of the sediments is an effective means for understanding the depositional mechanism, especially the direction of the transporting agent. Previous AMS research of the Late Pleistocene loess sediments in Chinese Loess Plateau has indicated that the declination of the maximal susceptibility axis（K ）coincides with the paleo-wind direction. This method can also be used to reconstruct the paleo-monsoon. This study is designed to test the potential application of AMS in paleo-wind reconstruction in older aeolian deposits formed during the transitional period of Late Pliocene and Early Pleistocene in Chinese Loess Plateau. AMS of the Late Pliocene-Early Pleistocene wind-blown sediments from three sections in northern China has been investigated. These sections are located in central（Lingtai, Gansu） , southern（Weinan, Shaanxi） , and eastern（ Baode, Shanxi） parts of the Loess Plateau. Samples selected from four different stratigraphic units including paleosol unit （ S32 ） , loess unit（L33）, transitional unit（TU）, and red clay unit（RC）around the transition from the Late Pliocene red clay to Early Pleistocene loess were measured. The results show weak lineations and strong foliations, which are consonant with the results of former AMS studies on the Early Pleistocene loess deposits. Also, the values of foliation, degree and shape of AMS of the four different stratigraphic units in three sections decrease with the increasing degree of pedogenesis, which indicated that the soil formation processes seem to have caused a decrease in the values of foliation, degree and shape of AMS. Inconsistent with the results of previous AMS studies on the Early Pleistocene loess deposits, the declination of the maximal susceptibility axis （Kmax） of the four different stratigraphic units is incapable of reflecting the direction of the transporting agent, namely the paleo-wind direction, during the transitional period of Late Pliocene and Early Pleistocene. Post-depositional compaction and soil formation processes are suggested to be one of the reasons of this result.