末次间冰期以来黄土-古土壤序列的磁组构特征及其指示的古风向

STUDY OF THE MAGNETIC FABRICS IN CHINESE LOESS-PALEOSOLS SINCE THE LAST INTERGLACIAL:IMPLICATION OF THE PALEOWIND DIRECTION

黄土高原粉尘物质的搬运、沉积过程与来自中高纬度地区的季风密切相关。研究冬季风演化历史将有助于深入理解全球变化背景下,东亚地区气候变化的动力机制。通过对黄土高原的磁组构研究发现:1)各向异性度(P)与磁面理度(F)相关性较高,因此各向异性主要由磁面理引起,磁化率椭球体为压扁状;2)磁面理大致水平,K3近垂直于水平面;3)黄土平均磁面理为1.007,古土壤为1.004,古土壤层的磁面理与各向异性度均要低于其下伏的母质黄土层。成土作用在某种程度上破坏了黄土的原生磁组构;4)P,F和粉尘粒度间具有一定内在联系,一般而言冬季风越强,搬运的粉尘颗粒越粗,从而导致在沉积过程中形成较高的磁面理;5)K1方向的等面积赤平投影图和玫瑰花图解表明,末次间冰期以来该地区的主导风向为NW向冬季风。

The transportation and deposition of eolian materials of Chinese loess are correlated and effected by the monsoon from the mid-high latitude. Studies of the winter monsoon evolution can help us to understand the dynamic mechanism to climate change in the east-Asian areas. The anisotropy of magnetic susceptibility (AMS) measurements of 220 loess-paleosols samples from the loess profile of Baicaoyuan, Gansu province, all showed original magnetic fabric for sediments. The data indicated that: 1) The degree of anisotropy (P) always show a strong correlation with the foliation (F) rather than with the lineation (L), therefore the anisotropy is controlled by the F; 2) The foliation is nearly horizontal and the axis K_(3)is perpendicular to the horizontal plane respectively; 3) Mostly values of the shape parameter (T) are in the area 0<T<1 and correspond to the oblate shape of the magnetic ellipsoid; 4) The AMS features between loess and paleosols are somewhat different. The average value of F in loess unites is 1.007, while 1.004 in paleosols units. Also we found that the F, P values of paleosols is lower than that of its parent loess respectively. It is the pedogenesis that reduced or destroyed the primary fabric of the loess; 5) AMS foliation is controlled by the grain size distribution of ferromagnetic minerals and by post-depositional compaction. The grain sizes of eolian inputs are in turn determined by the intensity of winter monsoon, thus the intensity of winter monsoon, grain size of the eolian inputs, the F and the P are somewhat inter-related. Generally, the higher intensity of the winter monsoon will carry coarser-grained eolian material, thereby resulting in a larger foliation during deposition. To the first-order, the median grain size and the F are positive interrelation in the study profile; 6) Many studies have demonstrated that the declination of long axis (K_(1)) is parallel to paleowind direction. The average declination of K_(1) is 308.9° and the rose diagram of K_(1) implies that the dominated wind direction for winter monsoon at this site is NW since the last interglacial. The result presented here is the whole-effect of the wind field in this temporal span and the difference among the declination of K_(1) maybe correspond to the wind direction changes in different periods.