汾渭盆地黄土剖面0.9Ma前后的粒度突变及其环境意义

Grain Size Abrupt Shift around 0.9 Ma in Loess and Its Environmental Effect in Fenwei Basin,China

通过对汾渭盆地老庄黄土剖面的古地磁、磁化率、粒度分析,认为该剖面具有1.2 Ma以来的完整风成沉积系列。该剖面最为显著的特点为其粒度组成中粗粒组分含量较高,自L9(对应年龄约为0.9 Ma)以来粒度突然变粗并且地层中共发育了11层厚度较大的埋藏古风成砂,研究区远离沙漠,黄土中的砂粒及风成砂是近源搬运造成的,其中黄河河漫滩是其砂粒的主要供给源。这一特点显示汾渭盆地在距今0.9 Ma以来气候显著变干,冰期时发生严重土地沙漠化,这可能是汾渭盆地对中更新世气候转型的响应,还可能与当时青藏高原急剧隆升而激发的亚洲内陆干旱化加剧有关。

The field investigations and the analyses of paleomagnetic polarity reversals, magnetic susceptibility and grain size on Laozhuang section which is on the fifth terrace of the Yellow River in Fenwei basin show that this section contains most of the aeolian stratigraphic units over the past 1.2 Ma. This section contains abundant sand （〉63 μm） and the curve of the percentage of the grain size exhibits an abrupt shift around 0.9 Ma. And since then there are several aeolian sand units in loess. Because of far from the desert, the abundant sand （ 〉63 μm） and the aeolian sand units are caused by the sand from near source. The sand beach of the Yellow River is the main source of sand in loess in Laozhuang section. The abrupt shift recorded in the section around 0.9 Ma possibly responded to the world-wide middle Pleistocene climatic transition event. This transition event may be related to the aggravation of drought in interior Asian triggered by the accelerated uplift of the Tibetan plateau.