黄土高原小流域植被变化和侵蚀产沙的孢粉示踪研究初探

如同137Cs和210Pbex核尘埃,土壤和坝库沉积物中的孢粉也源于大气沉降,是否能用于侵蚀产沙的示踪研究?在陕北吴旗周湾水库采集了一些表层土壤和现代淤地坝、古代聚湫坝沉积泥沙的样品,尝试土地利用/覆被变化和侵蚀产沙的孢粉示踪研究。初步研究表明:草地表土孢粉丰富,平均浓度为26077粒/g;有33个科属的花粉,孢粉组合反映了当地以蒿属为主的半干旱温带草原植被;坡耕地表土的孢粉组合和草地一致,但孢粉平均浓度远低于草地,仅5844粒/g。坝库沉积剖面中,和137Cs核素一样,孢粉也富集于每一洪水沉积旋迴的上部层位,可用于沉积旋迴的区分。现代沉积旋迴的孢粉平均浓度,远低于草地和坡耕地表土,这表明草地和坡耕地土壤不是沉积泥沙的主要来源。古代沉积物的孢粉平均浓度高于现代沉积,表明流域内300年前的植被好于现代,或300年前沟谷地的相对产沙量低于现代。古代孢粉组合中十字花科花粉比例高达36·85%,明显高于现代的0·58%含量,这表明草地退化严重,十字花科植物比例急剧降低。孢粉示踪技术在植被环境恢复,沉积旋迴区分,和泥沙来源确定等方面具有一定的潜力,但应建立针对土地利用/覆被变化和侵蚀产沙研究的孢粉示踪技术方法。

A TEST OF THE POLLEN TRACING TECHNIQUE FOR STUDIES OF VEGETATION CHANGES, EROSION AND SEDIMENTATION IN A SMALL CATCHMENT IN THE LOESS PLATEAU OF CHINA

Like radionuclide fallouts of ¹³⁷Cs and ²¹⁰Pb_ex, pollens in sediment deposits and soil come from the atmosphere. Therefore, can pollens be used as tracers for soil erosion and sedimentation studies? Surface soils, modern and ancient reservoir deposits were sampled as a test of the pollen tracing technique for studies of vegetation changes, erosion, and sedimentation in the Zhouwan Reservoir Catchment of the Loess Plateau, Wuqi County, North Shaanxi, China. Surface soils on grassland are rich in pollens and have an average pollen concentration of 26 077 grains/g, and 33 taxa of pollens, which are correlated to the local vegetation of semi-arid temporal steppe rich in Artemisia as well. Surface soils on cultivated land have similar taxa, but the pollen concentration is only 5 844 grains/g, much less than those on the grassland. Like ¹³⁷Cs, pollen concentration is higher in the upper horizon of a flood deposit couplet. Average pollen concentration in the modern deposits is much less than those in surface soils on both the grassland and the cultivated land. Average pollen concentration in the ancient deposits from 300 years ago is higher than concentration in the modern deposits. It indicates that the vegetation in the catchments from 300 years ago was better than today or the relative contribution of sediment derived from the gully area was less than today. The proportion of Cruciferae pollens compared to total pollens in the ancient deposits reaches 36.85%, which is much higher than the concentration in the modern deposits （only 0.58%）. It indicates that degradation of the grassland is severe and has resulted in reduction of Cruciferae. This study shows great potential of the pollen tracing technique for reconstruction of vegetation environments, distinguishing of flood deposit couplets, and investigation of sediment sources. However, specific sampling methodology should be developed for the pollen tracing technique.