特征时期黄土高原风化成壤强度的 空间特征与气候梯度*

文章基于15个剖面,采用土壤学使用的风化成壤强度指数(游离铁FeD/全铁FeT),对黄土高原S₀,L₁,S₁,S₄,S_5-1层位的风化成壤强度的空间变化进行研究,并与磁化率反映的特征进行对比。结果表明,冰期(黄土层)和间冰期(古土壤层)的风化成壤强度均呈现由东南向西北减弱的特征,与现代季风气候的基本格局一致。然而,冰期时(以L₁为代表)整个黄土高原南北气候梯度很弱,间冰期南北气候梯度远大于冰期,反映了冰期时夏季风环流对黄土高原影响很小,南北气候梯度更多体现了气候带的纬度效应。结果同时表明,间冰期时黄土高原地区风化成壤强度和南北气候梯度不具严格的对应关系。S₁和S_5-1指示了当时较强的夏季风环流,冬季风的影响相对亦较强,而S₀和S₄发育时期高原南北风化成壤强度的梯度明显减小,反映了当时黄土高原在夏季风影响的背景下,冬季风的影响亦减弱,导致南北梯度的减小。因此,黄土高原风化成壤强度的空间变化指示了冬、夏季风环流不同的消长关系,对研究高低纬度气候驱动力的相互作用具有重要意义。

SPATIAL VARIATIONS OF WEATHERING INTENSITY OF THE LOESS PLATEAU AND THE CLIMATE GRADIENTS WITHIN CHARACTERISTIC TIMESLICES

We examine spatial variations of chemical weathering intensity within five timeslices(S_(0),L_(1),S_(1),S_(4) and S_(5-1)) based on fifteen loess-soil sections along N-S,W-E and NW-SE transects,using the ratio of CBD extractable free Fe_2O_3(FeD) versus total iron(FeT).These data are compared with those reflected by magnetic susceptibility.The results show an overall increase from southeast to northwest of paleo-weathering intensity during interglacial periods(paleosol units) and glacial periods(loess units) but with rather weak gradients along the(W-E.) The reflected climate patterns are essentially similar to that under the present-day monsoonal climate regime,suggesting that the East-Asian summer monsoon was constantly the main moisture carrier over the past 600ka.During glacial periods,both weathering intensity and climate gradients between the northern and southern Loess Plateau are quite weak,indicating a negligible effect of summer monsoon during glacial periods.The observed weak N-S gradients mainly reflect the latitudinal effects of climatic zones.Our results also reveal two distinct combinations of climate gradient and weathering intensity for interglacial periods.The first is characterized by an overall strong weathering intensity associated with strong climate gradients as is evidenced by S_(1) and S_(5-1) soil units.Our data and the available grain-size data indicate that this case was resulted from the interactions between a strong summer monsoon circulation and a stronger-than-average interglacial winter monsoon.The second case is observed from the S_(0) and S_(4) soil units,characterized by an overall strong weathering intensity,but weak(S-N) gradients.Available data indicate that this case is resulted from the effects of a strong summer monsoon,but with a relatively weaker winter monsoon.The climate gradients for S_(1) and S_(5-1) were about twice lager than for the S_(0) and S_(4)soils.Thereby,the spatial variations of chemical weathering intensity can be explained as an indication of the interactions between the summer and winter monsoon circulations.