黄土高原黄土序列松山-布容地磁极性倒转界线空间分布特征与影响因素探讨

SPATIAL CHARACTERISTICS AND INFLUENCING FACTORS OF MATUYAMA- BRUNHES POLARITY REVERSAL BOUNDARY （MBB） IN EOLIAN SEQUENCES FROM THE CHINESE LOESS PLATEAU

松山-布容（M/B）地磁极性倒转是第四纪发生的最近一次地磁极性倒转事件，其确切位置是黄土-古土壤序列磁性地层学的关键。然而，松山-布容地磁极性倒转界线（MBB）在黄土中记录的位置在不同区域存在一定差异，制约了高分辨率黄土磁性地层学和黄土古气候学的区域格局分析以及海陆气候对比研究。本文对黄土高原不同黄土序列MBB的记录位置进行总结，分析了MBB在黄土高原的空间分布特征，并对MBB的记录位置及厚度与L8黄土厚度的相关关系及其影响因素进行了探讨。通过对比黄土高原不同黄土-古土壤序列研究结果发现，MBB的记录位置差异较大，黄土高原西北部多位于L8黄土顶部甚至S7古土壤，而东南部则位于L8黄土底部或L8/S8过渡带附近。代表性剖面中MBB的空间分布特征显示，MBB的记录位置与L8黄土受成壤作用影响强度以及沉积速率之间关系密切。黄土高原西北部沉积速率高，L8黄土受成壤作用影响较弱，MBB的记录位置基本不受成壤作用影响；黄土高原中部和东南部沉积速率相对较低，L8黄土受成壤作用影响显著，并呈现一定的穿时性特征。MBB记录位置对成壤作用影响较为敏感，L8厚度的微小变化都可能会引起MBB记录位置的显著移动。因此在黄土高原不同区域MBB的记录位置受成壤作用强度的影响，并影响到黄土与古土壤的地层划分而使MBB的记录位置产生区域差异；除此之外还受Lock-in效应、地磁场信息记录等在不同区域差异性响应，使MBB的位置呈现多因素影响，相互叠加放大造成了显著的区域差异。

Matuyama-Brunhes（M/B） polarity reversal is the last geomagnetic polarity reversal event happened in the Quaternary.Its exact position is the key issue to the magnetic stratigraphy research of loess-paleosol sequence.However, the position of Matuyama-Brunhes polarity reversal boundary（MBB） recorded in the loess sediments is apparent discrepant in different regions, which has seriously hampered the understanding of high-resolution magnetic loess stratigraphy and paleoclimatology of loess-paleosol sequence.In this paper, the reported recording positions of MBB in different loess sequence from the Chinese Loess Plateau（CLP） were summarized.We also analyzed the spatial distribution of MBB over the CLP, and discussed relative relationship between L8 loess and MBB.The recording positions of MBB are quite different in various studies.In northwest of the CLP, the recorded position is located near the top of the L8 loess or even the bottom of S7 paleosol, while it is located near the bottom of the L8 or L8/S8 transition zone in the southeastern part of the CLP.Spatial distribution characteristics of representative cross-sections of the MBB showed that the recording position of MBB is closely related to the pedogenic intensity and sedimentary rate of L8 loess.Sequences from northwest of the CLP present a high sedimentary rate, L8 loess is less affected by pedogenesis.Comparatively, sequences from the central and southeastern CLP have relatively low sedimentary rate, and L8 loess is significantly affected by pedogenesis, which showed some diachronism characteristics.MBB recording position is sensitive to pedogenesis.Significant shift of MBB recording position could be induced by small changes in the thickness of L8 loess.We therefore believed that the MBB recording positions in different areas of the CLP were influence by the intensity of pedogenesis which furtherly influence the stratigraphic division of loess-paleosol sequence, and leading to the regional differences of MBB recording position.In addition, overlapping of Lock-in effect and magnetic field information recorded during the polarity transitional period amplified the regional difference of the recording position of MBB.