摘要:
位于黄土高原与华北平原过渡带上的中原黄土地层,以邙山赵下峪剖面为其典型代表。据光释光和热释光测年及磁性地层研究结果,该剖面从邙山塬面至黄河河床出露S0-S10黄土-古土壤序列,总厚度172.1m,B/M界线记录于S8古土壤层顶部,以厚层晚更新世S1古土壤(15.7m)和巨厚L1黄土(77.3m)为其特色。赵下峪剖面上末次间冰期以来的平均沉积速率明显增大,其中以末次冰期晚冰阶L1LL1黄土的沉积速率最大,高达34.5mm/a。在邙山黄土堆积过程中,倒数第二冰期末(相当于L2顶部),约150ka B P,发生了风尘沉积速率的突变,其原因是此时黄河贯通三门峡东流,给风尘源区带来丰富的物质。同时赵下峪剖面的磁化率曲线所示,黄土-古土壤的磁化率强弱,并不简单地反映夏季风强度,也要受到沉积速率变化的影响。
关键词:
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中原
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邙山黄土
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晚更新世
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磁化率
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沉积速率
Abstract:
The Zhaoxiayu loess section in Mangshan Yuan which is located at the transitional zone between the North China Plains and the Loess Plateau is typical of the loess stratigraphy in China Central Plains.The section is exposed from the top of Mangshan Yuan to the valley bottom of the Huanghe River,the loess-paleosol sequence from S0 to S10,with a total thickness of 172.1 m.It is known from the results of optically stimulated luminescene dating and thermoluminescene dating and the magnetostratigraphy that,the B/M boundary was recorded at the top of S8.The sequence is characterized by a relatively thick paleoso S1 (15.7m) and a thick loess L1 (77.3m) of the Late Pleistocene.Mangshan loess is a kind of near-source sandy dust deposit with the source area lying just at the head of an alluvial fan at the lower reaches of Huanghe River.There is great difference in the average rate of sedimentation of loess in different glaciations or stades and of paleosol in different interglaciations or interstades in Zhaoxiayu loess profile.The sedimentation rate of loess L1LL1,paleosol L1SS 1,and loess L1LL2 in the Last Glaciation is as high as 3.45 mm/a,0.38 mm/a,1.51mm/a,respectively,and that of paleosol S1 of the Last Interglaciation is 0.28mm/a,a rate not only higher than that of paleosol S2(0.05 mm/a),but also higher than that of loess L2 (0.18 mm/a).It has been shown that an abrupt change of the dust sedimentation rate happened in the end of penultimate glaciation,about 150 ka B P (equivalent to the top of L2).The reason for this may be that the Huanghe River might have cut through the Sanmenxia Gorge to flow eastwards transporting abundant materials to the source area since that time.The magnetic susceptibility of loess and paleosol is related to the strength of pedogenisis,and can be served as a proxy index for variations of summer monsoon.Magnetic susceptibility values of loess in Zhaoxiayu section,30×10-5 SI or so,approximates to the background values of dust deposit that has undergone no little or no pedogenisis.And that of paleosol,70-170×10-5 SI,and that of immature paleosol,50-60×10-5 SI,can not be considered as a simple reflection of the strength of summer monsoon in interglaciation or interstades.They should also be affected by sedimentation rate.
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