黄土高原东北缘矾山剖面的古地磁新结果

NEW PALEOMAGNETIC RESULTS FROM THE FANSHAN LOESS DEPOSITS IN THE BEIJING REGION

黄土高原不同地区发育的黄土-古土壤序列在东亚冬、夏季风记录上存在一定的差异性,而目前的研究多集中在黄土高原腹地。本文以黄土高原东北缘北京邻区矾山剖面S_6～L_9段地层为研究对象,开展了详细的古地磁、岩石磁学、粒度及地球化学测试分析。结果表明:1)与黄土高原腹地典型剖面同时段地层相比,矾山黄土成壤程度偏低,所受夏季风的影响较黄土高原内部剖面偏弱;2)在黄土高原腹地L_8～S_8地层中广为记录的布容-松山地磁极性倒转界线(M/B)及其高频极性转换特征在该剖面并未被清晰地记录,却在"上粉砂层"—L_9的顶部记录了一个明显的正、反磁极性转换,而相对古强度参数均指示M/B可能应位于L_8的下部;3)该剖面L_9段地层记录了3个较稳定的正极性段落,推测可能是布容期重磁化的结果,而非短期地磁极性漂移;4)L_9下部约0.8m的地层记录了一段天然剩磁异常高值带,可能是瞬间极强磁场(如雷电)磁化的结果。

Although a wealth of detailed paleomagnetic and paleoclimatic records has been successfully retrieved from the hinterland of the CLP,few high-resolution data are available from its marginal areas,where have experienced much stronger/weaker（contrasting） East Asian winter/summer monsoon strengths compared to the hinterland of the CLP. The loess-paleosol sequences in these marginal areas with contrasting（ significantly different/highly varying）degrees of environmentally controlled pedogenesis and sedimentation rates provide ideal sites for assessing fidelity of polarity reversals and discerning the exact reason for the discrepant stratigraphic occurrence of polarity boundaries cross the CLP. We have therefore carried out a high-resolution paleomagnetic and rock magnetic study coupled with grain size and geochemical analyses cross the interval of S6 - L9 of the Fanshan loess section in the Beijing region. The Fanshan loess profile （40°11 ＇39＂N, 115°23 ＇5 I＂E） , located in Fanshan town in the Yanqing-Huailai-Zhuolu Basin, northeast margin of the CLP,about 90km northwest of Beijing. Although the NE extremity of the CLP has been less influenced by the East Asia winter and summer monsoons, loess-paleosol sequences were well retained in this section. The thickness of the interval of S6 - L9 of the Fanshan section is - 19m. After removing weathered surface sediments,paleomagnetic blocks（8×8×10cm3） covering S6 -L9 were collected and were marked the North Pole direction at the top of blocks. We collected oriented blocks at 20cm intervals for S6-1. To get the whole Matuyama/Brunhes boundary （M/B）, 76 oriented blocks from the bottom of L6-1 to the bottom of Ss were continuously collected. From the bottom of Ss to the top of S9 ,89 oriented blocks were collected at 10cm intervals. In the laboratory, all blocks were cut into 2×2×2cm3 cubic specimens, and two sets of parallel specimens （ sets A and B）were retrieved. Set A specimens were used for thermal demagnetization, and set B were used for determinations of rock magnetic parameters, grain size and geochemical analyses. The thickness between S6 and L9 in Fanshan is generally comparable to that of Luochuan. However, magnetic susceptibility peak values for certain paleosol layers in Fanshan are evidently smaller than that in Luochuan, indicating that the NE CLP has experienced weaker summer monsoon intensities compared to the hinterland of the CLP. Surprisingly,we failed to constrain the exact position of the Matuyama/Brunhes boundary（M/B） either in Ls or Ss ,but an abrupt reverse-normal polarity reversal was recorded at the top of L9. Preliminary relative paleointensity analysis suggests a paleointensity minimum at the lower part of Ls,which may represent the ＇true＇ position of the M/B. L9 carries three stable normal polarity intervals,which we propose represent extensive remagnetization rather than short-term geomagnetic excursions. An exceptionally high NRM interval（O. 8m thick）at the lower part of L9 may reflect a transient strong magnetic field record （e. g., lightning）. In summary, missing M/B and/or significant downward displacement of the M/B together with extensive remagnetization of L9 at Fanshan strongly imply complicated remanence acquisition and retain process（es） of Chinese loess.