| Although a wealth of detailed paleomagnetic and paleoclimatic records have 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-L9of the Fanshan loess section in the Beijing region. The Fanshan loess profile (40°’39"N,115°23’51"E) is located in Fanshan town in Yanqing-Huailai-Zhuolu basin, northeastern margin of the CLP, about90km 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 characteristic remanent magnetizations in Fanshan profile were mainly carried by magnetite and maghemite, and the grain sizes of magnetic minerals are in the PSD range. The data of anisotropy of magnetic susceptibility indicate a primary sediment fabric without evident disturbance after dust deposition. The thickness between S6and L9in 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. The paleosols display higherχ%, Jv, Jv%but lower Hc and Hcr, indicative of a significant increase in the concentration of ferrimagnetic minerals. This is consistent with previous studies from the hinterland of the CLP that the great precipitation of fine-grained ferrimagnetic minerals were formed during pedogenesis, resulting in magnetic enhancement of paleosol. Grain size data indicate that the East Asian winter monsoon was strongest during the accumulation of L9and the the distance between the source area and deposition area was shortest in the Early-Middle Pleistocene transition period. The<2μm fraction and geochemical analyses suggest that the influence of East Asian summer monsoon and pedogenesis in Fanshan profile is weaker than in the hinterland of Loess Plateau. Major and trace elements and REE element analysis indicate that the provenance of this profile is consistent with the typical profile from the hinterland of Loess Plateau (such as Xifeng and Lingtai), which provenance are the major deserts in northern and northwest China.Surprisingly, we failed to constrain the exact position of the Matuyama/Brunhes boundary (M/B) either in L8or S8, 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 L8, which may represent the ’true’ position of the M/B. L9carries three stable normal polarity intervals, which we propose represent extensive remagnetization rather than short-term geomagnetic excursions. An exceptionally high NRM interval (0.8m thick) at the lower part of L9may 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 L9at Fanshan strongly imply complicated remanence acquisition and retain process (es) of Chinese loess. |
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