【作者】 陈蕴；

【导师】 王喜生；

【作者基本信息】 中国地质科学院 ， 固体地球物理学， 2012， 硕士

【摘要】 黄土高原不同地区记录东亚冬夏季风存在一定的差异性,而目前的研究多集中在黄土高原腹地。黄土高原东北缘边缘区虽受东亚季风影响较小,但黄土-古土壤序列发育层次清晰,质地均一且地层中的钙质结核并不像黄土高原腹地广泛发育,是探究黄土古地磁可靠性的理想材料。为此,本论文选择了北京邻区矾山黄土剖面S6-S9段地层进行了高分辨率古地磁和岩石磁学研究,试图对布容/松山极性转换(MBB)的产出层位以及“上粉砂层”—L9等关键层位记录原生剩磁的可靠性进行了分析。此外,本论文还开展了较系统的环境磁学、粒度及代表性样品的地球化学指标分析,来试图揭示早、中更新世过渡期的磁气候记录及其区域古环境意义。矾山剖面(40°11’39"N,115°23’51"E)的主要载磁矿物为磁赤铁矿和磁铁矿,并含少量赤铁矿。磁化率各向异性分析结果显示,矾山黄土呈以磁面理占主导的典型沉积磁组构,表明黄土沉积后未受明显的后期改造或扰动。S6-L9段地层总厚度与洛川经典剖面相当,磁化率值比洛川经典剖面相应层位略低,这暗示了矾山剖面相对于黄土高原腹地受东亚夏季风影响和成土作用要弱。χ%、Jv、Jv%、Hc和Hcr等磁参数与χ和中值粒径对比表明,在古土壤形成时强烈的成土作用生成了大量细粒亚铁磁性矿物,导致磁信号增强。这些环境磁学参数在S6-1和L9中达到极大和/或极小值,说明在早中更新世过渡期,矾山一带在S6-1时气候最为适宜,而L9则记录了一个气候极端干冷期。粒度分析结果进一步表明,在中更新气候转型初期的L9堆积期气候最为干冷,东亚冬季风最强,物源区与沉积区的距离最近,沙漠扩张最大。<2μm细颗粒百分含量和地球化学分析结果也清晰表明,该剖面受东亚夏季风和成土作用较黄土高原腹地要偏弱。主微量元素和稀土元素分析结果显示,该剖面和黄土高原腹地典型剖面(如西峰和灵台)的物源相似,均是来自于中国北部、西北部的沙漠和沙漠边缘区。MBB在该剖面L8～S8地层中并未被清晰地记录,却在L9的顶部记录了一个明显的正、反极性转换。相对古强度参数分析可观察到L8下部有一个相对古强度明显降低带,而这一特征极有可能暗示了M/B应该出现的位置；与近年来对黄土高原其他剖面L9的古地磁研究结果相似,矾山剖面L9也经历了广泛而复杂的重磁化,记录了3个正极性带和2个反极性带,而相对古强度参数并未随L9记录的极性变化表现出明显的趋势性变化；L9下部存在一个约0.8m厚的NRM异常高值带,由于该带在岩石磁学性质和地球化学特征上均未表现出异常,初步推测可能是瞬间极强磁场(如雷电)磁化的结果。矾山剖面缺失的MBB和L9复杂的热退磁结果,在某种程度上反映了中国黄土记录原生剩磁的复杂性。更多还原

【Abstract】 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.更多还原