黄土高原西北部黄土岩心记录的松山-布容极性转换过程及其剩磁获得机制

Matsuyama-Brunhes (M/B) polarity transition is the latest geomagnetic polarity reversal event; its boundary (MBB) position is the most important age reference point and hot problem of the magnetic stratigraphy research on eolian sequences. With great thickness and high deposition rate, the eolian loess-paleosol sequences from the northwest part of Chinese Loess Plateau provide high-resolution information record of the geomagnetic field variation and paleoclimate changes; and its relatively weak pedogenesis largely reduces the transformation of magnetic minerals and "smoothing" of the magnetic field information underwent during the deposition process, hence could be regard as the ideal material for high-resolution magnetostratigraphic study. Our project proposes to select the drilled loess cores obtained from Jingyuan and Gulang sites which located at the northwest part of Chinese Loess Plateau, applying with high-resolution sampling, multiple magnetic parameters measurement, so to obtain the detail magnetostratigraphic frame result of loess-paleosol deposition span over loess-paleosol units S7-L9; identify the detailed rock magnetic characteristics and its remanent acquisition mechanism during the M/B polarity transition of loess-paleosol sequence; and analyze the features of geomagnetic polarity change and polarity transition process in details. By comparing with loess drill cores from Luochuan, Xifeng and Weinan sites located at the middle of Chinese Loess Plateau, we aim to summarize the regional differences of remanent acquisition mechanism in the loess-paleosol units of the M/B polarity transition zone. Furthermore, based on the predecessor research results, the other objective of this project is to systematically understand the similarities and differences of the M/B polarity transition process in different regions of the loess-paleosol sequences, so to contribute theoretical base and practical experience for further accurately confine on the position of Matsuyama-Brunhes boundary (MBB).

松山-布容(M/B)极性转换是最近一次地磁极性倒转事件，其边界位置是磁性地层学研究中最为重要的年龄参考点，也是黄土磁性地层学研究的热点和难点。黄土高原西北部风尘堆积厚度大，沉积速率高，能高分辨率记录地磁场及古气候变化信息；同时区内黄土堆积成壤程度较低，有效降低了沉积过程对原生磁性矿物的改造和地磁场信息的"平滑效应"，是高分辨率磁性地层研究的理想材料。本项目拟以靖远和古浪获得的黄土钻孔为研究对象，通过大密度古地磁采样和系统岩石磁学测试，获得S7-L9高分辨率磁性地层结果，查明M/B极性转换带内黄土堆积详细的岩石磁学特征及其剩磁获得机制，分析其地磁极性变化特征和详细过程；并通过与洛川、西峰和渭南黄土钻孔综合对比，揭示区域差异和气候变化对剩磁获得机制和M/B极性转换的影响；进一步总结前人研究成果，系统理解M/B极性转换过程在不同区域沉积记录中的异同点，为准确厘定其边界位置提供理论依据和实践经验。

松山-布容(M/B)极性转换是最近一次地磁极性倒转事件，其边界位置是磁性地层学研究中最为重要的年龄参考点，也是黄土磁性地层学研究的热点和难点。黄土高原西北部风尘堆积厚度大，沉积速率高，能高分辨率记录地磁场及古气候变化信息；同时区内黄土堆积成壤程度较低，有效降低了沉积过程对原生磁性矿物的改造和地磁场信息的“平滑效应”，是高分辨率磁性地层研究的理想材料。本项目通过古浪和靖远获得的黄土钻孔研究，获得以下认识：. 1）系统获得了松山-布容极性转换带内原生剩磁载磁矿物的种类和磁畴特征，基本探明剩磁获得机制及其对地磁场在沉积记录中的影响，探讨了以降雨量变化梯度带为特征的磁性矿物转化与剩磁获得机制；. 2）通过典型黄土-古土壤序列S7-L9 高分辨率磁性地层学研究结果分析，基本确定了黄土高原典型黄土序列松山-布容极性转换带的边界位置的区域变化与记录特征，系统研究了极性转换带的详细过程；. 3）综合黄土高原中部典型黄土序列古地磁结果，通过粒度变化与轨道调谐年代序列，发现BM界限在黄土高原呈现出相似的特征，均出现在L8/S8的界限附近，发生时间大致介于808-826ka，持续时间约14-16ka，并均呈现多次类似极性倒转事件的特征；. 4）结合获取的有效数据，综合整理黄土高原BM界限已发表的研究记录，对黄土高原不同剖面磁性记录的MBB结果以及在L8层位的深度进行归纳，分析MBB的记录位置在黄土高原的空间分布特征，并且对MBB距L8层底部的相对距离与L8层厚度的关系进行探讨。