中国黄土-古土壤剩磁获取机制的实验研究

The study of magnetization mechanisms in loess-paleosol is a key aspect in the paleomagnetic studies of Chinese loess-paleosol. It is also an important pathway to solve current debates about the reliability and accuracy of loess magnetization. Moreover, the lock-in depth of loess magnetization is directly related to the correlations between the oceanic and terrestrial paleoclimatic records during the Quaternary. In this project, loess-paleosol of different ages from three classic sections (Lanzhou, Xifeng and Luochuan) will be collected, and through redeposition and remagnetization experiments magnetization mechanisms in loess-paleosol will be studied. In the redeposition experiment, loose loess particles are deposited in Helmholtz-coils under a varying magnetic field simulating geomagnetic field reversal and excursion. Remanent magnetization of redeposited loess obtained in different polarities will be studied and analyzed in details, aiming to understand how loess magnetization records respond to the magnetic field changes, and to determine the lock-in depth of pDRM. Remagnetization experiment will combine two ways adopted in the preliminary studies, turning- burring loess blocks in-situ for a certain time or heating-cooling (from -4℃to 25℃) loess blocks in a reversed field generated by Helmholtz-coils. Loess L1 was found to be partially remagnetized after buried in-situ under a simulated reverse field for a short time about 1 year. By continuing this remagnetization experiment on loess-paleosol with different ages, we are going to investigate whether this partial remagnetization will continue with increasing experimenting time, whether it is common the Chinese Loess Plateau and mechanism of a pDRM obtained in air it suggested. Experiments will also be tried to find out other possible remagnetization mechanisms in loess-paleosol. By studying remanent magnetization with known-direction components obtained during redeposition and remagnetization experiments, how secondary components overprint primary components will be revealed.

黄土－古土壤剩磁获取机制研究是黄土古地磁学研究的关键问题之一，也是解决当前关于黄土剩磁可靠性与准确度争议的重要途径。黄土剩磁固锁的深度更是直接关系到第四纪海洋陆地古气候变化记录的对比。项目拟采取兰州、西峰、洛川三个经典剖面中不同年龄的黄土-古土壤样品，通过重沉积和重磁化实验来研究其剩磁获取机制。重沉积实验中利用Helmholtz线圈模拟极性倒转和漂移，研究重沉积黄土剩磁记录对磁场变化的响应以及剩磁固锁的深度。重磁化实验结合野外翻转实验和在Helmholtz线圈内的加热冷却(零下4℃-25℃)循环实验，分析实验中样品获取的次生剩磁特征，调查前期研究中发现的马兰黄土在模拟地球倒转条件下发生的部分不完全重磁化现象是否普遍存在，及其所指示的黄土气上pDRM产生的机制，并探索其他可能的重磁化途径。通过退磁分析重沉积和重磁化实验中样品所获取的已知不同方向剩磁分量，同时研究次生剩磁与原生剩磁的叠加规律。

项目对兰州九州台剖面L8-L12、西峰桃花咀剖面L1、洛川坡头村剖面S1，以及渭南阎村剖面L1的黄土-古土壤样品详细的古地磁学研究，并进行了野外黄土块样品倒转实验和室内重沉积实验。主要研究内容包括黄土-古土壤重磁化的普遍性及其机制以及次生剩磁对黄土古土壤序列记录地球磁场倒转或漂移的影响。研究表明兰州九州台黄土古土壤序列（L8-L12）受次生剩磁（pDRM，CRM，VRM）影响较小，其剩磁记录表明在MBB附近及其以下Jaramillo以下存在多次极性倒转，可能反映了该时期地球磁场的真实特征。渭南阎村黄土剖面L1详细的古地磁学研究发现中Laschamp极性漂移事件并没有前人报道那么显著，其中磁性矿物颗粒表面特征显示有大量次生磁性矿物的产生，重沉积实验结果表明这种在原生碎屑磁铁矿物之上次生磁性矿物颗粒生长形成的核壳结构可能携带不同方向的剩磁分量。化学剩磁的对古土壤以及西南区域黄土都存在较大的影响需要进一步的研究。渭南阎村黄土剖面L1黄土块野外翻转模拟地磁场倒转实验表明：在干燥条件下，对比原状样品，经过倒转样品剩磁记录未出现明显次生剩磁（VRM，pDRM)。其他条件（不同年代，不同含水量）样品还待测试和进一步的综合讨论。项目目前研究结果对今后研究有两点重要的启示：1）黄土古土壤特别是位于温暖潮湿区域的黄土和故土壤，次生磁性矿物颗粒所携带的化学剩磁对黄土剩磁记录的影响十分巨大，需要进一步对其进行评价和研究；2）寒冷干旱区域的黄土古土壤受次生剩磁影响较小，能更可靠地记录地磁场变化；3）地球磁场本身十分不稳定，其极性倒转转换期内磁场的频繁倒转和极性漂移事件可能远远超过了目前沉积物中获得的记录。