In situ remagnetization experiments of loess on the Chinese Loess Plateau: Evidence for localized post‐depositional remanent magnetization
In situ remagnetization experiments of loess on the Chinese Loess Plateau: Evidence for localized post‐depositional remanent magnetization
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DOI：
10.1029/2011gc003830
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发表时间：
2011-12
期刊：
影响因子：
3.7
通讯作者：
R. Løvlie;Ronghua Wang;Xishen Wang
中科院分区：
文献类型：
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作者：
R. Løvlie;Ronghua Wang;Xishen Wang
中国黄土高原(CLP)黄土/古土壤记录到的与极性反转有关的反转界面的不一致地层位置和虚假的古地磁变化，通常分别归因于推断的磁化延迟获取和地磁场的高频变化。这些结论部分是基于对黄土磁化获取过程的不完全认识。最近的实验室实验表明，干沉积黄土在第一次润湿(降水)后获得了稳定的剩余磁化强度，导致了显著的倾角误差，这在中国黄土中是不常见的。实验室实验只是远程模拟自然条件，因此，我们在中电两个相距约650公里的地点进行了现场实验，以阐明自然条件的变化是否会对黄土(L1)施加二次剩余磁化分量。实验是通过重新定位末次间冰期黄土块(体积∼5·10−3m~3)进行的，这些块在黄土裸露中被颠倒。分别于术后8个月和17个月取材进行分析。热退磁作用表明，黄土块的不同部位发生了部分再磁化。这种二次磁化的特征是阻塞温度与主成分的温度重叠，达到450°C以上，不能归因于粘性剩余磁化，因此被提议代表磁性颗粒的局部和部分重新排列。我们试探性地提出，黄土中不同矿物颗粒的差热膨胀所引起的压力释放可能有助于磁性颗粒的物理“解锁”和重新排列。局部化的再磁化作用可能解释了许多报道的来自CLP的虚假的古地磁结果。
Inconsistent stratigraphic positions of reversal boundaries and spurious paleomagnetic variations associated with polarity reversals recorded by loess/paleosols on the Chinese Loess Plateau (CLP) have in general been attributed to inferred delayed acquisition of magnetization and high‐frequency variations of the geomagnetic field, respectively. These conclusions are partly based on an incomplete knowledge of the magnetization acquisition process of loess. Recent laboratory experiments, which demonstrate that dry‐deposited loess acquires a stable remanent magnetization after its first wetting (precipitation), resulted in significant inclination errors that are not generally encountered in Chinese loess. Laboratory experiments only remotely mimic natural conditions, and we have therefore performed in situ experiments at two sites on the CLP separated by some 650 km to elucidate whether variations in natural conditions can impose secondary remanent magnetization components in loess (L1). The experiments were performed by relocating blocks (volume ∼5·10−3 m3) of last interglacial loess after being turned upside down inside loess exposures. Inverted blocks were retrieved and analyzed after 8 and 17 months respectively. Thermal demagnetization reveals that partial remagnetization has occurred within different parts of the loess blocks. This secondary magnetization, which is characterized by blocking temperatures that overlap with those of the primary component to above 450°C, cannot be attributed to a viscous remanent magnetization and is therefore proposed to represent local and partial realignment of magnetic grains. We tentatively propose that physical ‘unlocking’ and realignment of magnetic grains may be assisted by pressure release caused by differential thermal expansion of the different mineral grains in loess. Localized remagnetization may account for a number of reported spurious paleomagnetic results from CLP.