GJI Geomagnetism, rock magnetism and palaeomagnetism A statistical simulation of magnetic particle alignment in sediments
GJI Geomagnetism, rock magnetism and palaeomagnetism A statistical simulation of magnetic particle alignment in sediments
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发表时间：
2014
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通讯作者：
D. Heslop;A. Roberts;R. Hawkins
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作者：
D. Heslop;A. Roberts;R. Hawkins
摘要沉积磁化是古地磁学的基础，但控制剩磁获取的机制仍然受到很大的限制。所观察到的沉积物天然剩磁磁化强度通常比同一沉积物的饱和剩磁磁化强度小几个数量级，这表明剩磁采集效率低。我们提出了一个统计模型，冯米塞斯-费舍尔分布的基础上，其中被认为是对周围的磁场磁性粒子的重新取向，而不代表的磁化采集过程的物理。研究结果为沉积磁化的性质提供了新的认识。具体地说，随机取向的磁性颗粒的集合体可以获得高保真的古地磁信号，其中颗粒朝向环境场方向的旋转仅为小的旋转（在某些情况下<1 μ s）。这表明，单独的磁性矿物颗粒取向的地磁扭矩的作用可能是轻微的，和一个弱的方向偏差的颗粒组合可能是负责通常观察到的沉积剩磁采集效率低下。此外，我们证明了弱磁场产生的沉积磁化具有较大的方向不确定性。然而，对于天然沉积物，这些不确定性似乎足够小，可以可靠地记录弱场期间的定向地磁场行为，如古地磁反转和偏移。
SUMMARY Sedimentary magnetizations are fundamental to palaeomagnetism, but the mechanisms that control remanence acquisition remain poorly constrained. Observed sedimentary natural remanent magnetizations are often orders of magnitude smaller than the saturation remanent magnetization of the same sediment, which indicates inefficient remanence acquisition. We present a statistical model, based on the von Mises–Fisher distribution, in which magnetic particle reorientations towards an ambient field are considered, without representing the physics of the magnetization acquisition process. The results provide insights into the nature of sedimentary magnetizations. Specifically, an assemblage of randomly oriented magnetic particles can acquire a high-fidelity palaeomagnetic signal with only small rotations (in some cases <1 ◦ ) of particles towards the ambient field direction. This demonstrates that the action of a geomagnetic torque on individual magnetic mineral particle orientation may be minor,and that a weak directional bias on an assemblage of particles could be responsible for the typically observed inefficiency of sedimentary remanence acquisition. Additionally, we demonstrate that weak fields produce sedimentary magnetizations with larger directional uncertainties. For natural sediments, however, these uncertainties appear to be small enough to allow reliable recording of directional geomagnetic field behaviour during periods with weak fields, such as palaeomagnetic reversals and excursions.