A record of reversed polarity carried by the iron sulphide greigite in British early Pleistocene sediments
A record of reversed polarity carried by the iron sulphide greigite in British early Pleistocene sediments
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DOI：
10.1016/0012-821x(94)90032-9
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发表时间：
1994
影响因子：
5.3
通讯作者：
D. Hallam;B. Maher
中科院分区：
文献类型：
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作者：
D. Hallam;B. Maher
古地磁测量样品提取的一个短序列的早更新世河口粘土，现在暴露在谢林汉姆附近的海崖北诺福克海岸，英国。根据早期的孢粉学工作，这些粘土被归于Pastonian（晚期Tiglian）时代。粘土的颜色有明显的变化，从单元顶部的红棕色到中间的蓝灰色，再到底部的灰棕色。古地磁数据的变化与这些颜色变化密切相关。顶部和底部的棕色粘土显示分散的正常方向的低强度，而中间的蓝色粘土显示强烈的集群反向方向，高得多的强度。从中蓝粘土和上红粘土之间的边界上取的一些样品在退磁时显示出正常的叠加在稳定的反极性上。在这些精矿中的硫化铁矿物的存在下，确定使用能量色散X射线分析在扫描电子显微镜。更具体地说，X射线衍射鉴定出硼镁石是磁性精矿中唯一可检测到的亚铁磁性矿物。岩石磁性测量表明，三个粘土亚基的磁性矿物有明显的质的差异，但弱磁性的上部和底部棕色粘土的磁性矿物的绝对识别尚未成为可能。我们解释的序列作为一个主要的反极性记录。这一记录是由硫化铁云英岩作为化学剩磁期间获得的“同”沉积还原铁通过分解这些缺氧潮汐粘土中的有机物质。随后，粘土的上部和底部亚单元被邻近粗粒沉积物的地下水渗透所氧化。因此，粘土氧化边缘的大多数硅镁石都被改变为一种新的、效率较低的磁记录材料，因此带有较晚的、分散的、低强度的、正常的叠印。
Palaeomagnetic measurements were made on samples extracted from a short sequence of early Pleistocene estuarine clays, now exposed in a sea cliff near Sheringham on the north Norfolk coast, UK. On the basis of earlier palynological work, these clays had been ascribed a Pastonian (late Tiglian) age. The clays show marked changes in colour, from reddish-brown at the top of the unit, to blue-grey in the middle, and grey-brown at the base. The palaeomagnetic data vary in close association with these colour changes. The top and basal brown clays show scattered normal directions of low intensity, while the middle blue clays show strongly clustered reversed directions, of much higher intensities. Some samples taken from the boundary between the middle blue and upper red clays show upon demagnetisation a normal overprint on a stable reversed polarity.Using high-gradient magnetic extraction, magnetic concentrates have been obtained from the strongly magnetic middle blue clays. The presence of iron sulphide minerals in these concentrates was identified using energy-dispersive X-ray analysis during scanning electron microscopy. More specifically, X-ray diffraction identifies greigite as the only detectable ferrimagnetic mineral in the magnetic concentrates. Rock magnetic measurements show clear qualitative differences in the magnetic mineralogies of the three clay subunits, but absolute identification of the magnetic mineralogy of the weakly magnetic upper and basal brown clays has not yet been possible.We interpret the sequence as a primary reversed polarity record. This record is carried by the iron sulphide greigite as a chemical remanence acquired during ‘syn’-depositional reduction of iron via the decomposition of organic material in these anoxic tidal clays. Subsequently, the upper and basal subunits of the clay have been oxidised by permeation of groundwater from the adjacent coarse-grained sediments. Most of the griegite in the oxidised margins of the clay has been altered as a result, to a new, less efficient magnetic recording material which thus carries a later, scattered, low-intensity, normal overprint.