SUMMARY

The time-averaged geomagnetic field is generally purported to be uniformitarian across Earth history—close to a geocentric axial dipole, with average strength within one order of magnitude of that at present. Nevertheless, recent studies have reported that the field was approximately ten times weaker than present in the mid-Palaeozoic (∼410–360 Ma) and late Ediacaran (∼565 Ma). Here we present the first whole-rock palaeointensity determinations of Ediacaran age outside of Laurentia. These were obtained by the Thellier-Coe, Wilson and microwave methods for basaltic rocks of 560–580 Ma age of the Ediacaran traps, southwestern margin of the East European Craton, Ukraine. All four studied sites showed extremely low instantaneous field values of (3–7) μT with corresponding VDMs of (0.4–1) × 10²² Am². Summarizing all available data, the Ediacaran field appears to be anomalously characterized by ultra-low dipole moment and ultra-high reversal frequency. According to some geodynamo models, this state could indicate a weak dipole field regime prior to the nucleation of the solid inner core. However, given that ultra-low field intensities have also been detected in the Devonian, and that virtually no palaeointensity data exist for the intervening ∼150 Ma, the date of inner core nucleation remains extremely uncertain. Our new evidence of persistent ultra-weak magnetospheric shielding in the Ediacaran may be considered consistent with the recently hypothesized link between enhanced UV-B radiation in this interval and the subsequent Cambrian evolutionary radiation.

中文翻译：

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乌克兰东欧克拉顿的超低古强度支持了Ediacaran地区的全球古磁场异常

概要

一般认为，时间平均地磁场在整个地球历史上都是均匀的，接近地心轴向偶极子，平均强度在目前水平的一个数量级内。然而，最近的研究报告说，该油田的强度大约是古生代中期（〜410-360 Ma）和埃迪卡拉级晚期（〜565 Ma）的强度的十倍。在这里，我们介绍了Laurentia以外的Ediacaran年龄的第一个全岩石古强度测定。这些是通过Thellier-Coe，Wilson和微波方法获得的，位于乌克兰东部克拉通西南缘的Ediacaran圈闭，年龄为560-580 Ma，是玄武岩。所有四个研究站点均显示出（3–7）μT的极低瞬时场值，相应的VDM为（0.4–1）×10 ²² Am ²。总结所有可用数据，Ediacaran场似乎具有超低偶极矩和超高反转频率的异常特征。根据一些地球动力学模型，这种状态可能表明在固体内核成核之前，偶极子场状态很弱。但是，考虑到泥盆纪也已经检测到超低场强，并且实际上在〜150 Ma的中间没有古强度数据存在，内核成核的日期仍然非常不确定。我们在埃迪卡拉族中持续存在的超弱磁层屏蔽的新证据可能被认为与最近推测的在此间隔内增强的UV-B辐射与随后的寒武纪演化辐射之间的联系相一致。