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Archeomagnetism of Southern Africa: Implications for Longevity of the South Atlantic Anomaly

南部非洲的考古地磁学：对南大西洋异常长期存在的影响

基本信息

批准号：
1448227
负责人：
John Tarduno
金额：
$ 31万
依托单位：
University of Rochester
依托单位国家：
美国
项目类别：
Standard Grant
财政年份：
2015
资助国家：
美国
起止时间：
2015-03-01 至 2019-02-28
项目状态：
已结题

来源：
https://www.nsf.gov/awardsearch/showAward?AWD_ID=1448227&HistoricalAwards=false
关键词：
Archeomagnetism Southern Africa Implications Longevity

项目摘要

A broad low intensity area in Earth's recent magnetic field spans the southern Atlantic Ocean, Africa and South America. This is commonly called the South Atlantic Anomaly (SAA). The SAA allows a relatively close approach of Earth's radiation belts, affecting spacecraft operations. The low magnetic intensity decreases the efficiency of magnetic shielding in the region, which can influence atmospheric ozone. Many believe the SAA is linked to the dramatic decay of the dipole geomagnetic field intensity during the last 160 years, and the growth of an area of reversed magnetic field on Earth's core beneath South Africa. Some have even speculated Earth is heading toward a geomagnetic field reversal. But understanding these phenomena within the context of longer-term geomagnetic history has been limited by a lack of Southern Hemisphere archeomagnetic data (that is, data from archeological objects that were fired to high temperature and subsequently preserved a record of Earth's magnetic field as they cooled). Our research and education program is aimed at understanding the history of the geomagnetic field in the SAA region as recorded in southern Africa. The main focus of the work is the collection and subsequent analysis of archeomagnetic data from Iron Age burnt structures, and tests of models addressing how the nature of the boundary between Earth's core and mantle may be giving rise to recent changes seen in the geomagnetic field.Archeomagnetic data from Iron Age sites of southern Africa (ca. 1000-1650 AD) show a sharp intensity drop at 1300 AD, at a rate comparable to modern field changes in the present-day South Atlantic Anomaly (SAA), but to lower values. The recurrence of low field values may reflect magnetic flux expulsion from the core promoted by the unusual core-mantle boundary composition and structure beneath southern Africa defined by seismology (specifically the African Large Low Velocity Seismic Province, or LLVSP). Because the African LLVSP is a longstanding structure, this region might be a steady site of flux expulsion, and perhaps the triggering site for geomagnetic reversals, on time scales of millions of years. If correct, this conceptual model is transformative because it suggests reversals do not initiate at random locations, but instead nucleate at core-mantle boundary sites that promote flux to leak upward. The model predicts modulation of the field on time scales of the lifetime of an eddy in the core flow, and will be tested by extending the archeomagnetic record of Iron Age southern Africa. The research program is integrated with undergraduate and graduate education and will be conducted with geologists and archeologists from South Africa, Botswana, and Zimbabwe. This project was co-funded by the GVF fund in the Office of International Science Engineering.

地球近期磁场的一个大范围低强度区域横跨南大西洋、非洲和南美洲。这通常被称为南大西洋异常(SAA)。SAA允许相对近距离地接近地球辐射带，影响航天器的运行。低磁场强度降低了该地区的磁屏蔽效率，这可能会影响大气臭氧。许多人认为，SAA与过去160年来偶极子地磁强度的戏剧性衰减以及南非地核反转磁场区域的增长有关。一些人甚至猜测地球正朝着地磁场逆转的方向前进。但是，在较长期的地磁历史背景下理解这些现象受到了南半球考古地磁数据的缺乏的限制(即，来自考古物体的数据，这些物体被烧到高温，随后保存了地球冷却时的磁场记录)。我们的研究和教育计划旨在了解南部非洲记录的SAA地区的地磁场历史。这项工作的主要重点是收集和随后分析来自铁时代烧毁结构的古地磁数据，并测试解决地球核心和地幔边界的性质如何引起最近在地磁场中看到的变化的模型。来自南部非洲铁时代遗址(约公元1000-1650年)的古地磁数据显示，公元1300年强度急剧下降，速度与今天南大西洋异常(SAA)的现代磁场变化相当，但数值更低。低场值的重现可能反映了地震学定义的南部非洲(特别是非洲低速大地震省)之下不同寻常的核-地幔边界成分和结构促进了磁通量从核心排出。由于非洲LLVSP是一个长期存在的结构，该地区可能是一个稳定的通量喷发地点，也可能是地磁反转的触发地点，时间尺度为数百万年。如果正确，这个概念模型是变革性的，因为它表明反转不是从随机位置开始的，而是在促进通量向上泄漏的核-地幔边界处成核。该模型预测了磁场在核心流动中涡旋寿命的时间尺度上的调制，并将通过延长南部非洲铁器时代的考古地磁记录来进行测试。该研究计划与本科生和研究生教育相结合，将与来自南非、博茨瓦纳和津巴布韦的地质学家和考古学家一起进行。该项目是由国际科学工程办公室的全球科学基金会共同资助的。