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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.

最近地球磁场的一个广泛的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区域的磁屏蔽效率，影响了大气臭氧。许多人认为，南非大爆发与过去160年里偶极子地磁场强度的急剧衰减以及南非地下地核上一个反向磁场区域的增长有关。一些人甚至推测地球正在走向地磁场反转。但是，在长期地磁历史的背景下理解这些现象，由于缺乏南半球的考古地磁数据而受到限制（即来自考古物体的数据，这些物体被烧到高温，随后在冷却时保存了地球磁场的记录）。我们的研究和教育计划旨在了解南非南部地区地磁场的历史。这项工作的主要重点是收集和分析铁器时代燃烧结构的考古磁数据，并测试模型，以解决地核和地幔之间边界的性质如何引起最近地磁场的变化。来自南部非洲铁器时代遗址（约公元1000-1650年）的考古磁数据显示，在公元1300年，强度急剧下降，其速度与当今南大西洋异常（SAA）的现代磁场变化相当，但数值较低。低场值的反复出现可能反映了由地震学定义的南部非洲（特别是非洲大低速地震省，或LLVSP）下不寻常的核幔边界组成和结构促进的磁通量从核心排出。因为非洲LLVSP是一个长期存在的结构，这个地区可能是一个稳定的通量驱逐地点，也许是地磁反转的触发地点，在数百万年的时间尺度上。如果正确的话，这个概念模型是革命性的，因为它表明反转不是在随机位置开始的，而是在核心-地幔边界位置形成核，从而促进通量向上泄漏。该模型预测了地磁磁场在地核涡流寿命的时间尺度上的调制，并将通过扩展非洲南部铁器时代的考古地磁记录进行验证。该研究项目与本科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。本项目由国际科学工程办公室GVF基金共同资助。