Low Paleointensities and Ar/Ar Ages From Saint Helena Provide Evidence for Recurring Magnetic Field Weaknesses in the South Atlantic
Low Paleointensities and Ar/Ar Ages From Saint Helena Provide Evidence for Recurring Magnetic Field Weaknesses in the South Atlantic
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DOI：
10.1029/2021jb023358
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发表时间：
2022-02
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影响因子：
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通讯作者：
Y. A. Engbers;J. M. Grappone;D. Mark;A. Biggin
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作者：
Y. A. Engbers;J. M. Grappone;D. Mark;A. Biggin
南大西洋异常区（SAA）是一个地磁弱区，代表着当今磁场中最显着的异常区。尽管存在诸如此类的异常现象，但一个长期存在的假设是，如果在足够长的时间（104-106 年）内平均，地球磁场近似于地心轴偶极子 (GAD)。非 GAD 特征在时间平均场中的显着性如何是一个重要且尚未解决的问题。 SAA 在历史模型和古野外模型中并不总是可见。然而，据报道，南大西洋地区存在数百万年时间尺度的不稳定磁场。这项研究首次对圣赫勒拿岛进行了古强度研究，圣赫勒拿岛是南大西洋的一个火山岛，主要由 10 到 8 Ma 之间形成的熔岩组成。虽然古强度成功率较低，但我们能够从五个独立熔岩中恢复结果，这些结果共同表明 10.5 ± 3.0 μT 的低场强度，对应于 2.4 ± 0.7 × 1022 A m2 的虚拟轴向偶极矩 (VADM)。这些低古强度估计表明南大西洋的一个区域不仅方向不稳定，而且比预期的要弱得多。我们认为这构成了进一步的证据，表明 SAA 不是单一事件，而是该地区该领域一系列反复出现的弱点中的最新一个，可能是由核心地幔边界上的反向通量斑块引起的。
The South Atlantic Anomaly (SAA) is an area of geomagnetic weakness that represents the most significant anomaly in the present‐day field. Notwithstanding anomalies such as these, a long‐lived hypothesis is that, if averaged over sufficient time (104–106 years), the Earth's magnetic field approximates a geocentric axial dipole (GAD). The question of how significant the non‐GAD features are in the time‐averaged field is an important and unresolved one. The SAA has not always been visible in the historic and paleo‐field models; yet an unstable field was reported in the South Atlantic region on a multimillion‐year timescale. This study presents the first paleointensity study from Saint Helena, a volcanic island in the South Atlantic consisting primarily of lavas emplaced between 10 and 8 Ma. While paleointensity success rates were low, we were able to recover results from five independent lavas that together suggest a low field intensity of 10.5 ± 3.0 μT corresponding to a virtual axial dipole moment (VADM) of 2.4 ± 0.7 × 1022 A m2. These low paleointensity estimates suggest a field in the South Atlantic that was not only unstable in directions, but also substantially weaker than expected. We consider this to constitute further evidence that the SAA is not a single occurrence but rather, the latest in a series of recurring weaknesses in the field in this region, probably caused by Reversed Flux Patches on the Core Mantle Boundary.