Earth’s magnetic field is presently characterized by a large and growing anomaly in the South Atlantic Ocean. The question of whether this region of Earth’s surface is preferentially subject to enhanced geomagnetic variability on geological timescales has major implications for core dynamics, core−mantle interaction, and the possibility of an imminent magnetic polarity reversal. Here we present paleomagnetic data from Saint Helena, a volcanic island ideally suited for testing the hypothesis that geomagnetic field behavior is anomalous in the South Atlantic on timescales of millions of years. Our results, supported by positive baked contact and reversal tests, produce a mean direction approximating that expected from a geocentric axial dipole for the interval 8 to 11 million years ago, but with very large associated directional dispersion. These findings indicate that, on geological timescales, geomagnetic secular variation is persistently enhanced in the vicinity of Saint Helena. This, in turn, supports the South Atlantic as a locus of unusual geomagnetic behavior arising from core−mantle interaction, while also appearing to reduce the likelihood that the present-day regional anomaly is a precursor to a global polarity reversal.

目前，地球磁场的特征是南大西洋的异常现象正在不断扩大。地球表面的该区域是否优先受到地质时标上增强的地磁变异性的影响，对磁芯动力学，磁芯-地幔相互作用以及即将发生的磁极性反转的可能性产生了重大影响。在这里，我们呈现了圣海伦娜的火山磁数据，圣海伦娜是一个火山岛，非常适合检验以下假设：在数百万年的时间尺度上，地磁行为在南大西洋异常。我们的结果得到积极的烘烤接触和反向测试的支持，得出的平均方向近似于8到1100万年前的地心轴向偶极子所期望的方向，但伴随着很大的方向分散。这些发现表明，在地质时间尺度上，圣海伦娜附近的地磁长期变化持续增强。反过来，这也支持南大西洋作为由地幔相互作用产生的异常地磁行为的场所，同时也似乎减少了当今区域异常是全球极性反转的先兆的可能性。