Low Paleointensities and Ar/Ar Ages From Saint Helena Provide Evidence for Recurring Magnetic Field Weaknesses in the South Atlantic

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. Plain Language Summary The Earth's magnetic field is created by liquid iron convecting within the outer core. This field is highly variable. In the South Atlantic region, there is a large patch where the field is less strong than expected, causing us and our technology to be less protected against solar radiation. In that region lies Saint Helena, a volcanic island from which we collected samples that we used for paleomagnetic analysis. We performed experiments to derive the intensity of the field at that location when the rocks formed, which was 10–8 million years ago. Our results show that the magnetic field of Saint Helena was very weak during that interval suggesting that the modern irregular weak patch has been a recurring irregularity on a multimillion‐year timescale. This suggests that the weakness is most likely linked to heterogeneities in the lowermost mantle, which interfere with convective flow in the outer core. Key Points The first paleointensity results from Saint Helena suggest a low field in the South Atlantic betwe