Palaeomagnetic evidence for the persistence or recurrence of geomagnetic main field anomalies in the South Atlantic

- A unique paleomagnetic dataset from Tristan da Cunha in the South Atlantic.
- Lava flows Ar/Ar dated as 46 to 90 ka.
- Average paleointensity 18±6μT: weaker than global paleomagnetic average.
- Data suggest magnetic anomaly present in the Late Pleistocene in the South Atlantic.

We present a dataset of a full-vector palaeomagnetic study of Late Pleistocene lavas from the island Tristan da Cunha in the South Atlantic Ocean. The current day geomagnetic field intensity in this region is approximately 25 μT, compared to an expected value of ∼43 μT; this phenomenon is known as the South Atlantic geomagnetic Anomaly (SAA). Geomagnetic field models extending back to the last 10 ka find no evidence for this being a persistent feature of the geomagnetic field, albeit, all models are constructed from data which is particularly sparse in the southern hemisphere. New 40Ar/39Ar incremental heating dating indicates the studied lavas from Tristan da Cunha extruded between 90 and 46 ka. Palaeointensity estimations of eight lava flows made using the Thellier method yield an average palaeointensity of 18±6μT and virtual axial dipole moment (VADM) of 3.1±1.2×1022Am2. The lava flows demonstrate four time intervals comparable to the present day SAA, where the average VADM of the Tristan da Cunha lavas is weaker than the global VADM average. This suggests a persistent or recurring low intensity anomaly to the main geomagnetic field similar to the SAA existed in the South Atlantic between 46 and 90 ka.