Geomagnetic field behaviour in the Miocene and structural irregularities in the South Atlantic region

Convecting iron in the outer core forms the geodynamo that produces our geomagnetic field. This magnetic field protects us from harmful solar wind radiation and creates insight into the inner workings of our planet. The geomagnetic field approximates to a geocentric axial dipole (GAD) when averaged over sufficient time. Variations in the geomagnetic field over time are named secular variation (SV). The latitude dependence of this secular variation is used to create a statistical description of the variability of the geomagnetic field for certain time periods. The present-day field shows many deviations from the GAD field, specifically in the South Atlantic where we see the South Atlantic Anomaly (SAA). This SAA is a weak patch in the geomagnetic field linked to a reverse flux patch on the core mantle boundary. In this South Atlantic region, the Earth is less protected from solar wind radiation, leading to damaged satellites. The longevity of this SAA is an important topic of discussion. It has been implied that the SAA is a one-off and linked to the overall weakening of the geomagnetic field, which is a precursor of an impending reversal of the geomagnetic field. Others have implied that the SAA is one of many recurring anomalous features in the South Atlantic, linked to anomalous features in deep Earth. This thesis reports a local field study from Saint Helena, an island in the South Atlantic (8 – 10 Ma) and investigates the geomagnetic field, globally, and in the South Atlantic region, on a multi-million-year timescale. The longevity of the anomalous field behaviour in the South Atlantic is discussed alongside the global geomagnetic field behaviour in the Miocene. The field study on Saint Helena gave us a robust directional dataset showing an average field direction not far from the GAD expectation. However, when comparing the scatter of the directions with that expected on this latitude, it is significantly higher, suggesting a field that was anomalously variable in the South Atlantic. The palaeointensity results from Saint Helena showed weaker intensities in Saint Helena than the average from the Miocene. These two results showed us that the field in the South Atlantic was weaker and more variable when compared to the global geomagnetic field, suggesting the South Atlantic region has a recurring anomalous feature, most likely caused by a combination of deep Earth features including the African large low shear velocity province (LLSVP). The palaeosecular variation (PSV) in the Miocene (5 – 23 Ma) is studied by compiling a new palaeomagnetic directional database (PSVM) with all the high-quality published directional data from the Miocene. The statistical analyses on this database shows a geomagnetic field with significantly higher equatorial variability compared to the last 10 Myrs. This suggests a more strongly convecting outer core in that time, possibly linked to a higher heat flux on the core mantle boundary. The time-averaged field (TAF) spherical harmonic models created with the Miocene database (PSVM), showed us some differences but also some clear similarities between the field morphology in the Miocene and the field morphology in the last 5 Myrs. More importantly the models, collectively called MTAM1, showed no substantial difference between the normal and reverse fields for the Miocene, suggesting the entire field reverses and not just the dipolar components. The MTAM1 models created with the PSVM database showed a robust reverse flux patch in the South Atlantic on the core mantle boundary. The combination of the MTAM1 models showing a reverse flux patch in the South Atlantic and the anomalously high VGP dispersion from the Saint Helena directional study and the anomalously low palaeointensity results from Saint Helena compared to the global average in the Miocene together provides more evidence that the geomagnetic field has a frequently recurring anomalous feature in the South Atlantic, linked to features in the deep Earth, like the African LLSVP and possibly an eccentric gyre in the outer core that reaches the CMB under the Atlantic.