Palaeosecular variation in the Miocene and the first non-zonal time-averaged field model of that era.

Reconstructions of the geomagnetic field behaviour over long periods of time throughout history are important for understanding of geomagnetic field evolution and documenting the longevity of certain features. Statistical studies of palaeosecular variation inform us regarding the structure and behaviour of the geomagnetic field. Here we present a new data compilation, PSVM, of high-quality directional data from the Miocene era (5.3 – 23 Ma). Our compilation comprises 1454 sites from 44 different localities, each with at least 10 sites. We use this database to calculate new Model G parameters for PSVM with varying selection criteria. Our preferred database, , has the selection criteria of n ≥ 5, k ≥ 50, a Vandamme cutoff applied and at least 2 reversals shown within the 10 or more sites of a locality. This produced a Model G fit with a parameter of 15.7° (13.0° - 18.7°). This value is substantially higher than any of the Model G a parameters published for the past 10 Myrs or any other studied era, implying a less stable geomagnetic field in the Miocene. PSVM also enables the creation of the first non-zonal time-averaged field (TAF) models of the Miocene, called MTAM1. After separating our data into normal (PSVM_N) and reversed (PSVM_R) datasets, separate models for the two states were created. No substantial differences were found between the models (MTAM1_N and MTAM1_R, respectively), suggesting symmetry in the morphology of the magnetic field in the Miocene. There is no evidence for a previously hypothesised "memory" of the field after a reversal for this era. Instead, non-dipole structure appears to reverse simultaneously with the dipolar structures. After observing this symmetry, we compute a TAF model for the complete Miocene dataset (PSVM), enhancing the data distribution and thus the robustness of the model. In all versions of the models, a reverse flux patch (RFP) is seen under the South Atlantic. Our findings suggest a more variable magnetic field in the Miocene era compared to the past 10 Myrs, implying that the geodynamo was driven by a more strongly convecting liquid core producing a less dipole dominated field on average. In addition, we found a recurring RFP under the South Atlantic that was sufficiently frequent and stationary to appear in a TAF model, giving evidence for a recurring or consistent anomalous feature in the South Atlantic region in the Miocene, with longevity on a multi-million-year timescale.