Summary Palaeomagnetic field intensity measurements, derived from rocks with ages that span geological time, provide a crucial constraint on the evolution of Earth’s deep interior and its magnetic environment. The palaeointensity database PINT has been updated to version v.8.0.0  and includes palaeointensity site-mean records spanning an interval from 50 ka to 4.2 Ga, compiling efforts from the palaeomagnetic community spanning from 1959 to the end of 2019. Nearly all site-mean palaeointensity records have been assessed using the qualitative reliability of palaeointensity (Quality of Palaeointensity, QPI) framework. This updated database brings together and harmonizes prior QPI and PINT compilation efforts into a unified database referred to as the PINT database, incorporating recent efforts since 2014 to assess QPI. The spatio-temporal distribution of the PINT database is analyzed, revealing substantial biases towards young records (from the Brunhes chron) in the Northern hemisphere, and intervals with little to no palaeointensity data with a duration of 10s to 100s of millions of years in the Paleozoic and Precambrian. General QPI compliance is characterized for the PINT database, which shows that the median QPI scores range from 2 to 3 (out of a total possible score of 10), with a positive trend towards increasing QPI scores in studies published after the year 2000. This illustrates an increasing community awareness of what is required to establish confidence in palaeointensity data and an increasing robustness of the large scale interpretations that can be made with these data. We additionally present a description of the long-term average dipole field strength with descriptive statistics for distinct intervals of Earth history.

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.

Spatial and temporal geomagnetic field variations have been observed over different geological timescales. Ancient field measurements, mainly obtained from geological materials (sedimentary and igneous rocks), allow investigations of directional and intensity variability of the paleomagnetic field that result from processes operating in Earth's fluid core (see, e.g., Hulot et al., 2010). Particularly, information about paleosecular variation (PSV), long-term variations of the order of 10 5 -10 6 years (e.g., Johnson & McFadden, 2015), is essential to better understand temporal geomagnetic field evolution and to constrain numerical geodynamo models (

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.

Statistical studies of paleosecular variation (PSV) are used to infer the structure and behavior of the geomagnetic field. This study presents a new database, paleosecular variation of the Miocene (PSVM), of high‐quality directional data from the Miocene Epoch (5.3–23 Ma), compiled from 1,454 sites from 44 different localities. This database is used to model the latitude dependence of paleosecular variation with varying selection criteria using a quadratic form based on Model G. Our fitted model parameter for latitude‐invariant PSV (Model G a) is 15.6° and the latitude dependent PSV term (Model G b) is 0.23. The latitude invariant term is substantially higher than previously observed for the past 10 Myrs or any other studied ages. We also present a new stochastic model of the time‐average field, BB‐M22, using a covariant giant Gaussian process (GGP) which is constrained using data from PSVM, PINT and Earth‐like geodynamo numerical simulations. BB‐M22 improves the fit to PSVM data relative to prior GGP models, as it reproduces the higher virtual geomagnetic pole (VGP) dispersion observed during the Miocene. Our findings suggest a more variable magnetic field and more active geodynamo in the Miocene Epoch than the past 10 Myrs, perhaps linked to elevated core‐mantle heat flow. Our results suggest that the average axial dipole dominance of the time‐instantaneous field was lower than in more recent times. We note however, that the inclination anomaly estimates, suggest that it cannot be ruled out that the Miocene time averaged field resembles a geocentric axial dipole.

<p>A long-lived hypothesis is that, if averaged over sufficient time (ca 10 million years), the Earth’s magnetic field approximates a geocentric axial dipole (GAD). Despite this common assumption, the question of how significant the non-GAD features are in the time-averaged field is an important and unresolved one. In the present-day field, the South Atlantic Anomaly (SAA) is the biggest irregularity in the field. We know that this anomaly has not always been a part of the field, but in Engbers et al., 2020, it was shown that the magnetic field shows irregular behaviour in this region on a million-year timescale. The irregular behaviour was demonstrated through a substantially high VGP dispersion (21.9º) for lava flows from Saint Helena that are between 8 and 11 million years old. The island of Saint Helena is located at the margin of the present-day SAA and has declination -16.6º, inclination -57.5º relative to expected GAD values of 0.0º/-7.8º (Dec/Inc). We have now commenced the measurements of absolute palaeointensity data from this location. So far, we have performed thermal and microwave IZZI-Thellier experiments on 2 localities from Saint Helena. The site mean results show variable but generally very low field intensities, although further work is required to make these sufficiently robust. Our low field estimates suggest a field in the South Atlantic that is not only unstable, but mainly weaker than expected. This could mean that recurring reversed flux patches (RFP) are responsible for the irregularities and weaknesses in the field in this region, stretching back up to 11 million years ago.</p>

<p>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 <em>a</em> parameter of 15.7° (13.0° - 18.7°). This value is substantially higher than any of the Model G <em>a</em> 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<sub>N</sub>) and reversed (PSVM<sub>R</sub>) datasets, separate models for the two states were created. No substantial differences were found between the models (MTAM1<sub>N</sub> and MTAM1<sub>R</sub>, 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.</p>

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