Daniele Thallner

The Earth’s ancient magnetic field is challenging to constrain from the rock record in large part due to the presence of non-ideal magnetic recorders in addition to processes, like alteration, that affect the ability of a material to reliably record field strength. Of the magnetic minerals present on Earth’s surface, magnetite is one of the most commonly used to simultaneously recover palaeomagnetic direction and intensity. Recent work on archaeological artifacts and clinker deposits (sedimentary rocks baked by coal seam fires) has identified a potential new mineral capable of recording the full-vector magnetic field: ɛ-Fe₂O₃, a high-T metastable phase of hematite. The palaeomagnetic potential of ɛ-Fe₂O₃, specifically regarding palaeointensity, has not been studied in depth. Further, recent work on synthetic ɛ-Fe₂O₃ has raised questions about the reliability of this phase for palaeointensity recording. To understand whether ɛ-Fe₂O₃ is a trustworthy full-vector magnetic recorder, more work is needed to assess this phase in its natural form. Here, we present results from Thellier-style palaeointensity experiments using a lab-induced thermoremanent magnetization (TRM) on natural ɛ-Fe₂O₃ present in Quaternary age clinker samples from the Custer National Forest, Montana, USA. The experimental setup was designed in attempt to isolate the ɛ-Fe₂O₃ phase from other magnetic carriers. The results of our study suggest that natural ɛ-Fe₂O₃ can reliably record palaeointensity and palaeodirections, yielding palaeointensity estimates within 5% and directions consistent with the applied laboratory TRM field. These new results suggest that ɛ-Fe₂O₃ bearing artifacts and clinkers can be robust full-vector magnetic recorders. Overall, this study adds confidence to previously obtained archaeomagnetic data and to a novel palaeomagnetic recorder, clinkers, opening the door to a more detailed characterization of the recent field.

Heat flowing from the core to the mantle drives the geodynamo that produces Earth’s global magnetic field. Palaeomagnetic measurements record the behaviour of this field through time and have the potential to inform us about deep Earth structures and dynamics on either side of the core–mantle boundary. In practise, insights have proved difficult to obtain because of the limited spatiotemporal resolution of palaeomagnetic records and uncertainties in how to interpret them. Here we use palaeomagnetic datasets and models alongside numerical simulations of the geodynamo to show that certain observed characteristics of ancient magnetic field behaviour are uniquely or preferentially reproduced in the presence of strong lateral variability in core–mantle heat flux. Our findings suggest that strong contrasts in the spatial pattern of the temperature gradients and/or thermal conductivity of the lowermost mantle that are linked, today, to seismologically observed structures, have influenced the geodynamo for at least the last few hundred million years. The identified palaeomagnetic signatures provide a new means to constrain the properties and time evolution of the core–mantle boundary. Furthermore, our insights into how thermal heterogeneity at the base of the mantle can break the axial symmetry of the time-averaged magnetic field may help resolve longstanding palaeogeographic controversies.

Studies of the behaviour of the geodynamo through time, preserved in palaeomagnetic records, have a unique potential to inform us about the time-varying thermal structure of Earth’s core-mantle boundary region. In practise, however, such insights have proved difficult to obtain because of the limited spatiotemporal resolution of palaeomagnetic records and uncertainties in how to interpret them. Here we show that certain key characteristics of the palaeomagnetic field observed over the last 23 Myr can only be reproduced by numerical simulations of the geodynamo that have heterogeneous heat flux conditions imposed on their outer boundaries. We further demonstrate that a signal of azimuthal asymmetry in field behaviour, characteristic of such heterogeneity, is evident in the variance of records of palaeosecular variation recovered from low latitudes, that are up to 265 Myr old. Finally, we show that imposing strong heterogeneity in the heat flow boundary conditions significantly increases the parameter range over which simulations can reproduce aspects of palaeosecular variation that have been observed to be stable through much of geological time. In conjunction, these findings provide compelling new evidence that significant thermal heterogeneity at the core-mantle boundary has influenced the geodynamo for at least the last few hundred million years. The identified palaeomagnetic signatures of thermal heterogeneity at the core-mantle boundary provide a new means to constrain the properties and time-evolution of this region of Earth’s interior. Furthermore, our insights into how thermal heterogeneity at the base of the mantle can break the axial symmetry of the time-averaged magnetic field may provide an important step towards resolving longstanding palaeogeographic controversies.

The Earth’s magnetic field is generated by a dynamo in the outer core and is crucial for shielding our planet from harmful radiation. Despite the established importance of the core-mantle boundary heat flux as driver for the dynamo, open questions remain about how heat flux heterogeneities affect the magnetic field. Here, we explore the distribution of core-mantle boundary heat flux on Earth and its changes over time using compressible global 3-D mantle convection models in the geodynamic modeling software ASPECT. We discuss the use of the consistent boundary flux method as a tool to more accurately compute boundary heat fluxes in finite element simulations and the workflow to provide the computed heat flux patterns as boundary conditions in geodynamo simulations. Our models use a plate reconstruction throughout the last 1 billion years—encompassing the complete supercontinent cycle—to determine the location and sinking speed of subducted plates. The results show how mantle upwellings and downwellings create localized heat flux anomalies at the core-mantle boundary that can vary drastically over Earth’s history and depend on the properties and evolution of the lowermost mantle as well as the surface subduction zone configuration. The distribution of hot and cold structures at the core-mantle boundary changes throughout the supercontinent cycle in terms of location, shape and number, indicating that these structures fluctuate and might have looked very differently in Earth’s past. We estimate the resulting amplitude of spatial heat flux variations, expressed by the ratio of peak-to-peak amplitude to average heat flux, q*, to be at least 2. However, depending on the material properties and the adiabatic heat flux out of the core, q* can easily reach values >30. For a given set of material properties, q* generally varies by 30-50% over time. Our results have implications for understanding the Earth’s thermal evolution and the stability of its magnetic field over geological timescales. They provide insights into the potential effects of the mantle on the magnetic field and pave the way for further exploring questions about the nucleation of the inner core and the past state of the lowermost mantle.

Changes in the geomagnetic field over geologic timescales can be used to study the evolution and processes of its sources in Earth's deep interior. As a stark contrast to the geomagnetic field behaviour of the recent past, the field in the late Neoproterozoic is defined by ambiguous polar wander paths, ultralow field strengths and a period of extreme reversal hyperactivity. Palaeointensity data from this time are still scarce and conclusions made from these data suffer from large uncertainties brought by a low sampling density. In this study, we present new palaeomagnetic and palaeointensity data from quarry outcrops and five deep drill cores, covering the longest and most complete profiles through the Ediacaran units in the Volyn Basalt Province (NW Ukraine) to date. Palaeodirectional analysis of thermal and alternating field demagnetization experiments reveals at least six reversals in the profile as well as four palaeopoles that agree with previously published poles. Whole-rock palaeointensity estimates are derived from a multimethod approach that utilizes thermal and microwave Thellier experiments performed using Coe and IZZI protocols as well as Wilson and double-heating Shaw experiments. These produce instantaneous site-mean palaeointensity estimates in the range of 2.1 ± 0.4 to 11.1 ± 3.5 µT and virtual dipole moments in the range of 0.31 ± 0.06 × 10²² to 1.67 ± 0.53 × 10²² A m². Small-scale variations of palaeointensities throughout the profile not only show extremely weak field strengths around polarity changes but could also suggest that the field strength never fully recovered between reversals. These new results increase the density of intensity data coverage for Baltica in the Late Ediacaran and, in combination with previously published Ediacaran dipole moments, support an extremely weak field that seems to increase in strength towards the Ediacaran–Cambrian transition.

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, Q_PI) framework. This updated database brings together and harmonizes prior Q_PI and PINT compilation efforts into a unified database referred to as the PINT database, incorporating recent efforts since 2014 to assess Q_PI. The spatio-temporal distribution of the PINT database is analysed, 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 Palaeozoic and Precambrian. General Q_PI compliance is characterized for the PINT database, which shows that the median Q_PI scores range from 2 to 3 (out of a total possible score of 10), with a positive trend towards increasing Q_PI 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.

The geomagnetic field in the Ediacaran period appears to be characterized by anomalous directions, high reversal frequencies, and ultra‐low field strength. At the Ediacaran‐Cambrian transition, apparent polar wander paths become less controversial, but hyper‐reversing fields with reversal frequencies similar to earlier in the Ediacaran have been reported until the mid‐Cambrian. To understand the long‐term behavior of the magnetic field during this transitional interval, information about the field strength is vital but currently lacking. To improve the intensity record at that time, samples of volcanic rocks from the 550 Ma Skinner Cove Formation, western Newfoundland, have been used for paleointensity determination. This multi‐method paleointensity study, utilizing thermal and microwave Thellier as well as double‐heating Shaw and pseudo‐Thellier experiments produces paleointensity estimates of 2.6–10.3 μT, corresponding to virtual dipole moments of 0.65–2.25 × 10²² Am². Analyses using scanning electron microscopy and rock magnetic measurements, as well as a positive intraformational conglomerate test, suggest that the remanence is primary. These intensity estimates are higher than earlier Ediacaran intensities and could point to the geomagnetic field entering into a stronger, more dipole‐dominated state at the Ediacaran‐Cambrian boundary.

Palaeomagnetic investigations from the Ediacaran period (635-541 Ma) give anomalous results, which might indicate unusual behaviour of Earth’s magnetic field. In contrast to the conventional geomagnetic dipole field, geocentric and aligned with Earth’s rotational axis, records of the palaeomagnetic field from several locations in Laurentia and Baltica indicate that the Ediacaran geomagnetic field might have been exceptionally weak and spent extended periods of time with its poles close to the geographic equator. Multi-method palaeointensity determinations have been performed on rocks from the Grenville Dykes (Canada, 584-598 Ma), Skinner Cove volcanics (Newfoundland, 550.5 Ma) and the Volyn Traps (Ukraine, 560-580Ma), confirming that the field was exceptionally weak, with VDM values between 4 and 15 ZAm2. These values could correspond to considerably lower VDM strengths predicted by geodynamo simulations for fields with low dipolarity before the onset of nucleation of the solid inner core. In contrast, preliminary evaluation of published directional data indicates that palaeosecular variation in the Ediacaran might not be distinguishable from palaeosecular variation predicted from palaeomagnetic data of the last ten million years.These new palaeointensity results contribute to the elimination of data gaps in the Neoproterozoic palaeomagnetic record and will be used in combination with qualitatively assessed published directional and intensity data to capture the behaviour of the geomagnetic field of this time period in statistical field models. Comparisons of time averaged palaeosecular variation and VDM distribution with predictions of the geomagnetic field from geodynamo simulations will help to verify and improve models of deep Earth structures and dynamics.