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Precambrian palaeointensity measurements provide fundamental constraints on the evolution of the deep Earth. Core evolution models predict trends in dipole moment on billion-year timescales that can be tested by palaeomagnetic records. Here, we report new palaeointensity results from the recently identified ∼2.62 Ga Yandinilling dyke swarm of the Yilgarn Craton, Western Australia, and consider them alongside published measurements spanning 500 Myr across the late Archaean to earliest Proterozoic. Rock magnetic and scanning electron microscopy analysis confirm that the magnetic mineralogy is fine-grained magnetite, appearing mostly as exsolved lamellae with ilmenite. Six sites produced acceptable palaeointensity estimates from thermal and microwave IZZI protocol Thellier experiments and from double-heating technique Shaw experiments. These site mean values of 9–26 µT translate to virtual dipole moments of 11–44 ZAm2 that are considerably lower than today's dipole moment of ∼80 ZAm2 and the value predicted for this time period by some thermal evolution models. Their average (median = 41 ZAm2) is, however, similar to the long-term average during both of the intervals 2300–2800 Ma (median = 44 ZAm2; N  = 103) and 10–500 Ma (median 41 ZAm2; N  = 997). While there is little evidence for a substantial net change in average dipole moment between the late Archaean and Phanerozoic, there is preliminary evidence that its variance has increased between the two intervals. This lower variance more than two billion years ago supports the idea that the geodynamo, even while not producing a stronger magnetic field, was more stable on average at the Archaean–Proterozoic transition than it is today. [ABSTRACT FROM AUTHOR]

• New weak-field paleointensity results from 1.6 Ga Greenland dyke swarm. • Rigorous statistical analysis of dipole strength during the Paleoproterozoic. • Evidence for a billion-year paleomagnetic dipole low. The Paleoproterozoic era is the longest in Earth's history, with significant changes hypothesised to have occurred in the deep Earth's physical and chemical conditions at this time. It has been suggested that the paleomagnetic field became weaker at this time (∼2.4 Ga) and remained weak for the next billion years. Paleomagnetism is intrinsically linked to, and is able to inform on, ancient deep Earth processes; a weak dipole strength sustained over this time period may have implications for both core and mantle evolution. We test this hypothesis here in a two-fold approach: (1) A paleointensity study on the widespread ca. 1.6 Ga diabase/dolerite Melville Bugt dyke swarm. The swarm extends along the west coast of Greenland for more than 1000 km and intruded over ∼13 million years, capturing polarity reversals of Earth's magnetic field. (2) A detailed statistical analysis on the long-term trend in average dipole moment from an improved paleointensity dataset (PINT.org) that has recently undergone a major update. Five of the Greenland dykes produce paleointensity results ranging from 1.4 μT to 5.1 μT (virtual dipole moment range 0.3–1.2 × 1022 Am2) during the mid-point of this extended period of 'dipole low'. Our statistical study robustly confirms that this one-billion-year period was indeed associated with an anomalously weak dipole moment (2.7 × 1022 Am2) relative to 500-million-year intervals before and after, which were almost twice as strong. Sampling of more geographically diverse rocks from this time is needed to yield a clear picture of the long-term time evolution of the dipole moment. [ABSTRACT FROM AUTHOR]

Understanding the evolution of Earth's magnetic field can provide insights into core processes and can constrain plate tectonics and atmospheric shielding. The absolute paleointensity database PINT provides a curated repository of site mean (i.e., cooling unit), estimates of the strength of the magnetic field. We present a minor update to the PINT database to version 8.1.1 by adding 295 records from 34 studies. The PINT database is used to define a continuous model of the dipole field, using an approach combining non‐parametric and Monte Carlo (MC) resampling termed Monte Carlo Axial Dipole Average Model (MCADAM). Three dipole field strength models spanning 50 ka to 3.7–4.2 Ga (MCADAM.1a‐c) are presented, reflecting three tiers of increasingly more stringent data selection. The MCADAM models allow for the estimation of the magnetic standoff distance, constraining the shielding of Earth's atmosphere against solar wind erosion provided by the geodynamo. Plain Language Summary: The geomagnetic field is a long‐lived feature that provides critical shielding of Earth's atmosphere from solar wind erosion. Understanding changes in field strength can provide insight into the evolution of Earth's core. Here we use an updated database of paleointensity estimates to develop new continuous models of the strength of Earth's magnetic field. These models include plausible uncertainties, and capture variations in field strength spanning 50 thousand to over 3.7 billion years ago. Using our models, we suggest that the atmospheric shielding provided by the field was about 60% the present‐day shielding for most of the Precambrian. Key Points: Continuous dipole moment models for the past 3.7–4.2 billion years are presentedOur model reproduces salient features of the paleomagnetic dipole fieldPaleomagnetosphere estimates suggest Precambrian atmospheric shielding was much weaker than present day [ABSTRACT FROM AUTHOR]

TheMesozoic Dipole Low (MDL) is a period, covering at least ~80 My, of low dipole moment that ended at the start of the Cretaceous Normal Superchron. Recent studies of Devonian age Siberian localities identified similarly low field values a few tens of million years prior to the Permo-Carboniferous Reverse Superchron (PCRS). To constrain the length and timing of this potential dipole low, this study presents paleointensity estimates from Strathmore (~411 to 416Ma) and Kinghorn (~332 Ma) lava flows, United Kingdom. Both localities have been studied for paleomagnetic poles (Q values of 6 to 7), and the sites were assessed for their suitability for paleointensity from paleodirections, rock magnetic analysis, and microscopy. Thermal and microwave experiments were used to determine site mean paleointensity estimates of ~3 to 51 µT (6 to 98 ZAm²) and 4 to 11 µT (9 to 27 ZAm²) fromthe Strathmore and Kinghorn localities, respectively. These, and all the sites from 200 to 500 Ma from the (updated) Paleointensity database (PINT15), were assessed using the Qualitative Paleointensity criteria (QPI). The procurement of reliable (QPI = 5) weak paleointensity estimates from this and other studies indicates a period of low dipole moment (median field strength of 17 ZAm²) from 332 to 416 Ma. This "Mid-Paleozoic Dipole Low (MPDL)" bears a number of similarities to the MDL, including the substantial increase in field strength near the onset of the PCRS. The MPDL also adds support to the inverse relationship between reversal frequency and field strength and a possible ~200-My cycle in paleomagnetic behavior relating to mantle convection. [ABSTRACT FROM AUTHOR]

The timing of inner core nucleation is a hugely significant event in Earth's evolution and has been the subject of intense debate. Some of the most recent theoretical estimates for the age of nucleation fall throughout the Neoproterozoic era; much younger than previously thought. A young inner core requires faster recent core cooling rates and a likely hotter early core; knowledge of its age would be invaluable in understanding Earth's thermal history and total energy budget. Predictions generated by numerical dynamo models need to be tested against such data, but records are currently much too sparse to constrain the event to a precise period of time. Here, we present results from 720 Ma dolerite dykes (and one sill) from the Franklin Large Igneous Province, which fall within a crucial 300 Myr gap in palaeointensity records. This study uses three independent techniques on whole rocks from 11 sites spread across High Arctic Canada and Greenland to produce virtual dipole moments ranging from 5 to 20 ZAm2 (mean 11 ZAm2); almost one order of magnitude lower than the present-day field. These weak-field results agree with recent ultralow palaeointensity data obtained from Ediacaran rocks formed ∼150 Myr later and may support that the dynamo was on the brink of collapse in the Neoproterozoic prior to a young inner core formation date. [ABSTRACT FROM AUTHOR]

Since the establishment of direct estimations of the Earth's magnetic field intensity in the first half of the nineteenth century, a continuous decay of the axial dipole component has been observed and variously speculated to be linked to an imminent reversal of the geomagnetic field. Furthermore, indirect estimations from anthropologically made materials and volcanic derivatives suggest that this decrease began significantly earlier than direct measurements have been available. Here, we carefully reassess the available archaeointensity dataset for the last two millennia, and show a good correspondence between direct (observatory/satellite) and indirect (archaeomagnetic) estimates of the axial dipole moment creating, in effect, a proxy to expand our analysis back in time. Our results suggest a continuous linear decay as the most parsimonious long-term description of the axial dipole variation for the last millennium. We thus suggest that a break in the symmetry of axial dipole moment advective sources occurred approximately 1100 years earlier than previously described. In addition, based on the observed dipole secular variation timescale, we speculate that the weakening of the axial dipole may end soon. [ABSTRACT FROM AUTHOR]

Paleointensity data can yield insight on the state of the geodynamo, providing constraints on deep Earth events and enabling analysis of long-term trends in the paleomagnetic field. The Ediacaran (635 Ma–539 Ma) is a period of discrepant paleomagnetic behaviour that was recently characterised by sustained, extremely weak, paleointensity. The interval also coincides with some of the most recent estimates for Earth's inner core nucleation (ICN) age, determined from numerical geodynamo models and analysis of long-term paleointensity data. However, the field strength during the subsequent Cambrian period (540 Ma–485 Ma) is largely unknown with almost no data. Here, we provide high-quality paleointensity results for the Cambrian. A Grenville dyke (∼590 Ma) that was baked by the Chatham-Grenville stock (532 Ma), slowly cooled at a rate controlled by the stock and recorded the paleointensity averaged over this interval (up to several tens of thousands of years). The characteristic paleomagnetic directions of the dyke are well-defined and consistent with those previously obtained from the Chatham-Grenville and Mont Riguad stocks. Paleointensity data were obtained using multiple methods and indicate an extremely weak field during a period coincidental with evidence for hyper-reversing activity extending into the late Cambrian. The dipole strength is similar to that of the 'ultra-weak' Ediacaran and may suggest that this paleomagnetic behaviour persisted into the Cambrian. The cause of this weak-field interval remains enigmatic but an approximate 200-million-year quasi-periodicity in dipole strength extending across the entire Phanerozoic is not ruled out. • New high-quality paleointensity results obtained for the early Cambrian. • 'Ultra-weak' dipole moment recorded over tens of thousands of years at ∼532 Ma. • Weak dipole moment coincides with potential hyperactive reversal rate. • New data support an approximate 200 Myr quasi-periodicity in dipole strength. [ABSTRACT FROM AUTHOR]

Abstract: Despite the many advances recently made in palaeointensity (PI) studies, more high quality PI data are required for a better understanding of the full-vector evolution of the ancient geomagnetic field. In this study, we applied the recently proposed multispecimen parallel differential pTRM (MS) method for absolute PI determination to rocks older than those studied hitherto with this method. Fifty-one lava flows from within the Trans-Mexican Volcanic Belt, which are generally younger than 1Ma, returned 32 new PI estimates. Rock magnetic investigations revealed that in most cases, the remanence was carried by Ti-poor titanomagnetites of pseudo-single domain magnetic structure although titanomagnetites with higher Ti contents and additional contributions of (titano-) haematite were also occasionally present. Comparisons with reconstructions of the global dipole moment reveal that the PI estimates from this study were, on average, around 30% higher than expected. Other, mainly Thellier-type, PI data from Mexico were also observed to be high relative to global records, which could indicate that persistent non-dipole features might be responsible for the higher than expected results. However, the paucity of available data obscures the significance of this observation and the balance of evidence rather suggests an artificial biasing of most measurements towards high values. Our results seem to corroborate results from previous studies on historical lava flows and synthetic samples in which domain state effects were found to cause overestimates of the PI by up to 30% in the MS method. We expect that the degree of the overestimate in the majority of these new MS results is no larger than what might be expected from Thellier experiments performed on samples with a similar given degree of multidomain behaviour. However, seven of the lava flows which were studied here using the MS method were subjected to a preceding demagnetisation step of 200°C in order to erase viscous remanences and these may have produced results which were further biased towards higher intensities. This additional bias is expected from theoretical considerations and may represent a challenge to future application of the MS method to older rock units. It could, however, potentially be avoided in future studies by performing measurements of the sample moment at temperatures higher than the blocking temperature of any overprint. [Copyright &y& Elsevier]

• New palaeointensity data from six dykes of the Western Grenville Dykes in SE Canada. • Experiments reveal extremely weak geomagnetic field strengths in the mid-Ediacaran. • Results may suggest an extended period of weak and anomalous field behaviour. Long-term variations of the geomagnetic field, observed in the palaeomagnetic record, have the potential to shed much light on the evolution of Earth's deep interior. With a geomagnetic field characterised by anomalous directions and ultra-low intensities, the Ediacaran period (635-538 Ma) is a time of special interest. Steep and shallow directions, leading to virtual geomagnetic poles (VGPs) separated by angles of up to 90° and very close in age could have recorded a geomagnetic field switching between axial and equatorial dipole-dominated states. Alternatively, the field may simply have been highly nondipolar and subject to rapid reversals. Palaeointensity determinations of units that record the anomalous directions could potentially help to discriminate between morphologies but the spatial and temporal distribution of palaeomagnetic data require improvement. Here we present new palaeointensities from 6 dykes from the western end of the Grenville Dyke swarm that recorded directionally anomalous geomagnetic fields around ∼585 Ma. Results from double-heating Thellier experiments failed to satisfy the used selection criteria, but successful microwave Thellier, Shaw and pseudo-Thellier experiments lead to palaeointensities that show field strength values of 2.9 ± 2.2 μT and corresponding virtual dipole moments of 0.3- 1.7 × 10 22 A m2. These field strengths are an order of magnitude weaker than the present-day field. VGPs grouping in two distinct clusters with almost identical angular dispersions of VGPs (S B = 18.5 ° and 18.9°) may argue for the presence of an equatorial dipole. In contrast, the palaeointensities associated with the steep and shallow components are indistinguishable. Although based on a low number of results, this observation, together with the overall very large VGP dispersion may rather support that the Grenville Dykes have recorded enhanced secular variation linked to a highly unstable and rapidly reversing field. [ABSTRACT FROM AUTHOR]

Long-term temporal variations of the magnetic field (timescales >10 Myr), characterized from paleomagnetic data, have been hypothesized to reflect the evolution of Earth's deep interior and couplings between the core and mantle. By tying observed changes in the paleomagnetic record to mechanisms predicted from numerical geodynamo simulations, we have a unique tool for assessing changes in the deep interior back in time. However, numerical simulations are not run in an Earth-like parameter regime and assessing how well they reproduce the geomagnetic field is difficult. Criteria have been proposed to determine the level of spatial and temporal agreement between simulations and observations spanning historical and Holocene timescales, but no such criteria exist for longer timescales. Here we present a new set of five criteria (Quality of Paleomagnetic Modeling criteria, Q PM) that assess the degree of semblance between a simulated dynamo and the temporal and spatial variations of the long-term (∼10 Myr) paleomagnetic field. These criteria measure inclination anomaly, virtual geomagnetic pole dispersion at the equator, latitudinal variation in virtual geomagnetic pole dispersion, normalized width of virtual dipole moment distribution, and dipole field reversals. We have assessed 46 geodynamo simulations using the Q PM criteria. The simulations have each been run for the equivalent of at least ∼300 kyr, span reversing and non-reversing regimes, and include either homogeneous or heterogeneous heat flux boundary conditions. We find that none of our simulations reproduce all salient aspects of the long-term paleomagnetic field behavior for the past 10 Myr. Nevertheless, our simulations bracket Earth values, suggesting that an Earth-like simulation is feasible within the available computationally accessible parameter space. This new set of criteria can inform future simulations that aim to reproduce all aspects of Earth's long-term magnetic field behavior. • We present new assessment criteria for long-term geodynamo simulations. • Criteria chosen are based on variations in paleomagnetic field for the past 10 Myr. • Geodynamo simulations don't fully recreate long-term magnetic field behavior. • This framework can be extended to other time intervals beyond past 10 Myr. [ABSTRACT FROM AUTHOR]

The quantity of igneous material comprising the Siberian Traps provides a uniquely excellent opportunity to constrain Earth’s paleomagnetic field intensity at the Permo-Triassic boundary. There remains however, a contradiction about the strength of the magnetic field that is exacerbated by the limited number of measurement data. To clarify the geomagnetic field behavior during this time period, for the first time, a microwave paleointensity study has been carried out on the Permo-Triassic flood basalts in order to complement existing datasets obtained using conventional thermal techniques. Samples, which have been dated at ∼250 Ma, of the Permo-Triassic trap basalts from the northern extrusive (Maymecha-Kotuy region) and the southeastern intrusive (areas of the Sytikanskaya and Yubileinaya kimberlite pipes) localities on the Siberian platform are investigated. These units have already demonstrated reliable paleomagnetic directions consistent with the retention of a primary remanence. Furthermore, Scanning Electron Microscope analysis confirms the presence of iron oxides likely of primary origin. Microwave Thellier-type paleointensity experiments (IZZI protocol with partial thermoremanent magnetization checks) are performed on 50 samples from 11 sites, of which, 28 samples from 7 sites provide satisfactory paleointensity data. The samples display corresponding distinct directional components, positive pTRM checks and little or no zig-zagging of the Arai or Zijderveld plot, providing evidence to support that the samples are not influenced by lab-induced alteration or multi-domain behavior. The accepted microwave paleointensity results from this study are combined with thermal Thellier-type results from previously published studies to obtain overall estimates for different regions of the Siberian Traps. The mean geomagnetic field intensity obtained from the samples of the northern part is 13.4 ± 12.7 μT (Maymecha-Kotuy region), whereas from the southeastern part is 17.3 ± 16.5 μT (Sytikanskaya kimberlite pipe) and 48.5 ± 7.3 μT (Yubileinaya kimberlite pipe), suggesting that the regional discrepancy is probably due to the insufficient sampling of geomagnetic secular variation, and thus, multiple localities need to be considered to obtain an accurate paleomagnetic dipole moment for this time period. It demonstrates that the overall mean paleointensity of the Siberian Traps is 19.5 ± 13.0 μT which corresponds to a mean virtual dipole moment of 3.2 ± 1.8 × 10 22 Am 2 . Results indicate that the average magnetic field intensity during Permo-Triassic boundary is significantly lower (by approximately 50%) than the present geomagnetic field intensity, and thus, it implies that the Mesozoic dipole low might extend 50 Myr further back in time than previously recognized. [ABSTRACT FROM AUTHOR]