The selection of paleointensity data is a challenging, but essential step for establishing data reliability. There is, however, no consensus as to how best to quantify paleointensity data and which ...data selection processes are most effective. To address these issues, we begin to lay the foundations for a more unified and theoretically justified approach to the selection of paleointensity data. We present a new compilation of standard definitions for paleointensity statistics to help remove ambiguities in their calculation. We also compile the largest‐to‐date data set of raw paleointensity data from historical locations and laboratory control experiments with which to test the effectiveness of commonly used sets of selection criteria. Although most currently used criteria are capable of increasing the proportion of accurate results accepted, criteria that are better at excluding inaccurate results tend to perform poorly at including accurate results and vice versa. In the extreme case, one widely used set of criteria, which is used by default in the ThellierTool software (v4.22), excludes so many accurate results that it is often statistically indistinguishable from randomly selecting data. We demonstrate that, when modified according to recent single domain paleointensity predictions, criteria sets that are no better than a random selector can produce statistically significant increases in the acceptance of accurate results and represent effective selection criteria. The use of such theoretically derived modifications places the selection of paleointensity data on a more justifiable theoretical foundation and we encourage the use of the modified criteria over their original forms.
Key Points
Standard definitions for paleointensity statistics are proposed
A large paleointensity meta‐analysis is conducted to investigate data selection
Modifications based on SD predictions improve the effectiveness of selection
SUMMARY
In this study, we examine the role of palaeosecular variation (PSV) in the use of statistics for palaeomagnetic studies, and we provide new reliability criteria for palaeomagnetic poles or ...directions. We first conclude that Fisher statistics should not be applied to average palaeomagnetic directions but to virtual geomagnetic pole (VGP) distributions instead.
Secondly, we strongly advocate that typical properties of geomagnetic field behaviour are taken into account in the assessment of palaeomagnetic data sets. The latitude‐dependent properties (E, S, k) provide useful guidelines for the reliability of a palaeomagnetic data set. A reliable assessment of these properties depends on the (sufficient) number of palaeomagnetic samples being taken. Therefore, as an additional instrument of assessing data sets, we provide a N‐dependent A95 envelope, bounded by an upper limit A95max, and a lower limit A95min that helps to ascertain whether or not a distribution has sufficiently well‐sampled PSV and therefore geomagnetic field behaviour. Applying these criteria is indispensable for studies of geomagnetic behaviour, or for studies aiming at using TK03.GAD for inclination error correction through the elongation/inclination (E/I) method. For palaeomagnetic studies aimed at geological reconstructions, they form helpful guidelines and increase the confidence in the rocks having faithfully recorded the field.
An analysis of published Eastern Mediterranean data shows that the vast majority of studies do not conform to the Van der Voo criteria, in particular with respect to N and A95. We have provided criteria that are on the one hand more lenient (lower N may still provide relevant information), and on the other hand more strict (for high N the criterion of A95 < 16° should be adapted to a requirement of lower A95, e.g. A95 < 5° for N > 80).
Ascertaining the cause of variations in the frequency of geomagnetic polarity reversals through the Phanerozoic has remained a primary research question straddling paleomagnetism and geodynamics for ...decades. Numerical models suggest the primary control on geomagnetic reversal rates on 10 to 100 Ma timescales is the changing heat flux across the core-mantle boundary and that this is itself expected to be strongly influenced by variations in the flux of lithosphere subducted into the mantle. A positive relationship between the time-dependent global subduction flux and magnetic reversal rate is expected, with a time delay to transmit the thermal imprint into the lowermost mantle. We perform the first test of this hypothesis using subduction flux estimates and geomagnetic reversal rate data back to the early Paleozoic. Subduction area flux estimates are derived from global, full-plate tectonic models, and are evaluated against independent subduction flux proxies based on the global age distribution of detrital zircons and strontium isotopes. A continuous Phanerozoic reversal rate model is built from pre-existing compilations back to ~320 Ma plus a new reversal rate model in the data-sparse mid-to-early Paleozoic. Cross-correlation of the time-dependent subduction flux and geomagnetic reversal rate series reveals a significant correlation with a time delay of ~120 Ma (with reversals trailing the subduction flux). This time delay represents a value intermediate between the seismologically constrained time expected for a subducted slab to transit from the surface to the core-mantle boundary (~150–300 Ma), and the much shorter lag time predicted by some numerical models of mantle flow (~30–60 Ma). While the reason for this large discrepancy remains unclear, it is encouraging that our novel estimate of lag time represents a compromise between them. Although important uncertainties in our proposed relationship remain, these results cast new light on the dynamic connections between the surface and deep Earth, and will help to constrain new models linking mantle convection, the thermal evolution of the lowermost mantle and the geodynamo.
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•Subduction area flux estimates derived from tectonic model•Geomagnetic polarity reversal rates determined for the Phanerozoic•Statistical comparison shows link between lithospheric flux and reversal rate.•Subduction flux modulates geomagnetic polarity reversal rate after a 120 Ma delay.
A defining characteristic of the recent geomagnetic field is its dominant axial dipole which provides its navigational utility and dictates the shape of the magnetosphere. Going back through time, ...much less is known about the degree of axial dipole dominance. Here we use a substantial and diverse set of 3D numerical dynamo simulations and recent observation-based field models to derive a power law relationship between the angular dispersion of virtual geomagnetic poles at the equator and the median axial dipole dominance measured at Earth's surface. Applying this relation to published estimates of equatorial angular dispersion implies that geomagnetic axial dipole dominance averaged over 10
-10
years has remained moderately high and stable through large parts of geological time. This provides an observational constraint to future studies of the geodynamo and palaeomagnetosphere. It also provides some reassurance as to the reliability of palaeogeographical reconstructions provided by palaeomagnetism.
SUMMARY
Studying historic lava flows provides a rare comparison between direct measurements of the Earth's magnetic field and the field information recorded in the rock record. Connecting direct and ...indirect measurements provides a way to bridge the gap between historic data in the last 50–100 yr to geologic data over kyr to Gyr. The field strength in Hawaii in 1960 was directly measured at the Honolulu observatory to be 36.47 µT, so our palaeointensity analysis of the 1960 flow is expected to give the same value. Two vertical sections of the 1960 flow (section 1 and section 2) were the focus of a previous microwave palaeointensity study. The microwave experiments were run using the non-standard, perpendicular modified Thellier-type protocol and produced either apparently good quality data that were biased to low values (section 1) or more scattered results averaging close to the expected value (section 2). The cause of the non-ideal behaviour observed in the data from the 1960 flow is a long-standing mystery that it is important to resolve to confirm the reliability of palaeointensity measurements in general, and the microwave demagnetization mechanism in particular. Here, we test the hypothesis that higher quality, unbiased (only random noise) measurements are possible using an improved Thellier-type protocol coupled to an updated microwave system. New palaeointensity experiments were performed primarily using the IZZI protocol (which allows alteration checks during the experiment) adapted for the microwave system. The specimens from section 1 produced more linear Arai plots and gave an estimate of $36.8 \pm 3.4$ µT, whereas those from section 2 gave an estimate of $39.1 \pm 4.6$ µT. Our new experiments demonstrate the microwave system's ability to produce accurate results and efficiently run any Thellier-style experiment. We investigate correcting perpendicular data for undetected alteration and find that using too strict selection criteria can be counterproductive to obtaining accurate and precise microwave palaeointensity results.
Nearly three decades ago paleomagnetists suggested that there existed a clear link between latitude dependence of geomagnetic paleosecular variation (PSV) and reversal frequency. Here we compare the ...latitude behavior of PSV for the Cretaceous Normal Superchron (CNS, 84–126 Ma, stable normal polarity) and the preceding Early Cretaceous‐Jurassic interval (pre‐CNS, 126–198 Ma, average reversal rate of ~4.6 Myr−1). We find that the CNS was characterized by a strong increase in the angular dispersion of virtual geomagnetic poles (VGPs) with latitude, which is consistent with the results of earlier studies, whereas the VGP dispersion for the pre‐CNS period was nearly invariant with latitude. However, the PSV behavior for the last 5 or 10 million years (average reversal frequency of ~4.4–4.8 Myr−1) shows that the latitude invariance of VGP scatter cannot be considered as a characteristic feature of a frequently reversing field and that a strong increase in VGP dispersion with latitude was not restricted to the long periods of stable polarity. We discuss models describing the latitude dependence of PSV and show that their parameters are not reliable proxies for reversal frequency and should not be used to make inferences about the geomagnetic field stability. During the pre‐CNS interval, the geodynamo may have operated in a regime characterized by a high degree of equatorial symmetry. In contrast, more asymmetric geodynamos suggested for 0–10 Ma and the CNS were evidently capable of producing a very wide range of reversal frequencies.
Plain Language Summary
In the geologic past, the changes in the Earth's magnetic field have led to numerous polarity reversals, causing the field directions over the entire Earth to be opposite to those observed today. More subtle changes during periods of stable field polarity are referred to as “secular variation.” It is widely thought that the manner in which secular variation changes with geographic latitude provides an indirect way of assessing the field stability with regard to its propensity to reverse. Here we derived estimates of paleosecular variation (PSV) for a long interval of stable polarity in the Cretaceous (84–126 Ma) and the preceding Early Cretaceous‐Jurassic interval (126–198 Ma), during which geomagnetic reversals were frequent. We found that the latitude behavior of PSV during these two intervals was significantly different, but the comparison with PSV estimates for the last 5 and 10 million years showed that a strong latitude dependence of PSV, or its invariance with latitude, cannot be considered as a characteristic feature of a stable or frequently reversing field. Our analysis suggests that models describing the latitude dependence of PSV do not provide reliable proxies for reversal frequency and should not be used to make inferences about the geomagnetic field stability.
Key Points
Estimates of geomagnetic paleosecular variation (PSV) in Cretaceous and Jurassic time were derived using a new paleomagnetic database
Different PSV latitude patterns were observed for periods of stable geomagnetic polarity (84–126 Ma) and frequent reversals (126–198 Ma)
Comparison with the PSV behavior for the last 10 Ma indicates that its latitude dependence is not a reliable proxy for reversal frequency
SUMMARY
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.
Plain Language Summary
Nearly synchronous global changes in geomagnetic polarity give both a detailed irregular pacing to geological time and provide a glimpse into heat transfer processes across the ...core—mantle boundary which drives the Earth's geodynamo. Although the Late Carboniferous is characterized by some well‐studied reversals, details of the tempo of polarity changes in the Early Carboniferous are unknown. This work addresses this by providing a detailed record of polarity changes over a ∼2 million year interval at around 334.5–332.5 million years ago‐from the Trowbarrow Quarry section in NW England. We demonstrate that these limestones likely preserve magnetization from close to their time of formation and record at least 31 polarity reversals. These observations support the idea that the Earth's dynamo was in a hyperactive reversing state similar to those sustained for tens of Myr in the Late Jurassic, parts of the Cambrian and the Late Ediacaran. It further corroborates a ∼200 Myr cyclicity in paleomagnetic field behavior since the Precambrian, potentially linked to variable core heat flow forced by mantle convection.
The pattern of geomagnetic polarity changes during the Early Carboniferous (Mississippian) is not known in detail. This information sparsity is addressed by determining a magnetostratigraphy from the late Asbian (late Visean at ∼333 Ma) in Trowbarrow Quarry, UK. This is the stratotype section of the late Asbian and has a detailed foraminiferal zonation based on the same set of paleomagnetic samples, establishing a detailed biostratigraphy. The 195 m‐thick section was sampled at an average spacing of 1.1 m, yielding a detailed magnetostratigraphy comprising nine major magnetozone couplets, and seven submagnetozones. The section dataset has a 78% bias to normal polarity determined from 177 sampling levels. The magnetization is carried by a mixture of hematite and detrital magnetite, with 68% of specimens dominated by hematite magnetizations. The primary magnetization passes a fold test showing its age was prior to the latest Carboniferous. The hematite is inferred to be largely of detrital, eolian origin, although some reddened levels are associated with emergent surfaces, suggesting that a small fraction of hematite is associated with platform emergence. The Mississippian age magnetization is partly overprinted with Kiaman Superchron‐age and Brunhes‐age magnetizations. Using the duration of the section based on astrochronology indicates a reversal frequency of 15.7 ± 0.75 Myr−1, indicating that the geodynamo was in a hyperactive reversing state between 335 and 333 Ma.
Key Points
A 195 m section, representing ∼2 Myr, yields a magnetostratigraphy comprizing nine major magnetozone couplets and seven submagnetozones
Primary magnetizations are predominantly carried by hematite with lesser important detrital magnetite. Data pass regional fold tests
A reversal frequency of 15.7 ± 0.75 Myr−1 is calculated, suggesting that the geodynamo was in a hyperactive state at ∼333 Ma
The state of the geomagnetic field throughout the Precambrian era is largely unknown. Approximately 8% of global paleointensity records account for ∼4 billion years of Earth history. Despite this ...severe sparsity, the data are used to constrain models that predict the timing of significant deep earth events such as inner core nucleation. This carries with it the assumption that the Precambrian paleomagnetic field was less variable when compared to the Phanerozoic, or at least that the sparse data can be averaged to accurately represent a particular time period. This study reports new paleointensities from the West Australian Craton at 755 Ma (the Mundine Wells dyke swarm) and 1,070 Ma (the Bangemall Sills); both of which occurred within ∼30 Ma from times at which extremely weak and anomalously strong fields, respectively, have been reported. Virtual dipole moments of 6.3 ± 0.1 Am2 × 1022 and 1.8 ± 1.2 Am2 × 1022 have been obtained from the two suites of mafic rock units which are substantially different to the previous measurements for the two respective ages. The findings suggest that field variability over tens of Myrs in the Precambrian was greater than has previously been assumed. This is supported by comparisons of paleosecular variation and distributions of virtual dipole moments. If variability in the Precambrian field is similar to that observed in the Phanerozoic, spatial or temporal anomalies may introduce significant bias to statistical analyses and model constraints, implying that caution should be employed in the interpretation of the Precambrian dipole moment records.
Key Points
New unexpected paleointensity data are obtained for key periods of the Precambrian which are in stark contrast to existing data
Combined data now suggest high Precambrian field variability, similar to that observed in the Phanerozoic
Implies that caution should be applied when using sparse data sets to infer or constrain key events in Earth history
SUMMARY
Elucidating the processes in the liquid core that have produced observed palaeointensity changes over the last 3.5 Gyr is crucial for understanding the dynamics and long-term evolution of ...Earth’s deep interior. We combine numerical geodynamo simulations with theoretical scaling laws to investigate the variation of Earth’s magnetic field strength over geological time. Our approach follows the study of Aubert et al., adapted to include recent advances in numerical simulations, mineral physics and palaeomagnetism. We first compare the field strength within the dynamo region and on the core–mantle boundary (CMB) between a suite of 314 dynamo simulations and two power-based theoretical scaling laws. The scaling laws are both based on a Quasi-Geostropic (QG) force balance at leading order and a Magnetic, Archimedian, and Coriolis (MAC) balance at first order and differ in treating the characteristic length scale of the convection as fixed (QG-MAC-fixed) or determined as part of the solution (QG-MAC-free). When the data set is filtered to retain only simulations with magnetic to kinetic energy ratios greater than at least two we find that the internal field together with the root-mean-square and dipole CMB fields exhibit power-law behaviour that is compatible with both scalings within uncertainties arising from different heating modes and boundary conditions. However, while the extrapolated intensity based on the QG-MAC-free scaling matches Earth’s modern CMB field, the QG-MAC-fixed prediction shoots too high and also significantly overestimates palaeointensities over the last 3.5 Gyr. We combine the QG-MAC-free scaling with outputs from 275 realizations of core–mantle thermal evolution to construct synthetic true dipole moment (TDM) curves spanning the last 3.5 Gyr. Best-fitting TDMs reproduce binned PINT data during the Bruhnes and before inner core nucleation (ICN) within observational uncertainties, but PINT does not contain the predicted strong increase and subsequent high TDMs during the early stages of inner core growth. The best-fitting models are obtained for a present-day CMB heat flow of 11–16 TW, increasing to 17–22 TW at 4 Ga, and predict a minimum TDM at ICN.