Geocentric Axial Dipole Research Articles

Conformably Variable Geocentric Axial Dipole at ca. 2.1 Ga: Paleomagnetic Dispersion of the Indin Dyke Swarm, Slave Craton

AbstractPrecambrian paleomagnetic studies are critical for testing paleogeographic reconstructions in deep time but rely on the fidelity of the assumption of the geocentric axial dipole (GAD) hypothesis. With high‐reliability data from mafic dykes and volcanic rocks, the scatter of individual virtual geomagnetic poles (VGPs) can be used to test simple GAD models. In order to conduct such a test, the VGPs must be adequate in number and in spatial coverage of the sampling sites. In this study, we targeted the 2.1 Ga Indin dyke swarm of the Slave craton. Building on previous sampling of the Indin dyke swarm in the western and central parts of southern Slave craton, we report results from 9 additional sites in the central and eastern parts of the craton, sites that significantly expand the width of the dyke swarm across the entire craton. The VGPs obtained from 7 of 9 newly identified Indin dykes are broadly similar to previously reported directions, expanding the total of VGPs for individual Indin dykes to n = 28, which is sufficient for a test of the GAD‐based statistical models using VGP scatter. The high VGP scatter of the Indin swarm can be attributed to the relatively high paleolatitude of 56° ± 6° for the Slave craton at the time of dyke emplacement. The Indin data have VGP scatter that is consistent with field models associated with the GAD hypothesis for the indicated paleolatitude, thus confirming the fidelity of the GAD field at ca. 2.1 Ga.

Coherent flow structures and magnetic field patterns in rotating spherical shell convective dynamos: A data-driven approach

The Earth's outer core dynamics involve convective fluid motion generating an observable geomagnetic field. The velocity and magnetic fields exhibit characteristic spatiotemporal features possessing geophysical significance for which extensive datasets are available from direct observations and computational simulations. This study demonstrates the robustness of proper orthogonal decomposition (POD), a data-driven technique, in detecting prominent and relevant features in these datasets. Improvising on previous practices, the POD efficiently detects infinitesimal instabilities at the onset of convection, providing an accurate and objective methodology to determine the convective threshold, even for heterogeneous buoyancy forcing. Time evolution of paired, phase-shifted modes efficiently reconstructs the azimuthally drifting of traveling wave instabilities. Simultaneously reduced order modeling of velocity components clearly distinguish the equatorial and polar coherent flow structures. Supercritical convection-driven magnetic field data over long periods, generated using numerical simulations, produce dominant modes that are more accurately representative of time-averaged patterns than geocentric axial dipole patterns. Moreover, the quantitative significance of the dominant modes determines the extent of dimensional reduction complementing established diagnostics for dipolarity. Finally, analysis of observational geomagnetic field data reveals long-lived dominant patterns influenced by thermal core–mantle interaction consistent with numerical models employing tomographic heat flux boundary conditions in present as well as previous studies.

Pliocene–Pleistocene Paleomagnetic Secular Variation and Time‐Averaged Field From the Northern Volcanic Zone of the Andes, Colombia

AbstractPaleomagnetic results obtained from 38 Pliocene—Pleistocene volcanic flows from the Knot of the Pastos and surroundings of Puracé volcano and Popayán (southwestern Colombia) are presented. Using stringent quality criteria and excluding sites that classify as representatives of transitional states of Earth's field, a selected group of 27 sites (16 with normal polarity and 11 with reversed polarity) was obtained with a mean direction (Dec = 357.8°, Inc = 6.4°, α95 = 7.5° and κ = 15) that coincides with the geocentric axial dipole field (GAD: Dec = 0o, Inc = 3.2o) and, unlike similar studies at similar latitudes, does not coincide with the GAD plus a 5% axial quadrupolar component (Dec = 0°, Inc = −1.08°). However, when serial correlation (SC) among several sites with high inclination anomalies is taken into consideration, the mean direction of two resulting groups of sites shows greater consistency with a field that includes a quadrupolar component. It is interpreted that the sites treated for SC record states of the field similar to today's field in the area of study, which is being affected by the South Atlantic Anomaly.

The palaeomagnetic field recorded in Eyjafjarðardalur basalts (2.6–8.0 Ma), Iceland: are inclination-shallowing corrections necessary in time-averaged field analysis?

SUMMARY The geocentric axial dipole (GAD) hypothesis is key to many palaeomagnetic applications, for example plate-tectonic reconstructions; however, the validity of this hypothesis at high latitudes is not fully resolved. To address this, in this paper we determined the palaeomagnetic directional data of 156 lava units in Eyjafjarðardalur, Iceland, with the aim of determining the validity of the GAD hypothesis at high latitudes using time-averaged field (TAF) analysis. In addition to the palaeomagnetic directional data, we constructed an age model for the sequences using new 40Ar/39Ar dates, magnetostratigraphy and field data. The sequence age range is 2.6–8.0 Ma. We show that the mean virtual geomagnetic pole (VGP) for our data does not agree with the GAD theory at 95 per cent confidence, when only the standard tilt and tectonic corrections are made; however, when inclination-shallowing processes are accounted for, for example thermoremanence (TRM) anisotropy and refraction effects, the mean VGP can align with GAD at 95 per cent confidence. These inclination-shallowing processes are shown to reduce the inclination by up to 14° for some of the basaltic units. Applying the inclination-shallowing correction also reduces VGP dispersion to levels that agree with global model predictions. We propose that much of the scatter within the palaeomagnetic directional databases are due to inclination-shallowing process effects, which become more important as the natural remanent magnetization (NRM) intensity is high, for example >2 A m−1. We propose that inclination-shallowing processes can be identified and corrected for by examining the NRM intensity and dispersion.

Simulations of 7Be and 10Be with the GEOS-Chem global model v14.0.2 using state-of-the-art production rates

Abstract. The cosmogenic radionuclides 7Be and 10Be are useful tracers for atmospheric transport studies. Combining 7Be and 10Be measurements with an atmospheric transport model can not only improve our understanding of the radionuclide transport and deposition processes but also provide an evaluation of the transport process in the model. To simulate these aerosol tracers, it is critical to evaluate the influence of radionuclide production uncertainties on simulations. Here we use the GEOS-Chem chemical transport model driven by the Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) reanalysis to simulate 7Be and 10Be with the state-of-the-art production rate from the CRAC:Be (Cosmic Ray Atmospheric Cascade: Beryllium) model considering realistic spatial geomagnetic cutoff rigidities (denoted as P16spa). We also perform two sensitivity simulations: one with the default production rate in GEOS-Chem based on an empirical approach (denoted as LP67) and the other with the production rate from the CRAC:Be but considering only geomagnetic cutoff rigidities for a geocentric axial dipole (denoted as P16). The model results are comprehensively evaluated with a large number of measurements including surface air concentrations and deposition fluxes. The simulation with the P16spa production can reproduce the absolute values and temporal variability of 7Be and 10Be surface concentrations and deposition fluxes on annual and sub-annual scales, as well as the vertical profiles of air concentrations. The simulation with the LP67 production tends to overestimate the absolute values of 7Be and 10Be concentrations. The P16 simulation suggests less than 10 % differences compared to P16spa but a significant positive bias (∼18 %) in the 7Be deposition fluxes over East Asia. We find that the deposition fluxes are more sensitive to the production in the troposphere and downward transport from the stratosphere. Independent of the production models, surface air concentrations and deposition fluxes from all simulations show similar seasonal variations, suggesting a dominant meteorological influence. The model can also reasonably simulate the stratosphere–troposphere exchange process of 7Be and 10Be by producing stratospheric contribution and 10Be/7Be ratio values that agree with measurements. Finally, we illustrate the importance of including the time-varying solar modulations in the production calculation, which significantly improve the agreement between model results and measurements, especially at mid-latitudes and high latitudes. Reduced uncertainties in the production rates, as demonstrated in this study, improve the utility of 7Be and 10Be as aerosol tracers for evaluating and testing transport and scavenging processes in global models. For future GEOS-Chem simulations of 7Be and 10Be, we recommend using the P16spa (versus default LP67) production rate.

A stacked record of relative palaeointensity for past 500 ka from western equatorial Indian Ocean sediments

SUMMARY Relative palaeointensity (RPI) records can reflect the evolution of the Earth's axial dipole field and provide a suitable template for global stratigraphic correlation. Current RPI records are primarily obtained from the Atlantic and Pacific Oceans, particularly in the mid-latitudes of the Northern Hemisphere. Fewer RPI records originate from the Indian Ocean, particularly in low latitude areas, which limits sediment dating and geomagnetic evolution studies in these regions. In this study, we conduct a palaeomagnetic study on four sediment cores recovered from the western equatorial Indian Ocean (WEIO) to establish a new regional RPI stack from the past 500 ka for the global coverage of palaeointensity data, as well as a regional reference for palaeointensity-assisted stratigraphy. To estimate the RPI, the isothermal remanent magnetization is used as a normalizer. A chronological framework is constructed by correlating RPI records and environmental magnetic parameters (anhysteretic remanent magnetizationsaturation/isothermal remanent magnetization, ARM/SIRM) with PISO-1500 and LR04 δ18O curves, respectively. Our RPI stacked curve (WEIO-500) varies according to global and other regional high-resolution records. The five lows in the obtained RPI curve can be correlated with previously reported excursions. In addition, the mean inclination direction of each core is similar to the expected hypothetical geocentric axial dipole direction, with a negative inclination anomaly (mean ΔI) of −2.18° to −4.86°. The low mean ΔI and its reproducibility reflect the reliability of our chronological framework and stacked curve, thereby providing a new reference for correlating and calibrating RPI records from WEIO as well as other equatorial areas.

A Bootstrap Common Mean Direction Test

AbstractPaleomagnetic statistical inference is underpinned by a family of parametric null hypothesis tests. In many cases, however, paleomagnetic data do not meet the distributional assumptions of these tests, which can lead to spurious inferences. Earlier studies have proposed the bootstrap as a nonparametric alternative for paleomagnetic analysis, which can be applied even when the distributional form of the data is unknown. Key among these approaches is the bootstrap test for a common mean direction, which relies on assessment of the overlap of estimated confidence regions. In its current form, the bootstrap test for a common mean paleomagnetic direction does not consider a null hypothesis and can yield outcomes that cannot be interpreted in terms of a statistical significance level. To resolve these issues, we use recent advances to place such bootstrap tests within a null hypothesis significance testing framework, and unify them with the existing family of paleomagnetic statistical tests. Furthermore, using numerical experiments we demonstrate the applicability of such a nonparametric approach to moderately sized paleomagnetic data sets typical of modern and legacy studies. Finally, we demonstrate how a confidence region can be estimated for the common mean of two sets of directions and how known directions, such as the expected field produced by a geocentric axial dipole, can be compared to that mean.

Late Cambrian geomagnetic instability after the onset of inner core nucleation

The Ediacaran Period marks a pivotal time in geodynamo evolution when the geomagnetic field is thought to approach the weak state where kinetic energy exceeds magnetic energy, as manifested by an extremely high frequency of polarity reversals, high secular variation, and an ultralow dipole field strength. However, how the geodynamo transitioned from this state into one with more stable field behavior is unknown. Here, we address this issue through a high-resolution magnetostratigraphic investigation of the ~494.5 million-year-old Jiangshanian Global Standard Stratotype and Point (GSSP) section in South China. Our paleomagnetic results document zones with rapid reversals, stable polarity and a ~80 thousand-year-long interval without a geocentric axial dipole field. From these changes, we suggest that for most of the Cambrian, the solid inner core had not yet grown to a size sufficiently large to stabilize the geodynamo. This unusual field behavior can explain paleomagnetic data used to define paradoxical true polar wander, supporting instead the rotational stability of the solid Earth during the great radiation of life in the Cambrian.

Changes in Non‐Dipolar Field Structure Over the Plio‐Pleistocene: New Paleointensity Results From Hawai'i Compared to Global Data Sets

AbstractA foundational assumption in paleomagnetism is that the Earth's magnetic field behaves as a geocentric axial dipole (GAD) when averaged over sufficient timescales. Compilations of directional data averaged over the past 5 Ma yield a distribution largely compatible with GAD, but the distribution of paleointensity data over this timescale is incompatible. Reasons for the failure of GAD include: (a) Arbitrary “selection criteria” to eliminate “unreliable” data vary among studies, so the paleointensity database may include biased results. (b) The age distribution of existing paleointensity data varies with latitude, so different latitudinal averages represent different time periods. (c) The time‐averaged field could be truly non‐dipolar. Here, we present a consistent methodology for analyzing paleointensity results and comparing time‐averaged paleointensities from different studies. We apply it to data from Plio/Pleistocene Hawai'ian igneous rocks, sampled from fine‐grained, quickly cooled material (lava flow tops, dike margins and scoria cones) and subjected to the IZZI‐Thellier technique; the data were analyzed using the Bias Corrected Estimation of Paleointensity method of Cych et al. (2021, https://doi.org/10.1029/2021GC009755), which produces accurate paleointensity estimates without arbitrarily excluding specimens from the analysis. We constructed a paleointensity curve for Hawai'i over the Plio/Pleistocene using the method of Livermore et al. (2018, https://doi.org/10.1093/gji/ggy383), which accounts for the age distribution of data. We demonstrate that even with the large uncertainties associated with obtaining a mean field from temporally sparse data, our average paleointensities obtained from Hawai'i and Antarctica (reanalyzed from Asefaw et al., 2021, https://doi.org/10.1029/2020JB020834) are not GAD‐like from 0 to 1.5 Ma but may be prior to that.

Palaeosecular variation in Northern Patagonia recorded by 0–5 Ma Caviahue–Copahue lava flows

SUMMARY Palaeosecular variation (PSV) determinations and studies of the geometry of the Earth’s main magnetic field provide important information about the field evolution, and to constrain numerical geodynamo models. Palaeomagnetic directional data from lava flows over the last few million years is of particular interest because the regional and global tectonic effects are minimal. However, the distribution of this type of palaeomagnetic data is uneven where the Southern Hemisphere is the destitute side. Therefore, the better knowledge of the geomagnetic field behaviour depends on the increased availability of high-quality data, especially in the Southern Hemisphere. A PSV and the time-averaged field (TAF) study was then performed in 0–5 Ma lava flows from the Caviahue–Copahue Volcanic Complex located in Northern Patagonia, Argentina (37°0′S, 71°10′W). The magnetic mineralogy of lava flows was investigated through thermomagnetic susceptibility curves, isothermal remanent magnetization (IRM) acquisition curves, hysteresis loops and first-order reversal curves (FORCs). Samples are essentially comprised of titanomagnetite with different Ti contents and magnetic domain structures typical of vortex state particles. A total of 50 volcanic sites were sampled, which provided 42 reliable palaeomagnetic site-mean directions after alternating field and thermal demagnetization. From these 42 sites, 36 are of normal and 6 are of reversed polarity. The mean direction from normal (reversed) sites is D = 356.2°, I = −50.1°, α95 = 4.0° and N = 36 (D = 176.5°, I = 59.5°, α95 = 14.1° and N = 6). Using only site-level data with the precision parameter k ≥ 100, we obtain 26 palaeomagnetic sites for PSV and TAF investigations in the study region. The filtered data set has a mean direction (D = 354.4°, I = −53.2° and α95 = 5.1°) close to the expected direction for a geocentric axial dipole (GAD) field (IGAD = −57.3°). The palaeopole (Plat = 84.4°, Plon = 229.1° and A95 = 5.7°) coincides with the Earth’s spin axis within the 95 per cent confidence interval. Virtual geomagnetic pole scatter ($S_{B} = {15.8^{18.9}_{11.8}}^{\circ }$) and the inclination anomaly ($\Delta I = 4.1_{-1.0}^{{9.2}^{\circ }}$) are both consistent at the 95 per cent confidence level with recent PSV and TAF models, respectively. Our results support the presence of small non-dipole field contributions (<3 per cent) superimposed on the GAD term, as reported by South American studies at mid southern latitudes.

Paleomagnetism of the Taseeva Group (Yenisei Ridge): on the Issue of the Geomagnetic Field Configuration at the Precambrian–Phanerozoic Boundary

Abstract —We report results of a detailed study of the paleomagnetic record in the sedimentary rocks of the Taseeva Group of the Yenisei Ridge in three typical sections in the lower courses of the Angara, Taseeva and Irkineeva rivers. Our results confirm that the geomagnetic field was in an anomalous state at the Precambrian–Phanerozoic boundary. It is well known that Ediacaran rocks in general have preserved several different paleomagnetic directions that do not conform to the geocentric axial dipole model. For example, Siberian sections display two equally valid groups of paleopoles that cause many debates over the geometry of the geomagnetic field and whether any of the components correspond to its dipole configuration. The paleomagnetic record we studied is unique in that the rocks of the Chistyakovka and Moshakovka formations have captured both these components, which is factual evidence of a synchronous existence of two sources. To explain these findings, we propose an original hypothesis in which the bipolar component that is widely present in the rocks and corresponds to the Madagascar group of paleomagnetic poles is associated to the field of the geocentric axial dipole. The less widespread monopolar component corresponding to the Australian–Antarctic group of poles is reflective of a stationary anomalous source. The recording of this source became possible due to the abrupt decrease in the strength of the virtual dipole moment that probably was at its lowest during the accumulation of the Chistyakovka and Moshakovka formations. The new paleomagnetic pole calculated for the bipolar component – 39.2°N, 61.1°E – plots on the apparent polar wander path for Siberia and can be considered a key determination for the age ~570 Ma.

Further evidence of unstable reverse polarity geomagnetic field across the Olduvai subchron: paleomagnetic and multispecimen paleointensity study on Khertvisi lava flows (Lesser Caucasus)

Goguitchaichvili et al. (Physics of the Earth and Planetary Interiors, doi.org/10.1016/j.pepi.2020.106641, 2021) reported a detailed rock-magnetic and paleointensity study of a Lesser Caucasus lava sequence comprised between 1.93 ± 0.09 and 1.78 ± 0.11 Ma. Reverse polarity magnetization determined for 20 consecutive lava flows permitted to suggest that the Olduvai subchron is probably disrupted by a short reverse-polarity episode. No paleointensity was obtained because of thermally unstable samples. Here, we report a detailed rock-magnetic, paleomagnetic, and multispecimen parallel differential pTRM absolute paleointensities performed on the nearby parallel Khertvisi lava succession. New isotopic age determinations indicate that the lavas of the Khertvisi section erupted in the Early Pleistocene, at the boundary of Gelasian and Calabrian ages in a time span of 1.88 ± 0.10 to 1.71 ± 0.12 Ma. All lava flows yielded well-defined reverse polarity magnetization with site mean directions Inc. = −56.1°, Dec = 189.5°, N = 20 (lava flows), α95 = 2.3°, k = 187. These directions are close to the Toloshi mean paleodirections, and both slightly deviate clockwise from the geocentric axial dipole and expected Plio-Quaternary directions. The values of the virtual geomagnetic pole (VGP) scatter parameters indicate that the studied sequence was formed during a very short time, most probably insufficient to average paleosecular variation. The multispecimen methodology provided a relatively low paleointensity for most analyzed lava flows, comparable to the transitional field intensity in Georgia during the Bruhnes and Matuyama chrons. These results definitively confirm a rather unstable, reverse polarity geomagnetic field regime across the Olduvai normal superchron. Thus, we propose to consider the Khertvisi section as the firm, volcanic evidence of Olduvai's short-lived geomagnetic event.

New archaeomagnetic directions from Late Neolithic sites in Shandong province, China

SUMMARY Archaeomagnetism provides important constraints to help us understand the past behaviour of the geomagnetic field. For archaeologists, archaeomagnetic dating has become a potential supplement to traditional dating methods (e.g. radiocarbon dating). Although China has a long history with numerous archaeological discoveries, the collection of archaeomagnetic data remains scarce. In this paper, we provide new archaeomagnetic directions from four late Neolithic (c. 2000 BC) sites in Shandong province, China. After a careful characterization of magnetic mineralogy and a detailed alternating-field demagnetization of the oriented samples, a total of nine archaeomagnetic directions (each with both declinations and inclinations) were obtained, which fill the large gap at c. 2000 BC in the Chinese palaeomagnetic secular variation (PSV) curve. Combining these new results with previously published data, we updated the Chinese PSV curve for the last 7 kyr. We have compared the updated curve with several global geomagnetic models (e.g. pfm9k.1a, ARCH10k.1, CALS10k.1b). Comparisons show that the CALS10k.1b model does not yield a reasonable fit of the data, and the fit becomes worse for older intervals. This poor fit could be explained by the fact that the CALS10k.1b model consists of a large amount of sedimentary data. Therefore, the PSV pattern is difficult to determine due to the strong aliasing effect. On the contrary, the ARCH10k.1 model gives a much better fit than other models because its data are mostly from archaeological materials and the data are mainly from the Northern Hemisphere. The field intensity and PSV are potentially correlated, with a weak field corresponding to an enhanced PSV. However, due to the lack of data for certain time intervals, the proposed correlations need to be further tested. To explore if PSV exhibits longitudinal symmetric or latitudinal antisymmetric patterns like those of the geocentric-axial-dipole (GAD) model, we compiled and compared data from three East Asia countries (China, Korea and Japan) and from four areas (East Asia, North America, Europe and the Middle East, and Australia and New Zealand) of which the latitudes are between 30° and 40°. In the East Asia region, the PSV patterns shown in each data set are consistent because of the geographic proximity of these three countries. However, when comparing the PSV curves from the four global areas, we suggest a potential declinational minimum between 0 AD and 2000 AD. Although further confirmation and investigation are needed, this declination minimum could be diachronous, sweeping from East Asia to Australia and New Zealand, and then North America. Future studies should focus on adding more reliable and precisely dated data to better delineate the PSV trends. Archaeomagnetic dating is promising when a PSV curve can be continuously reconstructed.

Paleomagnetism and paleolatitude of igneous drill core samples from the Izu-Bonin forearc and implications for Philippine Sea plate motion

The Philippine Sea plate is an integral part of the east Asian plate mosaic but its past motions can only be reconstructed from paleomagnetic data. Moreover, because the plate is largely submarine, few fully-oriented paleomagnetic data are available. International Ocean Discovery Program (IODP) Expedition 352 cored at four sites on the Izu-Bonin forearc, penetrating ∼1170 m of forearc igneous crust radiometrically dated at ∼51 Ma. Paleomagnetic measurements on 361 samples produced paleo-inclination data that were used to estimate paleolatitude. Twenty five independent magnetic units give a paleocolatitude of 90.1° ±4.4° (2σ uncertainty), implying a paleolatitude exactly on the equator. Unit variance agrees with global paleosecular variation models, implying that these variations have been properly sampled and averaged. The paleolatitude implies northward drift of ∼28°, which agrees with other Izu-Bonin forearc sites of similar age. A slight difference in northward drift with southern Philippine Sea plate sites is consistent with plate rotation. Certain parts of the cored sections yield samples that were remagnetized to the present geocentric axial dipole inclination after ∼30 Ma, implying a tectonic event of unclear origin.

Paleosecular variation record from Pleistocene-Holocene lava flows in southern Colombia

Improvements in the spatial and temporal coverage of paleomagnetic data are essential to better evaluate paleofield behaviour over the past 10 Myr, especially due to data scarcity at low latitudes in the South American region. Here, we provide new Pleistocene-Holocene (0–2 Ma age interval) paleodirectional data from three volcanic systems (Doña Juana Volcanic Complex, Galeras Volcanic Complex and Morasurco Volcano) in southwestern Colombia between latitudes 1.2 and 1.4°N. A total of 38 paleodirectional sites were studied using progressive alternating field and thermal demagnetization treatments. After excluding transitional data, we obtain thirty site-mean directions for analysis of paleosecular variation (PSV) and the time-averaged field (TAF) in the study area. The mean direction (Dec = 351.2°, Inc = −3.4°, α95 = 6.2°, k = 20.0) and the paleomagnetic pole (Plat = 80.7°N, Plon = 173.1°E, A95 = 5.2°, K = 29.1) of these sites are not statistically compatible with the expected geocentric axial dipole (GAD) field direction and geographic north pole, respectively. Virtual geomagnetic pole dispersion (SB) for our filtered dataset (SB(2Ma) = 15.212.017.6°) and the Brunhes chron (SB(Bru) = 16.011.619.1°) are consistent at the 95% confidence level with South American studies at equatorial latitudes and recent PSV models for the 0–10 Ma and Brunhes intervals. Likewise, the corresponding inclination anomaly (ΔI) for two age groups ΔI2Ma = − 5.9−12.10.3° and ΔIBru = − 5.3−13.73.1° suggests large deviations relative to the GAD model, in accordance with predictions from zonal TAF models. The high VGP dispersion could be linked to strong longitudinal variability of the magnetic equator position over South America. This feature reflects the presence of significant non-dipole field components in this region that have been detected in geomagnetic field models for the most recent centuries and millennia, probably associated with the presence of the South Atlantic Magnetic Anomaly in the South American region.

PSVM: A Global Database for the Miocene Indicating Elevated Paleosecular Variation Relative to the Last 10 Myrs

AbstractStatistical 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.

First Pre‐Miocene Paleomagnetic Data From the Calabrian Block Document a 160° Post‐Late Jurassic CCW Rotation as a Consequence of Left‐Lateral Shear Along Alpine Tethys

AbstractThe Calabrian block, along with Alboran, Kabylies, and Peloritani form isolated and enigmatic igneous/metamorphic terranes (AlKaPeCa) stacked over the Meso‐Cenozoic sedimentary successions of the Apennines and Maghrebides. They are commonly interpreted as fragments of the Hercynian chain rifted apart from Europe during Jurassic Alpine Tethys spreading, drifted southward during Neogene roll‐back of (Neo) Tethyan slab fragments for hundreds of kilometers on top of nappe piles. We report on the paleomagnetism of upper Triassic‐lower Miocene sedimentary rocks from the Longobucco succession that is transgressive over the crystalline Sila Massif (NE Calabria). Well‐defined magnetization directions carried by hematite were isolated in 10 sites (122 samples) in Jurassic rocks. Nine Toarcian and one Tithonian Ammonitico Rosso sites yielded a dual polarity “A” magnetization component whit a direction over 40° from the geocentric axial dipole (GAD) field direction, that supports a positive fold test. Five sites yielded a “B” normal polarity component NE (<40°) of the GAD direction characterized by a negative fold test. We interpret the B component as a Miocene magnetic overprint later clockwise rotated by ∼20° during the well‐known Pleistocene (1–2 Ma) rotation of Calabria. When corrected for such rotation, the A component defines a ∼160° counterclockwise (CCW) rotation of the Calabrian block with respect to Europe. Of these, ∼90° likely occurred along with Corsica‐Sardinia block during its Eocene‐Miocene rotation from the Provençal margin. Thus, the Calabrian block underwent an additional Cretaceous‐Eocene 70° CCW rotation that we relate to Early‐mid Cretaceous >500 km left‐lateral transcurrent motion between Africa and Europe.

Low Paleointensities and Ar/Ar Ages From Saint Helena Provide Evidence for Recurring Magnetic Field Weaknesses in the South Atlantic

AbstractThe 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.

Characterizing the Geomagnetic Field at High Southern Latitudes: Evidence From the Antarctic Peninsula

AbstractDue to a dearth of data from high‐latitude paleomagnetic sites, it is not currently clear if the geocentric axial dipole (GAD) hypothesis accurately describes the long‐term behavior of the geomagnetic field at high latitudes. Here we present new paleomagnetic and paleointensity data from the James Ross Island (JRI) volcanic group, located on the Antarctic Peninsula. This data set addresses a notable lack of data from the 60°–70°S latitude bin and includes 251 samples from 31 sites, spanning 0.99–6.8 Ma in age. We also include positive fold, conglomerate, and baked contact tests. Paleointensity data from three methods (Thellier‐Thellier, pseudo‐Thellier, and Tsunakawa‐Shaw) were collected from all sites. The Thellier‐Thellier method had low yields and produced unreliable data, likely due to sample alteration during heating. Results from the Tsunakawa‐Shaw and pseudo‐Thellier methods were more consistent, and we found a bimodal distribution of paleointensity estimates. Most sites yielded either <15 μT or >40 μT, which together span a range of estimates from long‐term geomagnetic field models, but do not favor any model in particular. Alternating‐field demagnetization of these samples, when combined with preexisting data, yields a revised paleomagnetic pole of −87.5°, 025°, α95 = 3.6° for the Antarctic Peninsula over the last ∼5 Ma, which suggests that the current data set is sufficiently large to “average out” secular variation. Finally, the C2r/C2n transition was probably found at a site on JRI, and further geochronological and paleomagnetic study of these units could refine the age of this reversal.

A Holocene palaeomagnetic secular variation record from Lake Pupuke, New Zealand

ABSTRACT Palaeomagnetic records from three cores of lake sediment have been merged, producing a new Holocene geomagnetic secular variation record for northern New Zealand. The cores are from Lake Pupuke, a maar lake in Auckland Volcanic Field (AVF). They contain tephra from Rangitoto Volcano (530 ± 10 yr BP), Taupō (1718 ± 10 yr BP), Tūhua/Mayor Island (7637 ± 100 yr BP) and Rotomā tephra from Ōkataina Volcanic Centre (9423 ± 120 yr BP). These tephra dates are supplemented by selected radiocarbon age estimates. The cores were correlated using tephra and fine-scale variations in magnetic susceptibility. The natural remanent magnetisation is strong, stable and carried by fine-grained titanomagnetite from AVF’s basaltic volcanoes. The resulting palaeosecular variation (PSV) record spans from 10,000 to 1500 yr BP. It shows well-defined, millennial-scale swings in direction between 10,000 and 8000 yr BP and between 4000 and 1500 yr BP, while between 8000 and 4000 yr BP variations are of higher frequency and lower amplitude and centre on the geocentric axial dipole field direction. This is in excellent agreement with published PSV records from Mavora Lakes, 1000 km further south, indicating the regional nature of PSV – resulting from broad-scale changes in the circulation of conducting iron-rich fluid of Earth’s outer core.