Axial Dipole Field Research Articles

Can one use Earth’s magnetic axial dipole field intensity to predict reversals?

SUMMARY We study predictions of reversals of Earth’s axial magnetic dipole field that are based solely on the dipole’s intensity. The prediction strategy is, roughly, that once the dipole intensity drops below a threshold, then the field will continue to decrease and a reversal (or a major excursion) will occur. We first present a rigorous definition of an intensity threshold-based prediction strategy and then describe a mathematical and numerical framework to investigate its validity and robustness in view of the data being limited. We apply threshold-based predictions to a hierarchy of numerical models, ranging from simple scalar models to 3-D geodynamos. We find that the skill of threshold-based predictions varies across the model hierarchy. The differences in skill can be explained by differences in how reversals occur: if the field decreases towards a reversal slowly (in a sense made precise in this paper), the skill is high, and if the field decreases quickly, the skill is low. Such a property could be used as an additional criterion to identify which models qualify as Earth-like. Applying threshold-based predictions to Virtual Axial Dipole Moment palaeomagnetic reconstructions (PADM2M and Sint-2000) covering the last two million years, reveals a moderate skill of threshold-based predictions for Earth’s dynamo. Besides all of their limitations, threshold-based predictions suggests that no reversal is to be expected within the next 10 kyr. Most importantly, however, we show that considering an intensity threshold for identifying upcoming reversals is intrinsically limited by the dynamic behaviour of Earth’s magnetic field.

A full sequence of the Matuyama\u2013Brunhes geomagnetic reversal in the Chiba composite section, Central Japan

Geological records of the Matuyama–Brunhes (M–B) geomagnetic reversal facilitate the development of an age model for sedimentary and volcanic sequences and help decipher the dynamics of the Earth’s magnetic field. However, the structure of the geomagnetic field during the M–B geomagnetic reversal remains controversial due to its complex field behavior. In this study, we conducted paleo- and rock-magnetic analyses of samples from the Chiba composite section (CbCS), a continuous and expanded marine succession in Central Japan, to reconstruct the full sequence of the M–B geomagnetic reversal. We define an average stratigraphic position of the M–B boundary and estimate its age based on three sections in the CbCS and a neighboring drill core, TB-2. The average stratigraphic position of the M–B boundary in the CbCS is established at 1.1 ± 0.3 m above a widespread volcanic ash bed (the Byk-E tephra). Assuming a chronological error associated with orbital tuning of 5 kyr and stratigraphic uncertainty of 0.4 kyr, the M–B boundary in CbCS is at 772.9 ± 5.4 ka (1σ). The virtual geomagnetic pole, which is calculated from the paleomagnetic directions, shows several short fluctuations between 783 and 763 ka, with concomitant decreases in geomagnetic field intensity index. After termination of the field instabilities, the field intensity recovered and became higher than before the M–B boundary, with a stable normal polarity direction. The paleomagnetic records in the CbCS exhibit a field asymmetry between the axial dipole decay and field recovery, providing a full sequence of the M–B reversal, suggesting that the non-axial dipole field dominated several times during periods ca. 20 kyr long across the M–B boundary, due to depletion in the main axial dipole component. Our results provide probably the most detailed sedimentary record of the M–B geomagnetic reversal and offer valuable information to further understand the mechanism and dynamics of geomagnetic reversals.Graphical abstract

Triassic (Anisian and Rhaetian) palaeomagnetic poles from the Germanic Basin (Winterswijk, the Netherlands)

In this paper, we provide two new Triassic palaeomagnetic poles from Winterswijk, the Netherlands, in the stable interior of the Eurasian plate. They were respectively collected from the Anisian (~ 247–242 Ma) red marly limestones of the sedimentary transition of the Buntsandstein Formation to the dark grey limestones of the basal Muschelkalk Formation, and from the Rhaetian (~ 208–201 Ma) shallow marine claystones that unconformably overlie the Muschelkalk Formation. The magnetization is carried by hematite or magnetite in the Anisian limestones, and iron sulfides and magnetite in the Rhaetian sedimentary rocks, revealing for both a large normal polarity overprint with a recent (geocentric axial dipole field) direction at the present latitude of the locality. Alternating field and thermal demagnetization occasionally reveal a stable magnetization decaying towards the origin, interpreted as the Characteristic Remanent Magnetization. Where we find a pervasive (normal polarity) overprint, we can often still determine well-defined great-circle solutions. Our interpreted palaeomagnetic poles include the great-circle solutions. The Anisian magnetic pole has declination D ± ∆Dx = 210.8 ± 3.0°, inclination I ± ∆Ix = − 26.7 ± 4.9°, with a latitude, longitude of 45.0°, 142.0° respectively, K = 43.9, A95 = 2.9°, N = 56. The Rhaetian magnetic pole has declination D ± ∆Dx = 32.0 ± 8.7°, inclination I ± ∆Ix = 50.9 ± 8.1°, with a latitude, longitude of 60.6°, 123.9° respectively, K = 19.3, A95 = 7.4°, N = 21. The poles plot close to the predicted location of global apparent polar wander paths (GAPWaPs) in Eurasian coordinates and are feasible for future apparent polar wander path construction. They confirm that the intracontinental, shallow-marine Germanic Basin, in which the Muschelkalk Formation was deposited, existed at a palaeolatitude of 14.1° [11.3, 17.1] N, in a palaeo-environment reminding of the Persian Gulf today. In Rhaetian times, palaeolatitudes of 31.6° [24.8, 39.8] N were reached, on its way to the modern latitude of 52°N.

Paleomagnetism and rock magnetic properties of Late Pleistocene volcanism from El Pinacate Volcanic Field, northwest Mexico

The Pinacate Volcanic Field is a Neogene volcanic sequence at the northwestern part of Mexico. This paleomagnetic study (directional and paleointensity) will continue to improve the global paleomagnetic databases. It is the first full vector paleomagnetic analysis within the more than 400 volcanic structures of the Pinacate Volcanic Field, e.g. scoria, cinder cones, large ‘pahoehoe’ and ‘aa’ lava flows. A total of 140 oriented standard paleomagnetic cores from 16 sites of lava flows were collected. Rock magnetic experiments such as thermomagnetic analyses show that Ti-poor titanomagnetite is the main magnetic carrier of the remanence. Microscopy of oxide minerals supports this observation. Parameters from hysteresis and FORC diagrams suggest pseudo-single domain grain sizes or a mixture of single and multidomain particles. The directional analysis of 11/12 volcanic structures provides a mean paleomagnetic direction and corresponding mean paleomagnetic pole (calculated from the site VGPs): Dec = 0.1°, Inc = 54.5°, n = 11, α95 = 6.7°, k = 48, Plat = 86.5°, Plong = 247.8°, A95 = 7.5°, respectively. The mean paleointensity, obtained by using the Thellier-Coe protocol, and estimated from seven volcanic structures is 42.2 ± 3.6 μT, and VDM is 8.1 × 1022Am2. The obtained directions are compatible with the Late Pleistocene paleodirection as expected from the geocentric axial dipole (GAD) field. The virtual geomagnetic pole scatter (Sb) was calculated at 13.5° which is consistent with the paleosecular variation value expected from El Pinacate latitude.

Paleomagnetism and Paleosecular Variations From the Plio‐Pleistocene Golan Heights Volcanic Plateau, Israel

AbstractStatistical analysis of geomagnetic paleosecular variation (PSV) and time‐averaged field has been largely based on global compilations of paleomagnetic data from lava flows. These show different trends in the averaged inclination anomaly (ΔI) between the two hemispheres, with small positive (<2°) anomalies in midsouthern latitudes and large negative (> −5°) anomalies in midnorthern latitudes. To inspect the large ΔI between 20°N and 40°N we augment the global data with a new paleomagnetic data set from the Golan‐Heights (GH), a Plio‐Pleistocene volcanic plateau in northeast Israel, located at 32–33°N. The GH data set consists of 91 lava flows sites: 40 sites obtained in the 1990s and 51 obtained in this study. The chronology of the flows is constrained by 57 40Ar/39Ar ages: 39 from previous studies and 18 from this study, which together cover most of the GH plateau. We show that the 1990s data set might be affected by block rotations and does not fully sample PSV. The Plio‐Pleistocene pole (86.3°N, 120.8°E, N = 44, k = 25, α95 = 4.4°), calculated after applying selection criteria with Fisher precision parameter (k) ≥ 100 and number of specimens per site (n) ≥ 5 is consistent with a geocentric axial dipole field and shows smaller inclination anomaly (ΔI = −0.4°) than predicted by global compilations and PSV models. Reexamination of the inclination anomaly in the global compilation using different calculation methods and selection criteria suggests that inclination anomaly values are affected by (1) inclusion of poor quality data, (2) averaging data by latitude bins, and (3) the way the inclination anomaly is calculated.

A comprehensive model for the kyr and Myr timescales of Earth's axial magnetic dipole field

Abstract. We consider a stochastic differential equation model for Earth's axial magnetic dipole field. Our goal is to estimate the model's parameters using diverse and independent data sources that had previously been treated separately, so that the model is a valid representation of an expanded paleomagnetic record on kyr to Myr timescales. We formulate the estimation problem within the Bayesian framework and define a feature-based posterior distribution that describes probabilities of model parameters given a set of features derived from the data. Numerically, we use Markov chain Monte Carlo (MCMC) to obtain a sample-based representation of the posterior distribution. The Bayesian problem formulation and its MCMC solution allow us to study the model's limitations and remaining posterior uncertainties. Another important aspect of our overall approach is that it reveals inconsistencies between model and data or within the various data sets. Identifying these shortcomings is a first and necessary step towards building more sophisticated models or towards resolving inconsistencies within the data. The stochastic model we derive represents selected aspects of the long-term behavior of the geomagnetic dipole field with limitations and errors that are well defined. We believe that such a model is useful (besides its limitations) for hypothesis testing and give a few examples of how the model can be used in this context.

Robust Characteristics of the Laschamp and Mono Lake Geomagnetic Excursions: Results From Global Field Models

Data-based global palaeomagnetic field models provide a more complete view of geomagnetic excursions than individual records. They allow the temporal and spatial field evolution to be mapped globally, and facilitate investigation of dipole and non-dipole field components. We have developed a suite of spherical harmonic (SH) field models that span 50 to 30 ka and include the Laschamp (~41 ka) and Mono Lake (~33 ka) excursions. Palaeomagnetic field models depend heavily on the data used in their construction. Variations in palaeomagnetic sediment records from the same region are in some cases inconsistent. To test the influence of data selection and reliance on age models, we have built a series of SH models based upon different data sets. A number of excursion characteristics are robust in all models, despite some differences in energy distribution among SH coefficients. Quantities such as field morphology at the core-mantle boundary (CMB) or individual SH degree power variations should be interpreted with caution. All models suggest that the excursion process during the Laschamp is mainly governed by axial dipole decay and recovery, without a significant influence from the equatorial dipole or non-dipole fields. The axial dipole component reduces to almost zero, but does not reverse. This results in excursional field behaviour seen globally, but non-uniformly at Earth's surface. The Mono Lake excursion may be a series of excursions occurring between 36 and 30 ka rather than a single excursion. In contrast to the Laschamp, these excursions appear driven by smaller decreases in axial dipole field strength during a time when the axial dipole power at the CMB is similar to the power in the non-dipole field. We suggest three phases for the 50 to 30 ka period: (1) a broadly stable phase dominated by the axial dipole (50-43 ka); (2) the Laschamp excursion, with the underlying excursion process lasting ~5 ka (43-38 ka) and the surface field expression lasting ~2 ka (42-40 ka); (3) a weak phase during which axial dipole and non-dipole power at the CMB are comparable, leading to more than one excursion between 36 and 30 ka.

Paleomagnetic Evidence for Inverse Correspondence between the Relative Contribution of the Axial Dipole Field and CMB Heat Flux for the Past 270 Myr

We provide an evaluation of the paleolatitudinal dependence of the paleosecular variation throughout the Paleozoic-Mesozoic transition – linked to the high geomagnetic reversal frequency interval Illawarra Hyperzone of Mixed Polarity (IHMP; ~266.7–228.7 Myr). Our findings were compared with those for intervals of distinctive geomagnetic reversal frequencies within the Phanerozoic. Our results for the IHMP were conducted through estimates of angular dispersion (SB) of virtual geomagnetic pole (VGP) data groups, taken from a high quality paleomagnetic database. Model G was fitted to these data, providing its shape parameters a and b (respectively related to the antisymmetric and symmetric harmonic terms for the time-average geomagnetic field). Results for the IHMP exhibited compatible patterns with two well-known intervals of higher reversal frequency – Jurassic and the last 5 Myr. A comparison of b/a ratio results – considered as an efficient indicator for the relative contribution of the axial dipole field – for the last 270 Myr, indicated an inverse correspondence with the relative core-mantle boundary (CMB) heat flux, according to recent discussions, clarifying the physical meaning of the Model G shape parameters a and b.

Probabilistic structure of the geodynamo

One of the most intriguing features of Earth's axial magnetic dipole field, well-known from the geological record, is its occasional and unpredictable reversal of polarity. Understanding the phenomenon is rendered very difficult by the highly non-linear nature of the underlying magnetohydrodynamic problem. Numerical simulations of the liquid outer core, where regeneration occurs, are only able to model conditions that are far from Earth-like. On the analytical front, the situation is not much better; basic calculations, such as relating the average rate of reversals to various core parameters, have apparently been intractable. Here, we present a framework for solving such problems. Starting with the magnetic induction equation, we show that by considering its sources to be stochastic processes with fairly general properties, we can derive a differential equation for the joint probability distribution of the dominant toroidal and poloidal modes. This can be simplified to a Fokker-Planck equation and, with the help of an adiabatic approximation, reduced even further to an equation for the dipole amplitude alone. From these equations various quantities related to the magnetic field, including the average reversal rate, field strength, and time to complete a reversal, can be computed as functions of a small number of numerical parameters. These parameters in turn can be computed from physical considerations or constrained by paleomagnetic, numerical, and experimental data.

Paleomagnetism of Holocene lava flows from Los Humeros caldera, eastern Mexico: Discrimination of volcanic eruptions and their age dating

Los Humeros caldera in eastern Mexico has been formed since about 164 ka, with important effusive activity during the upper Holocene, which covered more than 100 km2 with lava. Two lava flows were 14C dated at 2692-1950 cal BCE and 1117-930 cal BCE, but the detailed succession of eruptions is yet unknown even though the interest for volcanic risk assessment. All Holocene lava flows were studied paleomagnetically in 22 sites to determine which of them have been contemporaneous and to apply the paleomagnetic dating method. For most sites well defined mean directions could be defined, resulting in an overall paleodirection of D = 7.7° E, I = 16.9°, α95 = 6.7°. This direction is discordant compared to the geocentric axial dipole field and indicates that secular variation has not averaged out. Paleointensities were obtained using the Thellier IZZI protocol, with only a small success rate of 27% due to thermally induced alterations, the presence of multidomain magnetic minerals, or unstable paleomagnetic directions as indicated by the checks during the experiments. Accepted paleointensities of 37 samples varied between 21 and 63 μT, with site mean paleointensity of 27–54 μT. The F-test was applied to mean directions of sites potentially belonging to the same lava flow, and if these were indistinguishable they were combined in flow mean paleomagnetic directions. Otherwise, these sites were assumed to represent independent eruptions. This way, thirteen independent flow mean directions were obtained and used for paleomagnetic dating by means of the SHA.DIF.14k reference model and the archaeo_dating software. The dating often provided several possible age ranges, and erosion state, soil cover, lava flow contacts, and the available 14C ages were used all together for selecting the most probable age range. Two well defined eruptive periods bracketed between 2500 and 1900 BCE, and 1300-600 BCE were defined, suggesting that all studied lava flows were emplaced during two main eruptive phases, which may indeed have been much shorter than 600 years. Paleomagnetic data support those eruptive periods and time among the different lava flow units can be inferred from those data. Early settlers occupied this region since about 2000 BCE and thus were certainly affected by these voluminous eruptions. At least another 12 nearby Holocene monogenetic volcanoes are known in the surrounding area, and all together these eruptions indicate that this is a highly active volcanic region, which might be affected by eruptions in the near future.

The First Catalog of Archaeomagnetic Directions From Israel With 4,000 Years of Geomagnetic Secular Variations

The large and well-studied archaeological record of Israel offers a unique opportunity for collecting high resolution archaeomagnetic data from the past several millennia. Here, we initiate the first catalog of archaeomagnetic directions from Israel, with data covering the past four millennia. The catalog consists of 76 directions, of which 47 fulfill quality selection criteria with Fisher precision parameter (k) ≥ 60, 95% cone of confidence (α95) < 6° and number of specimens per site (n) ≥ 8. The new catalog complements our published paleointensity data from the Levant and enables testing the hypothesis of a regional geomagnetic anomaly in the Levant during the Iron Age proposed by Shaar et al (2016, 2017). Most of the archaeomagnetic directions show <15° angular deviations from an axial dipole field. However, we observe in the 10th and 9th century BCE short intervals with field directions that are 19°-22° different from an axial dipole field and inclinations that are 20°-22° steeper than an axial dipole field. The beginning of the first millennium BCE is also characterized with fast secular variation rates. The new catalog provides additional support to the Levantine Iron Age Anomaly hypothesis.

Spectral methods for analyzing energy balances in geodynamo simulations

The geomagnetic field displays complicated variations over a broad range of frequencies. These variations can be decomposed by frequency and linked to physical processes using frequency domain spectral methods. These spectral methods are well developed but have not previously been applied to study the energy balance of geodynamo simulations. We illustrate their potential by analyzing output from numerical dynamo simulations that have previously been studied for their apparently Earth-like properties. We show that high coherence between variations in axial dipole energy at the outer boundary of the simulation and total magnetic energy within the fluid shell occur at frequencies below ∼0.1 kyr−1. This suggests that paleomagnetically-observable signals with periods exceeding 10 kyrs contain information about magnetic energy changes in the bulk core. We then use spectral analysis to investigate differences in the rate of growth and decay of the axial dipole field. This behaviour, characterised by rapid growth and slow decay, is observed when signals with frequencies higher than 0.03 kyr−1 have been filtered out. The origin of this asymmetric growth and decay is assessed using coherence spectra between rates of change in kinetic and magnetic energy, ohmic and viscous dissipation, and work done by the buoyancy and Lorentz forces. We show that asymmetry is associated with an imbalance between ohmic dissipation and work done by the Lorentz force; when changes in magnetic energy are more coherent with ohmic dissipation the field grows rapidly and decay slowly. Variations in Ohmic dissipation reflect changes in field strength in our models, while changes in viscous dissipation are associated with amplitude fluctuations of the large-scale flow that exists on millennial timescales. Our work shows that spectral analysis coupling observable and global products of the dynamo process can elucidate the physical origin of periodic processes occurring on timescales exceeding 10 kyrs.

A generating process of geomagnetic drifting field

The geomagnetic field is comprised of drifting and standing fields. The drifting field has two remarkable features. One is predominance of sectorial harmonics when the field is expressed in a spherical harmonic series, and the other is uniform drift rate irrespective of harmonics. We consider that the drifting field is a product of interaction of the core flow with the axial dipole field near the surface of the core. The key to the predominance of sectorial harmonics is in the boundary condition on the electric current at the core–mantle boundary. If we take the mantle to be an electrical insulator, the electric current normal to the boundary must vanish. This strongly constrains the surface flow. The toroidal flow becomes the flow with the sectorial harmonics predominant. Then, the sectorial toroidal flow, interacting with the axial dipole field, induces the poloidal field in which the sectorial harmonics are predominant. This is the observed type of drifting field. The uniform drift rate, the second nature of the drifting field, seems to suggest that the surface part of the core is rotating westwards as a whole. Subsequently, the sectorial type toroidal flow embedded in the westward-rotating surface layer is considered as the cause of the drifting field.

Preliminary paleomagnetic and rock magnetic results from 17 to 22\xa0ka sediment of Jeju Island, Korea: Geomagnetic excursional behavior or rock magnetic anomalies?

Paleomagnetic and rock magnetic investigations were performed on a 64-cm-thick section of nonmarine unconsolidated muddy sediment from the Gosan Formation on Jeju Island, Korea. This sediment was recently dated to have been deposited between 22 and 17 kyr BP calibrated, with a sedimentation rate of 13–25 cm/kyr, based on many radiocarbon ages. Interestingly, stepwise alternating field (AF) demagnetization revealed characteristic natural remanent magnetizations with anomalous directions, manifested by marked deviations from the direction of today’s axial dipole field, for some separate depth levels. On the other hand, stepwise thermal (TH) demagnetization showed more complex behavior, resulting in the identification of multiple remanence components. For all TH-treated specimens, consistently two different components are predominant: a low-temperature component unblocked below 240–320 °C entirely having normal-polarity apparently within the secular variation range of the Brunhes Chron, and a high-temperature component with unblocking temperatures (Tubs) between 240–320 and 520–580 °C that have anomalous directions, concentrated in the ~ 13–34-cm-depth interval (~ 17–19 ka in inferred age) and possibly below ~ 53 cm depth (before ~ 20 ka). Rock magnetic results also infer the dominance of low-coercivity magnetic particles having ~ 300 and ~ 580 °C Curie temperature as remanence carriers, suggestive of (titano)maghemite and/or Ti-rich titanomagnetite and magnetite (or Ti-poor titanomagnetite), respectively. A noteworthy finding is that AF demagnetizations in this study often lead to incomplete separation of the two remanence components possibly due to their strongly overlapping AF spectra. The unusual directions do not appear to result from self-reversal remanences. Then, one interpretation is that the low-temperature components are attributable to post-depositional chemical remanences, associated possibly with the later formation of the mineral phase having Tub ~ 300 °C, whereas the high-temperature components are of primary detrital origin that survived later chemical influence. Accordingly, the unusual directions might record geomagnetic instability within the ~ 17–22 ka period manifested by multiple excursional swings, partly associated with the Tianchi/Hilina Pali excursion. However, further work is needed to verify this interpretation and distinguish it from alternative explanations that invoke rock magnetic complexities as the cause of the unusual directions.

A Probabilistic Assessment of the Next Geomagnetic Reversal

AbstractDeterministic forecasts for the next geomagnetic reversal are not feasible due to large uncertainties in the present‐day state of the Earth's core. A more practical approach relies on probabilistic assessments using paleomagnetic observations to characterize the amplitude of fluctuations in the geomagnetic dipole. We use paleomagnetic observations for the past 2 Myr to construct a stochastic model for the axial dipole field and apply well‐established methods to evaluate the probability of the next geomagnetic reversal as a function of time. For a present‐day axial dipole moment of 7.6 × 1022 A m2, the probability of the dipole entering a reversed state is less than 2% after 20 kyr. This probability rises to 11% after 50 kyr. An imminent geomagnetic reversal is not supported by paleomagnetic observations. The current rate of decline in the dipole moment is unusual but within the natural variability predicted by the stochastic model.

A high-resolution paleosecular variation record from Black Sea sediments indicating fast directional changes associated with low field intensities during marine isotope stage (MIS) 4

A total of nine sediment cores recovered from the Archangelsky Ridge in the SE Black Sea were systematically subjected to intense paleo- and mineral magnetic analyses. Besides 16 accelerator mass spectrometry (AMS) 14C ages available for another core from this area, dating was accomplished by correlation of short-term warming events during the last glacial monitored by high-resolution X-ray fluorescence (XRF) scanning as maxima in both Ca/Ti and K/Ti ratios in Black Sea sediments to the so-called ‘Dansgaard–Oeschger events’ recognized from Greenland ice cores. Thus, several hiatuses could be identified in the various cores during the last glacial/interglacial cycle. Finally, core sections documenting marine isotope stage (MIS) 4 at high resolution back to 69 ka were selected for detailed analyses. At 64.5 ka, according to obtained results from Black Sea sediments, the second deepest minimum in relative paleointensity during the past 69 ka occurred, with the Laschamp geomagnetic excursion at 41 ka being associated with the lowest field intensities. The field minimum during MIS 4 is associated with large declination swings beginning about 3 ka before the minimum. While a swing to 50°E is associated with steep inclinations (50–60°) according to the coring site at 42°N, the subsequent declination swing to 30°W is associated with shallow inclinations of down to 40°. Nevertheless, these large deviations from the direction of a geocentric axial dipole field (I=61°, D=0°) still can not yet be termed as ‘excursional’, since latitudes of corresponding virtual geomagnetic poles (VGP) only reach down to 51.5°N (120°E) and 61.5°N (75°W), respectively. However, these VGP positions at opposite sides of the globe are linked with VGP drift rates of up to 0.2° per year in between. These extreme secular variations might be the mid-latitude expression of a geomagnetic excursion with partly reversed inclinations found at several sites much further North in Arctic marine sediments between 69°N and 81°N. Thus, the pronounced intensity minimum at 64.5 ka and described directional variations might be the effect of a weak geomagnetic field with a multi-polar geometry in the middle of MIS 4.

Fast geomagnetic field intensity variations between 1400 and 400 BCE: New archaeointensity data from Germany

Thirty-five mean archaeointensity data were obtained on ceramic sherds dated between 1400 and 400 BCE from sites located near Munich, Germany. The 453 sherds were collected from 52 graves, pits and wells dated by archaeological correlation, radiocarbon and/or dendrochronology. Rock magnetic analyses indicate that the remanent magnetization was mainly carried by magnetite. Data from Thellier-Thellier experiments were corrected for anisotropy and cooling rate effects. Triaxe and multispecimen (MSP-DSC) protocols were also measured on a subset of specimens. Around 60% of the samples provide reliable results when using stringent criteria selection. The 35 average archaeointensity values based on 154 pots are consistent with previous data and triple the Western Europe database between 1400 and 400 BCE. A secular variation curve for central-western Europe, built using a Bayesian approach, shows a double oscillation in geomagnetic field strength with intensity maxima of ∼70µT around 1000-900 BCE and another up to ∼90µT around 600-500 BCE. The maximum rate of variation was ∼0.25µT/yr circa 700 BCE. The secular variation trend in Western Europe is similar to that observed in the Middle East and the Caucasus except that we find no evidence for hyper-rapid field variations (i.e. geomagnetic spikes). Virtual Axial Dipole Moments from Western Europe, the Middle East and central Asia differ by more than 2·1022A·m2 prior to 600 BCE, which signifies a departure from an axial dipole field especially between 1000 and 600 BCE. Our observations suggest that the regional Levantine Iron Age anomaly has been accompanied by an increase of the axial dipole moment together with a tilt of the dipole.

An activity index for geomagnetic paleosecular variation, excursions, and reversals

AbstractMagnetic indices provide quantitative measures of space weather phenomena that are widely used by researchers in geomagnetism. We introduce an index focused on the internally generated field that can be used to evaluate long term variations or climatology of modern and paleomagnetic secular variation, including geomagnetic excursions, polarity reversals, and changes in reversal rate. The paleosecular variation index, Pi, represents instantaneous or average deviation from a geocentric axial dipole field using normalized ratios of virtual geomagnetic pole colatitude and virtual dipole moment. The activity level of the index, , provides a measure of field stability through the temporal standard deviation of Pi. Pi can be calculated on a global grid from geomagnetic field models to reveal large scale geographic variations in field structure. It can be determined for individual time series, or averaged at local, regional, and global scales to detect long term changes in geomagnetic activity, identify excursions, and transitional field behavior. For recent field models, Pi ranges from less than 0.05 to 0.30. Conventional definitions for geomagnetic excursions are characterized by Pi exceeding 0.5. Strong field intensities are associated with low Pi unless they are accompanied by large deviations from axial dipole field directions. provides a measure of geomagnetic stability that is modulated by the level of PSV or frequency of excursional activity and reversal rate. We demonstrate uses of Pi for paleomagnetic observations and field models and show how it could be used to assess whether numerical simulations of the geodynamo exhibit Earth‐like properties.

Magnetic excursions in the late Matuyama Chron (Olduvai to Matuyama‐Brunhes boundary) from North Atlantic IODP sites

AbstractThe sedimentary record of late Matuyama magnetic excursions, between the end of the Olduvai Subchron and the Matuyama‐Brunhes boundary, has relied on North Atlantic Ocean Drilling Program (ODP) Sites 983/984, and individual recordings at ODP Site 1021 from the California Margin, and Core MD972143 from the west Philippine Sea. North Atlantic IODP Sites U1305, U1306, U1307 (Eirik Drift, off SE Greenland), and Site U1308 (central Atlantic) provide new records from u‐channel samples and discrete (8 cm3) cubic samples, collected from multiple holes drilled at each site. The results confirm the existence of the Intra‐Jaramillo, Punaruu, Bjorn, Gardar, and Gilsa excursions and possibly the Kamikatsura excursion. These excursions are not recorded at all sampled sites, although they do correspond to excursions recorded at ODP Sites 983/984 and to relative paleointensity minima recorded at all studied sites. For some records, including that of the Bjorn excursion (1251 ka), antipodal magnetization directions yield virtual geomagnetic poles that reach high northern latitudes, implying that the excursion involved reversal of the main axial dipole field. On the other hand, excursions are often partially and inadequately recorded, attributable to a combination of the brevity of geomagnetic excursions, bioturbation, and filtering of the magnetization signal by the detrital remanent magnetization acquisition process, nonuniform sedimentation rates at millennial/centennial scales, and possibly unrecognized drilling/sampling disturbance.

Paleomagnetic inclination variations during the last 200 kyr in the Okhotsk Sea and their relation to persistent non-axial-dipole field

Studies on geomagnetic paleointensity using marine sediments revealed that intensity fluctuations contain variations with timescales of 104 years and longer. In contrast, directional secular variations of such timescales were far less studied. In this paper we study inclination variations of longer than a millennial timescale using sediment cores at nine sites in the Okhotsk Sea. Relative paleointensity and magnetic susceptibility variations were used for inter-core correlations and age estimations. The average inclinations of individual cores were close to those of the geocentric axial dipole (GAD) field at the site latitudes. A stacked inclination curve for the last 200 kyr showed intervals of shallower inclinations at about 25–45, 75–90, 110–135, and 185–200 ka. These are synchronous with inclination shifts toward negative previously reported in the western equatorial Pacific, and temporally coincide with paleointensity lows in general. Both the Okhotsk Sea and western equatorial Pacific are within a region of outward directed flux in the persistent non-axial-dipole (NAD) field, and the synchronous inclination shifts may have been caused by a larger contribution of the NAD field when the GAD was weaker.