Geomagnetic Polarity Research Articles

The Jaramillo subchron in Chinese loess-paleosol sequences

The Chinese loess is one of the most important terrestrial records of the Quaternary climate changes. However, the positional discrepancy of geomagnetic polarity boundaries, such as Matuyama-Brunhes boundary (MBB), between Chinese loess and marine sediments remains controversial, hampering to establish an accurate chronological framework for the Chinese loess and its correlation with marine oxygen isotope records. The Jaramillo subchron within the Matuyama Chron is an important paleomagnetic age constraint for Quaternary magnetostratigraphic dating. Here, we present a high-resolution magnetostratigraphic, rock magnetism, and relative paleointensity (RPI) study of the stratigraphic interval between paleosols S9 and S18 in two parallel loess sections from the southeastern Chinese Loess Plateau, with an aim to explore the positions of the upper and lower boundaries of the Jaramillo in the Chinese loess. The results show that significant uncertainty and variability (more than one loess-paleosol cycle) for the positions of the measured upper and lower Jaramillo boundaries among different loess sections cannot be explained by the discrepancy of loess stratigraphy, sedimentation rate or regional climate. The measured upper and lower Jaramillo boundaries in the loess can be above or below their true positions, which cannot be explained by the lock-in effect. As the measured boundaries in the loess are mostly within the intervals of low-intensity geomagnetic field, we propose that the loess deposits may have acquired unstable primary remanent magnetization during the intervals of geomagnetic reversals or excursions with low-intensity geomagnetic field, and these deposits could be easily overprinted (remagnetizated) by the ensuing high-intensity field. This hypothesis is able to explain the phenomena that the measured MBB in the Eurasian loess is downward shifted by tens centimeters to several meters relative to the true boundary, and it can also explain the anomalistic polarity zones recorded in some loess sections.

How did the dipole axis vary during the first millennium BCE? New data from West Europe and analysis of the directional global database

Despite progress in the knowledge of secular variation during the first millennium BCE in Europe, data coverage remains poor at the earliest periods, especially in some regions as in the Central Mediterranean area. This study presents three new directional and six new intensity data between the 13th and the 4th centuries BCE on archaeological kilns, pottery and baked clay fragments from South Italy and France. Archaeodirections were determined after thermal demagnetizations and archaeointensities using the Thellier-Thellier protocol with corrections for the anisotropy and cooling rate effects. The new data confirm the large deviation of the direction from a Geocentric Axial Dipole field, the high geomagnetic field strength and the fast secular variation observed in Europe during the earliest half of the first millennium BCE. Another characteristic of this period is a difference of ~25° between the longitudes of the virtual geomagnetic poles inferred from European and Middle East data. This unusual behaviour can be mainly related to the Levantine Iron Age anomaly (LIAA) and its expansion from the Middle East to Europe. However, the review of the global directional database shows that almost all virtual geomagnetic poles, 96% of them coming from Europe, the Middle East, East Asia, North America and Hawaii, are 10–25° away from the rotation axis towards North Russia between 1000 and 600 BCE. The calculation of a mean global VGP curve suggests that the North geomagnetic pole followed a clockwise motion during this period with a dipole tilt up to around 14°. This study shows that a dipole axis tilt may have played an important role in the rapid secular variation in western Eurasia, although part of this variation may also be related to non-dipole fields associated with the LIAA.

Paleomagnetism of Permian-Triassic volcanic sequences from Son La province, northwest Vietnam

Nineteen sites with 198 oriented-core samples have been collected from the Upper Permian-Lower Triassic volcanic rocks of Vien Nam Formation at Quynh Nhai locality, Son La Province, northwestern Vietnam. The characteristic remanent magnetization components carried by magnetite and hematite were successfully isolated from secondary components reveal a mean paleomagnetic direction Ds = 48.3°, Is = -10.0°, a95 = 8.0°, corresponding to a virtual geomagnetic pole located at l = 35.7°N, f = 217.4°E and a paleo-latitude of study area situated at 5.1°S during the Permian time. Compared with the Late Permian-Early Triassic pole of the South China Block (SCB), the data show that crustal elements of NW Vietnam have been close to, but not unequivocally a coherent part of the SCB, since the Late Permian. Development of the parallel NW-SE striking Song Ma and Song Chay orogenic belts did not involve the closure of wide (> 500 km) ocean basins.

Paleomagnetism of Permian-Triassic volcanic sequences from Son La province, northwest Vietnam

Nineteen sites with 198 oriented-core samples have been collected from the Upper Permian-Lower Triassic volcanic rocks of Vien Nam Formation at Quynh Nhai locality, Son La Province, northwestern Vietnam. The characteristic remanent magnetization components carried by magnetite and hematite were successfully isolated from secondary components reveal a mean paleomagnetic direction Ds = 48.3°, Is = -10.0°, a95 = 8.0°, corresponding to a virtual geomagnetic pole located at l = 35.7°N, f = 217.4°E and a paleo-latitude of study area situated at 5.1°S during the Permian time. Compared with the Late Permian-Early Triassic pole of the South China Block (SCB), the data show that crustal elements of NW Vietnam have been close to, but not unequivocally a coherent part of the SCB, since the Late Permian. Development of the parallel NW-SE striking Song Ma and Song Chay orogenic belts did not involve the closure of wide (> 500 km) ocean basins.

Contrasting opacity of bridgmanite and ferropericlase in the lowermost mantle: Implications to radiative and electrical conductivity

Earth's lowermost mantle displays complex geological phenomena that likely result from its heterogeneous physical interaction with the core. Geophysical models of core-mantle interaction rely on the thermal and electrical conductivities of appropriate geomaterials which, however, have never been probed at representative pressure and temperature (P-T) conditions. Here we report on the opacity of single crystalline bridgmanite and ferropericlase and link it to their radiative and electrical conductivities. Our results show that light absorption in the visible spectral range is enhanced upon heating in both minerals but the rate of change in opacity with temperature is a factor of six higher in ferropericlase. As a result, bridgmanite in the lowermost mantle is moderately transparent while ferropericlase is highly opaque. Our measurements support previous indirect estimates of low (< 1 W/m/K) and largely temperature-independent radiative conductivity in the lowermost mantle. This implies that the radiative mechanism has not contributed significantly to cooling the Earth's core throughout the geologic time. Opaque ferropericlase is electrically conducting and mediates strong core-mantle electromagnetic coupling, explaining the intradecadal oscillations in the length of day, low secular geomagnetic variations in Central Pacific, and the preferred paths of geomagnetic pole reversals.

A geomagnetic polarity timescale for the Carboniferous

Abstract The geomagnetic polarity pattern for the Carboniferous is incompletely known with the best-resolved parts in the Serpukhovian and Bashkirian. Hence, data from both igneous and sedimentary units are also used in an additional polarity bias evaluation. In the Tournaisian to mid Visean interval polarity is mainly derived from palaeopole-type palaeomagnetic studies, allowing identification of polarity bias chrons. Seven polarity bias chrons exist in the Mississippian (MI1n B to MI4n B ) with an additional 33 conventional magnetochrons and submagnetochrons (MI4r to MI9r). The Moscovian and Gzhelian polarity is best resolved in magnetostratigraphic studies from the Donets Basin and the southern Urals. Dispute about the reliability of these data is ill-founded, since an assessment of supporting data from palaeopole-type studies suggests that these datasets currently provide the best magnetic polarity data through the Pennsylvanian. Polarity bias assessment indicates a normal polarity bias zone in the Kasimovian. In the Pennsylvanian there are 27 conventional magnetochrons and submagnetochrons (PE1n to CI1r) and one normal polarity bias chron (PE8n B ). The Kiaman Superchron begins in the mid Bashkirian, with clear data indicating brief normal polarity submagnetochrons within the Superchron. The magnetochron timescale is calibrated using 31 U–Pb zircon dates and a quantitative Bayesian-based age-scaling procedure.

A New Middle to Late Jurassic Geomagnetic Polarity Time Scale (GPTS) From a Multiscale Marine Magnetic Anomaly Survey of the Pacific Jurassic Quiet Zone

AbstractThe Geomagnetic Polarity Time Scale (GPTS) provides a basis for the geological timescale, quantifies geomagnetic field behavior, and gives a time framework for geologic studies. We build a revised Middle to Late Jurassic GPTS by using a new multiscale magnetic profile, combining sea surface, midwater, and autonomous underwater vehicle near‐bottom magnetic anomaly data from the Hawaiian lineation set in the Pacific Jurassic Quiet Zone (JQZ). We correlate the new profile with a previously published contemporaneous magnetic sequence from the Japanese lineation set. We then establish geomagnetic polarity block models as a basis for our interpretation of the origin and nature of JQZ magnetic anomalies and a GPTS. A significant level of coherency between short‐wavelength anomalies for both the Japanese and Hawaiian lineation magnetic anomaly sequences suggests the existence of a regionally coherent field during this period of rapid geomagnetic reversals. Our study implies the rapid onset of the Mesozoic Dipole Low from M42 through M39 and then a subsequent gradual recovery in field strength into the Cenozoic. The new GPTS, together with the Japanese sequence, extends the magnetic reversal history from M29 back in time to M44. We identify a zone of varying, difficult‐to‐correlate anomalies termed the Hawaiian Disturbed Zone, which is similar to the zone of low amplitude, difficult‐to‐correlate anomalies in the Japanese sequence termed the Low Amplitude Zone (LAZ). We suggest that the LAZ, bounded by M39–M41 isochrons, may in fact represent the core of what is more commonly known as the JQZ crust.

Comprehensive palaeomagnetic study of San Borja and Jaraguay monogenetic volcanic fields, Baja California (28–30°N): considerations on latitudinal corrections

SUMMARY We report 24 palaeomagnetic directions and 10 high-quality Thellier-derived palaeointensity (PI) values, obtained from 27 sites located in Baja California Peninsula, northwestern Mexico. Sampling was done in four rock units (magnesian andesites, calc-alkaline lavas, ignimbrites, adakites) belonging to San Borja and Jaraguay monogenetic volcanic fields. These units have erupted between ∼15 and 2.6 Ma (previous K-Ar and 40Ar/39Ar data); hence results are presented in two consecutive periods: middle-late Miocene and Pliocene. The identified main magnetic minerals in the sampled sites are titanomagnetite, magnetite and minor hematite, of variable grain size, present as intergrowths or surrounding grains, which reflect varying oxidation/reduction conditions during emplacement of high-temperature magmas. Based on previous geological and geophysical records, the kinematic evolution was carefully considered in the region, allowing for the independent restoration of the palaeoposition of each sampled site. Previous palaeodirections were also evaluated and corrected for tectonic motion in order to combine them with present data. Accordingly, a number of 15 and 36 directional data are used to calculate palaeopole position for Pliocene and middle-late Miocene periods, respectively, selected from a total of 74 data points. Pliocene (Plat = 87.8°, Plong = 147.5°, K = 41.06, A95 = 6.0°) and middle-late Miocene (Plat = 86.0°, Plong = 172.7°, K = 41.08, A95 = 3.8°) palaeopole positions, calculated after tectonic corrections, are not statistically different from expected North American reference pole. Tectonic correction for Middle-late Miocene virtual geomagnetic poles plays an important role in reducing the resultant tilting from 2.7° to –0.8°. PI mean were calculated for Pliocene and middle-late Miocene periods at 29.2 ± 9.1 and 23.2 ± 6.3 μT, respectively. Compiling global filtered PI data, together with our results, indicates that the strength of the geomagnetic field during middle-late Miocene was weak (virtual dipole moment = 5.0 ± 2.2 × 1022 Am2) compared to Pliocene (6.4 ± 2.8 × 1022 Am2), and also relative to the present-day value (7.6 × 1022 Am2). This indicates the global nature of the low dipole moment during the middle-late Miocene period. However, issues related to the spatio-temporal distribution of PI data still present an obstacle to validating these suggestions; therefore, more reliable data are still needed.

High-resolution palaeomagnetic results of Ethiopian trap series from Lima Limo section: implications for the Oligocene geomagnetic field behaviour and timing of volcanism

SUMMARY We present high-resolution palaeomagnetic and rock magnetic results from 92 successive volcanic units on a 2 km tall Lima Limo (LL) section of the Oligocene Ethiopian flood basalts (OEFB). A total of 85 well-defined flow-mean characteristic remanent magnetization directions representing seven stable magnetic polarity zones with six transitional/excursional directions were determined. These results indicate the identification of four more polarity reversals than previous palaeomagnetic studies; the resulting magnetostratigraphy has, therefore, more polarity zonation. Comparison of this result with the most recently revised geomagnetic polarity time scale (GPTS, hereafter GPTS2016), by using available radiometric age data, could not find a unique correlation. Instead, it strongly suggests the recording of at least one very brief polarity chron or putative geomagnetic excursion. Our magnetostratigraphic interpretation infers that the relative timing between the OEFB eruptions and the Oi-2 global cooling estimated two possible scenarios: one where the lowermost reverse-polarity (R1) interval for the LL section is correlated to Chron C11r of GPTS2016, same as the interpretations of earlier studies, would make a causal linkage between the OEFB eruptions and the Oi-2 cooling less possible; and, the eruption timing estimated in another scenario correlating the R1 to Chron C12r might preserve the probability of its causal influence in the Oi-2 cooling. The resulting overall mean pole for the LL section of this study, 78.2° N in latitude and 213.0° E in longitude (A95 = 3.4°, N = 79), is closer to other co-eval palaeomagnetic poles from different regions and the reference pole of Africa. Our analysis of geomagnetic palaeosecular variation (PSV) shows that, compared to the Plio-Pleistocene, the Early Oligocene has higher degrees of PSV but its slightly weaker latitude dependence.

Semicontinuous paleomagnetic record of the last 1 Ma from radiometrically dated igneous rocks (Trans-Mexican Volcanic Belt and surrounding areas)

The radiometric data available in the Trans-Mexican Volcanic Belt enables a quasi-continuous record of the Earth's magnetic field over the last million years to be obtained. Here, we analyze 149 well-defined paleomagnetic directions supported by an equal number of isotope ages. Eleven new paleomagnetic directions from the Tacambaro-Puruaran area (the Michoacán Guanajuato Volcanic Field) are incorporated into the database. Global mean paleomagnetic directions are calculated for one Ma after discarding transitional lavas. The mean direction agrees with the expected paleomagnetic directions obtained from the Pliocene-Quaternary reference poles within the range of uncertainty. The secular variation rate estimated through the scattering of the calculated virtual geomagnetic poles, and the inclination anomaly, both agree with recently reported latitudinal distribution models. An interesting feature of the paleomagnetic direction compilation is the possible evidence of relatively short geomagnetic events or excursions within Matuyama and Brunhes chrons, which may help to refine the geomagnetic instability time scale (GITS).

The Location of the Earth's Magnetic Poles From Circum‐Terrestrial Observations

AbstractThe magnetic poles or dip poles are the points on the Earth's surface and circum‐terrestrial area where the magnetic field is vertical, so the horizontal component vanishes. They are not coincident with geomagnetic poles, which are the points where the dipole axis, obtained by analytic models, intersects the Earth's surface, and their instrumental sampling determination from ad hoc ground surveys is difficult due to the harsh environmental conditions and remoteness of the areas where they are located. In this work, we use magnetic field measurements from ESA's Swarm satellites covering 5 years (2015–2019), and determine the position of the magnetic poles by modeling the yearly average horizontal magnetic field component through analytic two‐dimensional Taylor polynomial fit and finding the locations where this component is minimum. The yearly dip poles positions at average satellites geocentric altitudes are also projected at ground level based on WGS84 geodetic reference system. Reliability of our method is evaluated by an optimized Monte Carlo test applied to combined International Geomagnetic Reference Field (IGRF) model and Swarm data. The availability of several years of data allows us to investigate the long term variation and dynamics of the magnetic poles, also in comparison with the results provided by IGRF model (both IGRF12 and IGRF13). Our results agree with the model, in better accordance in the north hemisphere with IGRF13, indicating that both magnetic poles move in the north‐west direction, with a speed of ∼37–72 km/y (lower in year 2016) for the north dip pole and of ∼5–9 km/y for the south one.

Four‐Dimensional Paleomagnetic Dataset: Plio‐Pleistocene Paleodirection and Paleointensity Results From the Erebus Volcanic Province, Antarctica

AbstractA fundamental assumption in paleomagnetism is that a geocentric axial dipole (GAD) geomagnetic field structure extends to the ancient field. Global paleodirectional compilations that span 0–5‐million year support a GAD dominated field structure with minor non‐GAD contributions, however, the paleointensity data over the same period do not. In a GAD field, higher latitudes should preserve higher intensity, but the current database suggests that intensities are independent of latitude. To determine whether the seemingly “low” intensities from Antarctica reflect the ancient field, rather than low‐quality data or inadequate temporal sampling, we have conducted a new study of the paleomagnetic field in Antarctica. This study focuses on the paleomagnetic field structure over the Plio‐Pleistocene. We combine and reanalyze new and published paleodirectional and paleointensity results from the Erebus volcanic province to recover paleodirections from 98 sites that were both thermally and alternating field demagnetized and then subjected to a set of strict selection criteria and paleointensities from 26 sites from the Plio‐Pleistocene that underwent the IZZI modified Thellier‐Thellier experiment and were also subjected to a strict set of selection criteria. The paleopole (201.85°, 87.65°) and α95 (5.51°) recovered from our paleodirectional study supports the GAD hypothesis and the scatter of the virtual geomagnetic poles falls within the uncertainty of that predicted by TK03 paleosecular variation model. Our time‐averaged field strength estimate, 33.57 ± 2.71 μT, is significantly weaker than that expected from a GAD field estimated by the present field.

Magnetic Anomaly Map of Shatsky Rise and Its Implications for Oceanic Plateau Formation

AbstractShatsky Rise oceanic plateau was emplaced during a period of frequent geomagnetic polarity reversals, allowing reconstruction of its tectonic evolution using magnetic anomalies. Prior studies mainly focused on identifying magnetic isochrons and encountered difficulties in tracing magnetic lineations over high relief. We complied a large magnetic data set over Shatsky Rise and its environs, using 5.5 × 106 data points from 96 geophysical cruises spanning 54 years. The long‐time span and heterogeneity of component data sets made data merger a challenge. Contributions of internal and external fields, and spurious readings were removed during processing. A “backbone” method, using recent GPS‐navigated data as a foundation, was developed to improve the coherency of the data set. The singular characteristic of the new magnetic anomaly map is that linear magnetic anomalies are ubiquitous. In nearly every place where data are dense enough to delineate anomaly trends, the plateau and surrounding crust are characterized by linear anomalies. Discordant anomalies in some areas imply complex tectonics related to triple junction migration and ridge reorientation. Tamu Massif apparently formed along a segment of Pacific‐Farallon spreading ridge that rotated by 90° as a triple junction migrated through the edifice. Ori Massif appears to have formed on the Pacific‐Izanagi ridge between triple junctions. Shirshov Massif contains discordant lineations that may indicate a microplate. The pervasive occurrence of linear magnetic anomalies within Shatsky Rise implies that these volcanic edifices must have formed by spreading analogous to mid‐ocean ridges that formed anomalously thick crust.

Geomagnetic polarity during the early Silurian: The first magnetostratigraphy of the Llandovery

Magnetostratigraphic studies in the Silurian are absent, and what is understood about the geomagnetic polarity during this time is based on polarity bias-type data from palaeopole-type studies. We provide the first composite magnetic polarity record through the Lower Silurian (Llandovery) from the magnetostratigraphy of six sections. These are integrated with graptolite biostratigraphy and some carbon isotope chemostratigraphy. The palaeomagnetic signal is carried by both haematite and magnetite, with haematite dominating in red-coloured mudstones and mostly magnetite in non-red lithologies. The influence of possible tectonic disruption of the fabric is assessed using anisotropy of magnetic susceptibility. Only the most thermally mature section at Backside Beck shows the imprint of initial tectonic fabric formation. The Llandovery is divided into 6 major normal-reverse-polarity chron couplets (referred to as LL1 to LL6). An additional longer, exclusively normal polarity interval (referred to as WE1n), beginning in Telychian Stage slice Te3, runs into the lower Sheinwoodian. Within these five polarity couplets there are 10 further submagnetozones, and 10 tentative submagnetozones. Average reversal frequency (including the tentative submagnetozones) was ca. 3.0 Myr−1 in the Early Silurian, which is probably an underestimate, due to insufficient sampling density in some parts of the Rhuddanian and Aeronian. This reversal frequency is similar to that in the late Cenozoic, indicating the future potential utility of magnetostratigraphy for high-resolution correlation and dating in the Early Silurian.

Pleistocene magnetostratigraphy of four cores in the West Philippian Basin and regional sedimentary shift during the Mid‐Pleistocene transition

The Philippine Sea is a well‐preserved, extinct, marginal basin, and due to generally deeper than the carbonate compensation depth, the geochronology and its geological significance of the abyssal sediments are less studied in previous researches. In this work, we studied the magnetostratigraphy of four gravity cores from the centre of the West Philippian Basin and establish a reliable chronology for sedimentary sequences in the Pleistocene. The main results include: (a) several magnetozones are recognized in the studied four cores, which can be correlated to the geomagnetic polarity timescale from chrons C1n to C2An (0–3.0 Ma); (b) since ~1.2 Ma, the depositional rates across the West Pacific are relatively consistent, about 160–250 cm/Myr; (c) a transition in depositional rates around ~1.2 Ma was identified in the studied area, from a spatial variety in the Early Pleistocene to a persistent pattern since the Middle Pleistocene. Based on these findings, we report that this sedimentary shift in the Philippian Sea can be correlated to global climate changes during the Mid‐Pleistocene transition, representing a major change in the studied area. Therefore, we suggest that the sedimentary processes in the studied area can be linked to a common shift in palaeoclimatology across the inner Asian continent, the Philippian Sea, and the tropical Pacific during the late Early Pleistocene.

New insights on the age of the Mengyejing Formation in the Simao Basin, SE Tethyan domain and its geological implications

Knowledge on the origin and mechanism of the large potash formation in the southeast Tethyan tectonic domain are still under debate, one of which mainly due to the controversial ages of the potash-bearing strata. Here we present a detailed detrital zircon U-Pb geochronologic-magnetostratigraphic study of the potash-bearing Mengyejing (MYJ) Formation (Fm) from the Simao Basin. 948 paleomagnetic cores and 4 detrital zircon U-Pb samples were collected from a 932-m thick Jiangcheng section. Thermal demagnetization isolated 857 primary characteristic remnant magnetization (ChRM) directions, indicated by positive reversals and fold tests and petrologic examinations. Seven normal and seven reversed polarity zones were identified and correlated well with chrons C27r to C34n of the geomagnetic polarity timescale (GPTS2012), yielding the first known age sequence of >112 to ∼63 Ma for the MYJ Fm. The ages for the potash bearing strata in the Simao commenced ∼20 Ma earlier than those in the adjacent Khorat basins, opposite to the traditional viewpoint that the former would rather correspond to the middle and lower parts of the latter. Given the paleogeographic reconstruction of likely a pan-Simao-Khorat basin during the Cretaceous, ∼85 Ma would be another important potential period for future potash exploration in the Simao Basin.

Persistent shallow magnetic inclination in the past 5 million years with implications for regional tectonics in the Philippines

Despite the growing knowledge on geomagnetic paleosecular variations, the region around the Philippine archipelago remains to be a huge data gap in this regard. This work looked into rock magnetic information from <5 Ma volcanic rocks of Luzon Island to provide data for the statistical characterization of the time-averaged field in the region.A total of 21 sites with ages from 1 ka to 4.3 Ma yielded 13 normal, 7 reversed, and 1 intermediate polarity dataset. These were aggregated with previously published work to form a combined dataset of 59 normal + reversed sites with a mean virtual geomagnetic pole at 321.1°E and 83.7°N with α95 = 3.4, and an angular standard deviation of about 14.6°. This amount of scatter is slightly larger than that predicted by Model G but relatively comparable with other near-equatorial regions such as Java and Ecuador. This work is a first examination of paleomagnetic data from the Philippines in relation to the long-term behavior of the magnetic field in this part of Southeast Asia. The long-ranging shallow inclination observed may also have implications on how paleomagnetic data are interpreted for tectonic studies in the region.

A geomagnetic polarity stratigraphy for the Middle and Upper Ordovician

Magnetostratigraphic studies of the Ordovician provide evidence for the nature of core-mantle boundary interactions, and provide means for dating and correlation across differing environmental regimes. We provide new magnetostratigraphic data from the Middle and Upper Ordovician, compiling this into a polarity chronostratigraphic scale for the Dapingian to Hirnantian interval. The new data are derived from the Backside Beck and Cheney Longville sections in Britain, the Mójcza section in Poland and two cores from Poland and Lithuania. The chronology is provided by existing biostratigraphy, principally based on chitinozoans and conodonts for the Ordovician. Correlations between sections are supported by carbon isotope stratigraphy linked to Baltic isotopic zonations, along with lithological and local magnetic susceptibility correlations in Polish cores. The palaeomagnetic signal is carried by both haematite and magnetite, with haematite dominating in red-coloured lithologies (marls and limestones) and magnetite in non-red mudstones and limestones. A positive reversal test (class C) in the Cheney Longville section and positive fold tests in the Backside Beck section provide validation of the isolation of a primary palaeomagnetic signal. Palaeomagnetic directions from cores were re-oriented using Kiaman-age and Brunhes overprints. These new datasets in combination with existing Middle Ordovician data provides a near-complete magnetic polarity chronostratigraphic scale through the Middle and Upper Ordovician. Brief normal-polarity magnetozones extend well into the later parts of what has been considered the Moyero Superchron, which started in the late Tremadocian. Reversal frequencies for the mid and late Ordovician are 1.7 and 1.5 Myr−1 respectively, although that for the late Ordovician may be an underestimate.

Semi-annual, annual and Universal Time variations in the magnetosphere and in geomagnetic activity: 4. Polar Cap motions and origins of the Universal Time effect

We use theam,an, asand theaσgeomagnetic indices to the explore a previously overlooked factor in magnetospheric electrodynamics, namely the inductive effect of diurnal motions of the Earth’s magnetic poles toward and away from the Sun caused by Earth’s rotation. Because the offset of the (eccentric dipole) geomagnetic pole from the rotational axis is roughly twice as large in the southern hemisphere compared to the northern, the effects there are predicted to be roughly twice the amplitude of those in the northern hemisphere. Hemispheric differences have previously been discussed in terms of polar ionospheric conductivities generated by solar photoionization, effects which we allow for by looking at the dipole tilt effect on the time-of-year variations of the indices. The electric field induced in a geocentric frame is shown to also be a significant factor and gives a modulation of the voltage applied by the solar wind flow in the southern hemisphere that is typically a ±30% diurnal modulation for disturbed intervals rising to ±76% in quiet times. For the northern hemisphere these are 15% and 38% modulations. Motion away from/towards the Sun reduces/enhances the directly-driven ionospheric voltages and reduces/enhances the magnetic energy stored in the tail and we estimate that approximately 10% of the effect appears in directly driven ionospheric voltages and 90% in changes of the rate of energy storage or release in the near-Earth tail. The hemispheric asymmetry in the geomagnetic pole offsets from the rotational axis is shown to be the dominant factor in driving Universal Time (UT) variations and hemispheric differences in geomagnetic activity. Combined with the effect of solar wind dynamic pressure and dipole tilt on the pressure balance in the near-Earth tail, the effect provides an excellent explanation of how the observed Russell-McPherron pattern with time-of-yearFandUTin the driving power input into the magnetosphere is converted into the equinoctialF-UTpattern in average geomagnetic activity (after correction is made for dipole tilt effects on ionospheric conductivity), added to a pronouncedUTvariation with minimum at 02–10 UT. In addition, we show that the predicted and observedUTvariations in average geomagnetic activity has implications for the occurrence of the largest events that also show the nettUTvariation.

Magnetostratigraphy of West Kamchatka Marine Cenozoic Key Section (Kvachina Bay)

The results of paleomagnetic research of sediments from the Cenozoic key section of West Kamchatka at the Kvachina Bay are presented. In total, 152 samples collected from 121 stratigraphic levels of the 480-m-thick section are studied. The behavior of scalar magnetic parameters k, NRM, and Qn and magnetic anisotropy suggests that sedimentary rocks from the Kvachina Bay section were not subjected to significant deformations and substantial secondary alterations. A succession of 21 monopolar intervals combined into 10 magnetozones is identified within the section. Taking into account the correlation with reference levels of local zones based on diatoms, the correlation of recognized magnetozones with the Geomagnetic Polarity Time Scale (chrons C17 to C6) is outlined.