Geomagnetic Events Research Articles

Stratigraphic placement of the Santonian-Campanian boundary (Upper Cretaceous) in the North American Gulf Coastal Plain and Western Interior, with implications to global geochronology

Recent micro- and macropaleontological research in the northern Aquitaine Basin of France, the stratotypic area for the Upper Cretaceous Santonian and Campanian Stages, was used to resolve a long-standing discrepancy in the stratigraphic position of the Santonian-Campanian boundary in east-central Texas as based upon zonations of planktonic foraminifers versus ammonites. On the basis of the presence in Texas of species of ammonites restricted to the lower reaches of the basal Campanian Placenticeras bidorsatum Zone of the Aquitaine Basin, the Santonian-Campanian boundary in Texas is significantly lower than has generally been considered by micropaleontologists. Specifically, it is within the foraminifer-based Globotruncana (= Marginotruncana ) concavata Zone ( sensu lato ). Such placement involves middle reaches of the type Austin Chalk, perhaps within the Dessau Chalk Member. An assumption that underlies the following conclusions is that the boundaries of the G. concavata Zone as recognized in European Tethys, the Atlantic and Caribbean basins, and North American Gulf Coast represent approximately synchronous stratigraphic levels. The following represent additional conclusions of pertinence to global chronology of the Cretaceous. (1) Rather than being younger than the Globotruncana concavata Zone, the Placenticeras bidorsatum Zone should be thought of as being contemporary with at least the younger half of the G. concavata (species total range) Zone. (2) Most of the Merchantville and Magothy Formations on the Atlantic seaboard of New Jersey and Delaware were deposited during early Campanian time, rather than principally during the Santonian. (3) The traditional placement of the Santonian-Campanian boundary in the Western Interior of North America slightly below the zone of Scaphites hippocrepis I is essentially correct. (4) The chronology of the Niobrara Formation of the North American Western Interior as based upon planktonic foraminifers is shown to be in serious error; specifically, it has been considered much too young. (5) Paleomagnetically sampled parts of the Niobrara Formation probably correlate with upper reaches of the “Cretaceous Long Normal Interval”, below magnetochron 33R. (6) Although the Santonian Stage surely represents a surprisingly brief interval of earth history, the Coniacian-Santonian boundary probably should be placed below the G. concavata Zone. (7) The Santonian-Campanian boundary slightly post-dates initiation of the last general Cretaceous regression of the North American Western Interior Seaway. (8) The beginning of the North American Aquilan Land Mammal “Age” represents earliest Campanian time. (9) Rather than representing an earliest Campanian geomagnetic event, the temporal span of global magnetochron 33R equates with approximately the second quarter of the total duration of the Campanian.

Quaternary Paleomagnetic Studies in Japan.

Various geomagnetic phenomena have been revealed by paleomagnetic and archeomagnetic studies on the Japanese Quaternary. Archeo-secular variations of the geomagnetic field in Japan are clarified for the past 2, 000 years. Fairly big local gradients in the geomagnetic direction are found within Japan.Studies on tephra layers and marine and lake sediments, revealed that several geomagnetic events or excursions occurred during the Brunhes normal polarity epoch. These events and excursions will provide very good time markers to correlate with the middle and late Quaternary strata.Age of paleo-earthquakes can be determined by using remanent direction of sediments squeezed out by ancient earthquakes, because their magnetization recorded the geomagnetic field at the time when the earthquakes occurred.

Atmospheric density parameters from Seasat laser ranging

Analysis of Seasat laser range data over long arcs has identified geomagnetic modelling within atmospheric density models as a significant source of error. In particular, a major deficiency is the absence or mis-modelling of persistence of a geomagnetic event at heights near 800km. A logical step is to refine persistence and the geomagnetic model parameters within the least-squares differential correction procedure of the orbital refinement. To investigate the feasibility of this approach, persistence has been studied and parameters adjusted within the J71 atmospheric model and its variant due to Slowey, JS84. In J71 two parameters were derived by combination of 7 or 8 Seasat arcs. No advantage was found by refining the 3 extra parameters within JS84. The validity of the solution has been tested on 17 Starlette arcs for which the level of improvement is significant, particularly during levels of high geomagnetic activity. This is the first occasion that atmospheric density parameters have been derived within the orbital analysis data-reduction procedure and paves the way for more extensive analyses in the future.

The Blake geomagnetic event: transition geometry, dynamical characteristics and geomagnetic significance.

Two records of the Blake geomagnetic event have been obtained from marine cores in the Mediterranean. The upper bound for duration of the event, calculated from accurate oxygen isotope stratigraphy and/or tephrochronology, is ∼ 4000 years. One record is very detailed with 70 transitional directions and allows some characteristics of the event to be precisely retrieved. The directional changes do not occur at constant speed, rather a stop-and-go behaviour is observed as in detailed records of reversals. The virtual geomagnetic pole (VGP) path, confined to two bands of longitude over the Americas and antipodal to them, is virtually identical to the only other path of this event obtained from lacustrine sediments in the western USA, suggesting that a dipolar component may be present in the transitional field during the Blake event. The two bands of longitude coincide with those of many recent reversals, suggesting that the physical processes leading to reversals or events are basically the same and that the same mechanism is statistically observed over long periods of time.

Chronology and correlation of Quaternary magnetostratigraphy and nannofossil biostratigraphy in Norwegian-Greenland Sea sediments

Paleomagnetic analyses of a series of sediment cores from the Norwegian—Greenland Sea document the existence of seven short-lived geomagnetic events during the last about 500.000 years. Calcareous nannofossil biostratigraphy, correlated to oxygen isotope chronostratigraphy, provides a time framework to infer age and duration of these periods of extreme divergence of the earth's magnetic paleofield from the regular normal polarity configuration of the Brunhes Chron. The geomagnetic events typically extend over 5 to 10 kyr. Two events are observed within the oxygen isotope stage 2–4 interval. A short event is recorded at the stage 4/5 boundary and another just below the stage 5/6 boundary. Additional three geomagnetic events are identified in oxygen isotope stage 6, at the stage 9/10 boundary and near the top of oxygen isotope stage 13, respectively.

A magnetovariational study in the SW alpine arc: a short report

Magnetovariational studies are useful for the investigation of deep conductivity structures within the Earth. Equipment for geomagnetic time variations research, consisting of four sets of fluxgate magnetometers with microloggers for acquisition, storage and transfer of data, has been installed in northwestern Italy. Geomagnetic variation events along two profiles, covering the area roughly corresponding to the subduction and obduction system of the Ivrea body, have been recorded during Summers 1986 and 1987. This area is characterized by structural anomalies of the lithosphere and asthenosphere, at the contact between the European and African plates. After an analysis in time and frequency domains, results partially confirm some previous findings on the electrical conductivity structure in the inner part of the SW Alpine arc.

A comparison of geomagnetic activity models using SEASAT laser range data

The changes in air density linked with geomagnetic disturbance, as predicted by various thermospheric models at heights near 800 km are compared by analysis of laser range data acquired during the operational lifespan of SEASAT. Computations incorporate the satellite-drag-based model J71, its variants proposed by Slowey (1983, Adv. Space Res. 3, 67; 1984, NASA Conf. Report 3835) derived from ESRO 4 mass spectrometer data, and the 1983 MSIS model. Comparisons establish a preference for the simpler J71 model for geomagnetic activity, being superior to the ESRO 4 model and comparable with MSIS83 in most studies. The introduction of persistence of a geomagnetic event by a weighted average of the preceding geomagnetic activity leads to improvements in J71 for large well-defined storms.

Paleomagnetic correlation of Newark Supergroup volcanics

Three Lower Jurassic volcanic sequences are present in the Deerfield, Hartford, and Newark basins of eastern North America. Volcanics from the Deerfield and Newark basins display similar magnetic directions that also agree with magnetizations from associated Lower Jurassic intrusives. The intermediate volcanic sequence, including the Deerfield basalt, the Holyoke basalt, and the base of the Second Watchung basalt, yields anomalous directions that differ by more than 30{degrees} from the nominal Early Jurassic direction. These anomalous directions record a short-lived geomagnetic secular variation event that provides a chronostratigraphic marker. The simplest interpretation of the paleomagnetic and geochemical data is a one-to-one correspondence between successive volcanic units of the Hartford and Newark basis, a correlation that agrees with a previous scheme based upon geochemical inferences. Our data suggest that initiation of the second eruptive phase was synchronous over the 300 km from the Deerfield Basin to the Newark Basin. Reevaluation of the widely quoted N1 Early Jurassic paleomagnetic pole for North America indicates that the 72 virtual geomagnetic poles from which it was originally derived are not all independent observations. The N1 reference pole probably represents 7 independent samples of the geomagnetic field, each of which averages at best a small amountmore » of geomagnetic secular variation. As a consequence, the N1 paleomagnetic pole may not represent the time-average paleofield.« less

Global Thermospheric Modelling

Reviewing observations of the thermosphere, one may obtain a first impression of bewildering, even chaotic, variations of basic parameters such as the density, composition, temperature and wind velocity. Ordering these observations by altitude, geographic and geomagnetic location, local and Universal Time, by considering the season, and the evolution of solar EUV activity and geomagnetic disturbances, they fall into patterns which can be interpreted causally. The thermosphere can then be viewed as a highly variable and responsive medium, displaying classical signatures of a number of major driving forces. These variable driving forces, the complex mixture of local, global and weakly non-linear responses of the thermosphere to forcing, and the deeply-embedded feed-back mechanisms between the thermosphere and the ionosphere, and beyond to the magnetosphere, combine to cause its complex variations. The time-dependent response of the terrestrial thermosphere and ionosphere, and their interactions, under combined solar UV/EUV and geomagnetic forcing, as described by theoretical and semiempirical models, can now be simulated in self-consistent numerical models. The theoretical models, in basic agreement with observations, and with semi-empirical thermosphere models, can be used to dissect and analyse the major thermospheric driving forces, and provide a means of investigating the behaviour of the thermosphere and ionosphere in regions which are not easily observed, such as the lower thermosphere. In view of the limited number of spacecraft with suitable instrumentation and orbits, and the capabilities of ground-based instrumentation fixed at specific locations, the latest models also provide a valuable global picture of the reaction of the global thermosphere and ionosphere to specific geomagnetic events.

A palaeomagnetic secular variation record from c. 120 000 yr-old New Zealand cave sediments

Summary. A record of palaeomagnetic secular variations has been obtained from a 5 m section of cave sediments in New Zealand. The uppermost horizon has been dated at 120 000 yr, and the section probably spans about 10 000 yr. The mean direction of remanence is close to the field direction expected at the site from an axial geocentric dipole. In the upper part of the record the palaeomagnetic vector describes three open clockwise loops, ending in a swing to anomalously low inclinations. This swing is tentatively associated with a weakening of the main dipole field prior to the Blake geomagnetic event.

Comparison of plasma flow and thermospheric circulation over northern Scandinavia using EISCAT and a Fabry-Perot interferometer

The EISCAT incoherent scatter radar, operating in a full tristatic mode, provided data on the ionospheric plasma drift above northern Scandinavia, during the 24 h period, 11 UT 25 November to 11 UT 26 November 1982. For the hours of darkness, 14 UT until 05 UT, observations of thermospheric winds were made by means of a ground-based Fabry-Perot interferometer (FPI) operated at Kiruna Geophysical Institute (21° E, 68° N). During this period, the radar observations describe well the ebbing and flowing of regions of strong convective ion flow associated with the auroral oval. As individual geomagnetic disturbances occur, the overall ion flow pattern intensifies and moves equatorward. The zonal thermospheric wind observed by the FPI responds rapidly to surges of the local ionospheric convection, while the meridional wind response is slower and apparently to much larger-scale features of the geomagnetic input to the high latitude thermosphere. From the data base, periods of strong heating of the ionospheric ions and of the thermospheric gas can be identified, which can be compared with Joule and particle heating rates deduced from the observations of ionospheric drifts, neutral winds, electron densities and auroral emission rates. A three-dimensional, time-dependent global thermospheric model is used to distinguish local and global features of the thermospheric wind field. Meridional and zonal wind components at 312 km may be theoretically derived from the EISCAT data using an appropriate model (MSIS) for neutral temperature. The EISCAT-derived meridional wind is within about 50 m s −1 of the FPI observations throughout the period of joint observations. The EISCAT-derived zonal wind is systematically larger (by about 50%) than the FPI measurement, but the two independent measurements follow closely the same fluctuations in response to geophysical events until 03 UT, when the EISCAT solution is driven away from the FPI measurement by a sharp increase in both neutral and ion temperatures. Between 03 and 05 UT the EISCAT-derived zonal wind is 200–400 m s −1 westward. Allowance for the neutral temperature rise would reduce the EISCAT values towards the very small zonal winds shown by the FPI during this period. We describe the relatively straightforward analysis required to derive the meridional wind from the radar data and the limitations inherent in the derivation of zonal wind, using the ion energy equation, due to the lack of precise knowledge of the background neutral temperature from the EISCAT data alone. For analysis of EISCAT ion drift observations at 312 km, the ground-based FPI temperature measurements do not improve the accuracy of the analysis, since the median altitude of the FPI measurement is probably in the range 180–240 km throughout the observation period. This median altitude and the temperature gradient both fluctuate in response to local geomagnetic events, while the temperature gradient may be considerably greater than that predicted by standard atmospheric models. When the neutral temperature is well known, or when there is a large enhancement of the ion temperature, the EISCAT-derived zonal wind exceeds the FPI measurement, but the consistency with which they correlate and follow ion-drag accelerations suggests that the differences are purely due to the considerable altitude gradients which are predicted by theoretical models.

The blake geomagnetic event as inferred from late Brunhes ignimbrites in Southwest Japan and West Indonesia.

A reversed short polarity episode in the late Brunhes Epoch has been discovered in two dacitic welded tuffs located in Kagoshima, South Kyushu, Japan, and Siguragura, North Sumatra, Indonesia. Combined with the stratigraphic relations and both fission-track ages ranging 0.10-0.13m.y., each polarity episode is correlated with the Blake polarity event. If this is correct, then it is the first time that the Blake reversed event has been identified in igneous terrestrial rocks. Based on our results, along with the recently published, worldwide Blake episodes, it is strongly suggested that the hypothesis of a global geomagnetic reversal for the Blake event is more tenable than that of a local reversal model by DENHAM (1976).

Cosmic-ray variations related to solar, geomagnetic and interplanetary disturbances (23 March?7 April, 1976)

During the second interval of the Study of Travelling Interplanetary Phenomena (STIP, 20 March–5 May, 1976) a series of solar, interplanetary, geomagnetic and cosmic-ray events have occurred. These are surprising events, since this period falls into the minimum of the solar activity of the past solar cycle. The present analysis is concentrated on Forbush decreases, cosmic-ray increases, geomagnetic variations and the related solar wind disturbances recorded by the heliocentric satellites Helios-1, 2 and the geocentric IMP-8, in the period 23 March–7 April, 1976. The cosmic-ray enhancements on 26 March and 1 April were of geomagnetic origin and particularly expressed in middle latitude stations during the largeDst magnetic field depressions. The detected multiple Forbush decreases are related with the type IV solar flares, all produced by the same active region (McMath Plage 14143). The relative positions among the satellites Helios-1, 2, the Sun, and the Earth were very favorable in this period for studying these events, since Helios-1 approached the Sun to its perihelion and Helios-2 was lined-up with the Earth. Helios-2 detected two shock fronts on 30 March and 1 April, respectively, and Helios-1 detected a tangential discontinuity on 26 March. An attempt is made to relate these shock fronts with the erupted solar flares and Storm Sudden Commencements (SSC) recorded on the Earth and to estimate a lower limit of the deceleration distance of the involved shock waves.

Revised age estimates of Brunhes palaeomagnetic events: Support for a link between geomagnetism and eccentricity

Revisions in the dates of reported geomagnetic excursions during the Brunhes Epoch are proposed in light of possible correlations between a section at Gioia Tauro, Italy, deep‐sea cores, a core from Lake Biwa, Japan, and some lava flows. The anomalously long, double Blake Event reported at Gioia Tauro is here correlated with the Blake Event (∼110 kyr) and the Biwa 1 event (180 ± 5 kyr); an hiatus may be present in the section between these two events. The α event at Gioia Tauro is correlated with the Biwa 2 event at about 295 kyr; the β event with the "Biwa 3" event at about 400 kyr; the γ event with the Snake River event at 480 ± 50; and the δ event, not recorded elsewhere, is estimated to have occurred at ∼620 kyr. These proposed refinements in the age estimates of the excursions suggest an approximately 100 kyr cyclicity. If the events are real and the revised dating is correct, the timing of the geomagnetic events seems to coincide with times of peak eccentricity of the earth's orbit, suggesting a causal connection.

A magnetometer array study in northwest India

A magnetometer array study has been carried out in northwest India, from Rajasthan across the Indo-Gangetic Plain into the Himalayan foothills. Over the three months of operation, a wide variety of natural geomagnetic events has been recorded. The analysis of a simple substorm, polarized just west of north, shows a strong anomaly in the form of a reversal of the vertical component of the fluctuation, both in the Himalayan foothills and on the Ganga Plain. The magnetic fluctuations pattern observed is most directly interpreted in terms of a path of concentrated current flow in the Earth, striking across the Himalaya. It is evidently aligned with the Aravalli belt which outcrops further south, and may indicate that some geological structure in the sub-basement is of abnormally high electrical conductivity. The path of such a current concentration across the foothills raises the question whether some transverse structure in the Himalaya is not acting as a bridge to Peninsular India for current induced in the Tibetan plateau to the northeast.

Geomagnetic excursion in late‐glacial basalt outcrops in south‐western Iceland

Three hills in the active volcanic zone of the Reykjanes peninsula, SW‐Iceland, are largely composed of volcanic tuffs and thin lava flows having stable, shallow reverse directions of magnetization. These hills are most probably contemporaneous and of late Wisconsinan age. Their mean virtual pole position is at 254°E, 12°S. Along with a geomagnetic event of similar age recently discovered elsewhere in SW‐Iceland, this excursion may define a useful stratigraphic horizon in studies of the Mid‐Atlantic ridge crest in Iceland.

Deep Drilling Study of Lake Biwa

Lake Biwa is the third oldest lake in the world next to Lake Baikal and C aspian and Aral Seas. Its peculiar position in the Japanese Archipelago, Deep Seated Earthquake zone, striking by negative gravity anomaly, and many endemic species of animals and plants indicate its extremely long limnetic history. In other words, if we succeed in obtaining whole sediments columns by deep boring, we can clarify many unknown facts such as glacial and interglacial features, geomagnetic events, cosmic ray variations, sedimentological phenomena, chemical composition, biological evolutions…cand so on.On the basis of this idea, the writers and their collaborators undertook deep coring operations of 200 meters at the water depth of 65 m at the center of the lake in 1971 and of 1, 000 meters at the shore in 1975-1976. This paper summarizes these results together with a discussion of our future project of extremely deep drilling in order to introduce our pioneering work of human beings.

The Ivory Coast microtektite strewnfield: new data

Ivory Coast microtektites have recently been found in three piston cores from the Atlantic Ocean. In two of these cores, the microtektites occur in a layer above the base of the Jaramillo geomagnetic event indicating that they fell approximately 0.95 m.y. ago. Major element compositions have been obtained for forty-one of the microtektites by electron microprobe and energy-dispersive X-ray analysis. The geographic location, stratigraphic age and major element compositions indicate that these microtektites are part of the Ivory Coast tektite strewnfield. Present data suggest that the Ivory Coast strewnfield may cover an area of about 4.5 million square kilometers and contain about 20 million metric tons of microtektites.

Magnetostratigraphy of the Shungura and Usno Formations, southwestern Ethiopia: new data and comprehensive reanalysis

Summary. Six hundred new samples are added to those we previously reported from the Shungura and Usno Formations of southwestern Ethiopia, and the entire data set is reviewed. Limited mineralogical study of sands indicates maghemite and hematite as carriers of remanence. After AF cleaning the remanent directions are well polarized into normal and reversed groups, with greater scatter in reversed directions. The time-averaged magnetization axis corresponds to a ‘far-sided right-hand’ virtual pole, a pattern previously found by Wilson. Intrasite dispersion is commonly large but better for the new samples, which we attribute to field technique. Polarity zonation is accomplished by statistical smoothing on stratigraphic plots of angle from dipole axis. Magnetozones are correlated with the standard polarity-chronologic intervals using K-A dates at nine stratigraphic levels. The main change from our previous interpretation is that the reversed zone in the Basal Member of the Shungura Formation is correlated with the top of the Gilbert Epoch rather than with the Mammoth event of the Gauss Epoch. Thus the Basal Member and Member A are somewhat older than we previously believed, but age assignments for major fossil localities are unaffected. The inferred age ranges for the Shungura and Usno Formations are 3.5 to 0.8 Myr and 3.7 to 2.8 Myr respectively. The present analysis yields five zones of normal polarity in the lower Matuyama Reversed Epoch, that is, between the Matuyama/Gauss transition and the base of the Olduvai. All were found both in the previous data and in the new samples taken at different locations. For three of the five zones an unbroken series of normal samples extends over several lithologies, so a geomagnetic event is indicated.

Evidence from Iceland on geomagnetic reversal during the Wisconsinan Ice Age

THE discoveries of reversely magnetised basalts in the axial valley of the Mid-Atlantic Ridge1–3 and Cayman Trough4 have added a new facet to the debate about geomagnetic events during the recent Brunhes. Are these reversely magnetised rocks the legacy of a recent geomagnetic event or are they the product of some process of axial valley tectonics which we do not yet understand? We have confirmed the existence of two reversely magnetised lava flows which were erupted subglacially in southwestern Iceland (Fig. 1) and seem to be direct analogues of the reversed rocks in the axial valley. We report here that a recent geomagnetic event is the only mechanism which can explain both the Icelandic and axial valley reversed outcrops.