Geomagnetic Polarity Event Research Articles

Cosmogenic signature of geomagnetic reversals and excursions from the Réunion event to the Matuyama–Brunhes transition (0.7–2.14 Ma interval)

Long-term variations of the geomagnetic dipole moment (GDM) during periods of stable polarity and in transitional states (reversals and excursions) provide key information for understanding the geodynamo regime. Following several studies dealing with the Brunhes chron and the Matuyama–Brunhes transition, this study presents a new authigenic 10Be/9Be ratio (Be-ratio) record obtained from the MD97-2143 core (western Pacific Ocean). This new Be-ratio series yields a record of GDM variations covering the early Brunhes and mid to late Matuyama time period (i.e. 700–2140 ka), independently from the relative paleointensity (RPI) record obtained from the same core, that can be compared with available RPI records and stacks. Stratigraphic offsets measured between the Be-ratio peaks and the corresponding RPI minima reach 2 to 14 cm. They can be assigned to (post-) detrital remanent magnetization (pDRM) effects leading to magnetization locking-in delays varying from 2 to 12 ka in the studied core. 10Be overproduction episodes triggered by geomagnetic dipole moment lows (GDL) linked to polarity reversals and excursions confirm the global control exerted by the GDM on cosmogenic radionuclides production. A dipole moment reconstruction derived from the Beryllium-10 (BeDiMo) was compiled and calibrated using absolute paleointensity data. This independent record complements the available paleomagnetic RPI records, permitting 1) to overcome the pDRM lock-in offsets induced below the mixing layer, 2) to confront and increase the robustness and precision of GDM reconstructions and, 3) to better constrain the chronology of geomagnetic field instabilities during the mid to late Matuyama chron. Our new 10Be derived inventory is fully compatible with the GDL series linked to geomagnetic polarity reversals and events (Matuyama–Brunhes transition, Jaramillo and Olduvai subchron boundaries, Cobb Mountain, Réunion) and it strengthens the occurrence of several excursions (Kamikatsura, Santa Rosa, Punaruu, Bjorn, Gilsa, Gardar) that were until now reported from only sparse locations.

A suggested reverse geomagnetic polarity event from the Panga de Abajo magnetic anomalies in Mexicali Valley, Baja California, Mexico

Las características de tres conjuntos de datos aeromagnéticos observados en Panga de Abajo, al NE de Baja California, México, su modelación inversa 3D y la información geológica disponible, sugieren el registro de eventos geomagnéticos de polaridad normal e inversa en cuerpos volcánicos de composición máfica que intrusionan la columna sedimentaria a unos 1500 m de profundidad.

Intrusives, Extrusives and Linear Magnetic Anomalies

Summary The definition of narrow linear oceanic magnetic anomalies resulting from dikes and associated extrusive bodies is compared to anomalies resulting from dikes without extrusives. It is shown that if the extrusive has a tapering cross-section with maximum thickness over the centre of extrusion, then peaks of' the resulting linear magnetic anomalies will possess a sharper definition than the anomalies due to a dike alone, provided the horizontal extent of the extrusive is of the order of, or less than, the depth of the ocean. Geomagnetic polarity events recorded by dike injection into a spreading oceanic crust will therefore not necessarily become ' smeared ' by the presence of substantial associated extrusive phases.

Revised ages for tuffs of the Yellowstone Plateau volcanic field: Assignment of the Huckleberry Ridge Tuff to a new geomagnetic polarity event

40 Ar/ 39 Ar ages were determined on the three major ash-flow tuffs of the Yellowstone Plateau volcanic field in the region of Yellowstone National Park in order to improve the precision of previously determined ages. Total-fusion and incremental- heating ages of sanidine yielded the following mean ages: Huckleberry Ridge Tuff—2.059 ± 0.004 Ma; Mesa Falls Tuff— 1.285 ± 0.004 Ma; and Lava Creek Tuff— 0.639 ± 0.002 Ma. The Huckleberry Ridge Tuff has a transitional magnetic direction and has previously been related to the Reunion Normal- Polarity Subchron. Dating of the Reunion event has been reviewed and its ages have been normalized to a common value for mineral standards. The age of the Huckleberry Ridge Tuff is significantly younger than lava flows of the Reunion event on Re union Island, supporting other evidence for a normal-polarity event younger than the Reunion event.

Age and magnetism of lavas in Jökuldalur area, Eastern Iceland: Gilsá event revisited

We present results of paleomagnetic measurements and K–Ar age determinations of 38 lava flows collected in five separate sections in the Jökuldalur area of Eastern Iceland, including one section previously studied by Watkins et al. [Watkins, N.D., Kristjansson, L., McDougall, I., 1975. A detailed paleomagnetic survey of the type location for the Gilsá geomagnetic polarity event. Earth Planet. Sci. Lett. 27, 436–444]. These sites are close to the type locality of the normal “Gilsá event” in the Matuyama chron first identified by McDougall and Wensink [McDougall, I., Wensink, H., 1966. Paleomagnetism and geochronology of the Pliocene–Pleistocene lavas in Iceland. Earth Planet. Sci. Lett. 1, 232–236]. Using new experimental results as well as field observations, we could correlate the sequence of lava flows in the Jökuldalur area spanning the time interval between 1.8 and 0.5 Ma. The magnetic polarities and ages obtained in this study are quite consistent with the standard time scale for Brunhes–Matuyama ages given by Cande and Kent [Cande, S.C., Kent, D.V., 1995. Revised calibration of the geomagnetic polarity timescale for the Late Cretaceous and Cenozoic. J. Geophys. Res. 100, 6093–6095] based on marine magnetic anomalies, except that the Gilsá event needs to be added. Existence of Olduvai age lavas under Gilsá was inferred but not certain. Our results are fairly in good agreement with the former studies by Wensink [Wensink, H., 1964a. Secular variation of Earth magnetism in Plio–Pleistocene basalts of eastern Iceland. Geol. Mijnbouw 43, 403–413; Wensink, H., 1964b. Paleomagnetic stratigraphy of younger basalts and intercalated Plio–Pleistocene tillites in Iceland. Geol. Rund. 54, 364–384] and Watkins et al. [Watkins, N.D., Kristjansson, L., McDougall, I., 1975. A detailed paleomagnetic survey of the type location for the Gilsá geomagnetic polarity event. Earth Planet. Sci. Lett. 27, 436–444], but the assignment to the polarity zones is different because of the newly obtained K–Ar ages. Based on the present study, we propose that the Gilsá event is a short normal subchron in the Matuyama chron distinct from and above the more well-established Oldvai subchron.

Geomagnetic Polarity Events in China since 2.48 Ma

AbstractThis paper presents the results of palaeomagnetic studies of Cenozoic volcanic rocks and Quaternary strata in China, which have confirmed the occurence of some short‐period polarity events. After a correlation with the worldwide palaeomagnetic data, a geomagnetic polarity time scale for the time since 2.48 M*** has been compiled.

High-resolution magnetostratigraphy of four sediment cores from the Greenland Sea-I. Identification of the Mono Lake excursion, Laschamp and Biwa I/Jamaica geomagnetic polarity events

SUMMARY High-resolution magnetostratigraphic analysis of three sediment cores from the base of the volcanic seamount Vesteris Banken in the Greenland Basin and one core from the Jan Mayen Fracture Zone revealed records of three pronounced geomagnetic events within the last 200 ka. Dating by stable carbon and oxygen isotope analysis, AMS14C measurements and biostratigraphic data (foraminifera abundances) yielded ages of 28-27 ka for the Mono Lake excursion, 37-33 ka for the Laschamp event, and 189179 ka for the Biwa I event. In at least one of the cores the Laschamp event exhibits a full reversal of the local geomagnetic field vector. The same is true of the Biwa I event, documented in one of the cores.

A link between geomagnetic reversals and events and glaciations

The apparent duration of geomagnetic polarity events in Arctic Ocean sediments is much longer than in sediments from lower latitudes. In fact, while the remanence of Brunhes age sediment cores from the Yermak Plateau at 82°N is fully reversed for ∼ 30% of their lengths [1], the events often evade detection in many other continuously deposited sediments. For example, the Laschamp event is absent in an otherwise high-resolution record of secular variation from Lac du Bouchet [2], which is located near the Laschamp volcanics, where the event was first detected. Very short event durations of a few hundred years at the most have been suggested before [2,3]. Because sedimentation rates in the Arctic Ocean were increased during glaciations, the exaggerated proportion of reverse polarities in sediments from high latitudes suggests a link between glaciation and field reversals. This suggestion is supported by magnetostratigraphic results obtained from thick loess/paleosol sequences in China [4]. These demonstrate that all polarity boundaries separating chrons and subchrons since the Gauss-Matuyama field reversal have been recorded in loess, and thus during periods of cold climate, although conflicting evidence exists for some boundaries. Furthermore, the ages of 22 events and chron boundaries have been compared with the oxygen-isotope record [5], thought to represent global ice volume. All events and reversals younger than 2.6 Ma may have occurred during periods of global cooling or during cold stages; however, some ages are still too poorly dated for a definite correlation. Climatic signals also exist in the two longest relative paleointensity records [6,7] but these are suspected to be caused by climatically driven variations in the rock magnetic parameters. A mechanism for field reversals may be the acceleration of the Earth's rotation, caused by lowering of the sea level during glaciations. The short duration of events also implies that the geomagnetic field can reverse an order of magnitude faster than commonly assumed.

Magnetostratigraphic Data From Late Quaternary Sediments From the Yermak Plateau, Arctic Ocean: Evidence For Four Geomagnetic Polarity Events Within the Last 170 Ka of the Brunhes Chron

SUMMARY Palaeomagnetic investigations of two sediment cores recovered from RV Polarstern near the eastern slope of the Yermak Plateau (sites PS 1533 and PS 2212) reveal convincing evidence for four polarity events of the Earth's magnetic field during the last 170 Ka. A comprehensive rock magnetic study of the sediments proved that fine-grained magnetite is the principal carrier of the remanent magnetization. No changes in magneto-mineralogy across the polarity transitions in the sediments investigated were found. Calcareous nannofossil biostratigraphy, AMS-14C (accelerated mass spectrometry) and oxygen isotope data, and 10Be and 230Th stratigraphies yielded age ranges of 24–29 Ka for the Mono Lake event, 34–43 Ka for the Laschamp event, 72–86 Ka for the Norwegian-Greenland Sea event and 118–128 Ka for the Blake event. Two reverse polarity samples at the base of core PS 2212–3 KAL are interpreted as the termination of the Biwa I event (171-181 Ka). the events exhibit full inversion of inclination in both cores. the data suggest that the transition process of the Earth's magnetic field during such polarity events requires some 1 Ka.

Etude paléomagnétique des formations du Plio-Pléistocène de la région de la Peninj (Ouest du lac Natron, Tanzanie). Limites de l'interprétation magnétostratigraphique. Paleomagnetic study of Plio-Pleistocene formations in the Peninj area (west of Lake Natron, Tanzania ). Limits of the magnetostratigraphic interpretation

Complementary experiments have been performed on the sedimentary and volcanosedimentary fossiliferous deposits of the western margin of the Lake Natron basin in order to check the previous interpretations. The dominant reverse polarity was thought to be interrupted by five normal polarity zones possibly correlated with events in the Matuyama epoch. However, uncertainties remained on the depositional origin of the stable normal magnetization. According to the new results (Saturation Isothermal Remanent Magnetization and new thermal analysis), the normal polarity zones cannot be further considered as geomagnetic polarity events but resulting from complete or nearly complete remagnetizations probably due to chemical alteration. The only reliable polarity is the reverse one, attributed to the Matuyama epoch.

Paleomagnetic data from the Coso Range, California and current status of the Cobb Mountain normal geomagnetic polarity event

Two basalt flows which erupted about 1.08 m.y. ago in the Coso Range, California, have normal magnetic polarity and thus provide additional evidence for the Cobb Mountain normal polarity event. A review of available data confirms that this event was of geomagnetic origin. A mean age of 1.10 ± 0.02 m.y. B.P. for the Cobb Mountain normal polarity event was found to best fit all available radiometric and geologic data.

Age of the Olby–Laschamp geomagnetic polarity event

SEVERAL excursions or reversals of the Earth's magnetic field throughout the normal polarity Brunhes period which started 690,000 yr ago, have been reported recently; these were summarised by Verosub and Banerjee1. The first recent event to be reported was recognised by Bonhommet and Babkine2 in lavas from the Chaine des Puys (Massif Central, France) and was named the Olby–Laschamp magnetic event, but no precise date for this event has yet been obtained. An upper limit of 20,000 yr was suggested by K–Ar data3 and has led to attributing to this event several recently discovered young magnetic events4. However, subsequent thermoluminescence and K–Ar data5,6 indicate ages older than 30,000 yr. To clarify these age relationships I report here my dating of the Olby flow by the 230Th/238U radioactive disequilibrium method.

Geomagnetic polarity event recorded at 1.1 m.y. B.P. on Cobb Mountain, Clear Lake volcanic field, California

Research Article| November 01, 1978 Geomagnetic polarity event recorded at 1.1 m.y. B.P. on Cobb Mountain, Clear Lake volcanic field, California E. A. Mankinen; E. A. Mankinen 1U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar J. M. Donnelly; J. M. Donnelly 1U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar C. S. Grommé C. S. Grommé 1U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar Geology (1978) 6 (11): 653–656. https://doi.org/10.1130/0091-7613(1978)6<653:GPERAM>2.0.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation E. A. Mankinen, J. M. Donnelly, C. S. Grommé; Geomagnetic polarity event recorded at 1.1 m.y. B.P. on Cobb Mountain, Clear Lake volcanic field, California. Geology 1978;; 6 (11): 653–656. doi: https://doi.org/10.1130/0091-7613(1978)6<653:GPERAM>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Paleomagnetic studies show that a normal polarity event within the Matuyama reversed polarity epoch is recorded by one of the volcanic units on Cobb Mountain in northern California. K-Ar age determinations show that this event has an age of 1.12 ± 0.02 m.y. and clearly preceded the Jaramillo normal polarity event. These data provide the first confirmation from a subaerial volcano that a brief polarity event occurred at this time, as suggested by published studies of some marine sedimentary cores and magnetic anomalies. This new event is herein named the “Cobb Mountain normal polarity event.” This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

A detailed paleomagnetic survey of the type location for the Gilsa geomagnetic polarity event

A total of 163 cores have been taken from a maximum of 40 separate lavas in three separate sections of the Jökuldalur, southwest of Egilsstadir, Iceland, and subjected to paleomagnetic analysis and some K-Ar dating. Previous work on the sections by McDougall and Wensink (1966) led to the establishment of the Gilságeomagnetic polarity event, with an age of about 1.60 m.y., during the reversed polarity Matuyama epoch. This earlier study described a possible reversely magnetized lava separating the Gilsáevent from a second normal polarity lava, perhaps representing the Olduvai event. Such a possibility was subsequently a source of speculation from diverse sources concerning the polarity history for the lower Matuyama. The present study indicates clearly that there is no second normal polarity event represented in the sections. Only one normal polarity event is therefore represented in the lower Matuyama of the Jökuldalur, and the age of the lavas involved is confirmed to be approximately 1.58 ± 0.08 m.y. Because of uncertainties in the interpretation of the original K-Ar results from Olduvai Gorge, it is still not possible to be certain that the Olduvai and Gilsáevents are separated in time. An incidental result of the survey is evidence to show that, contrary to recent suggestions by Einarsson (1972), there is no substantial hiatus between the major lower parts of the section and lavas believed to represent extrusions after a regional tilting and peneplanation.

Age and duration of the réunion geomagnetic polarity event

Palaeomagnetic studis on oriented samples from two sequences of olivine basalt lava flows on the island of Réunion together record the Réunion normal polarity event within the Matuyama reversed polarity epoch. Detailed K−Ar dating of the lavas indicates that the Réunion event has a mean age 2.02±0.02 my with a duration estimated to lie within the range 10,000 to 50,000 yr. This could explain the rarity of detection of the event in deep sea sedimentary cores.

Identification of short polarity events by transforming marine magnetic profiles to the pole

A method is presented that permits marine magnetic anomaly profiles from all parts of the world to be stacked for the purpose of enhancing coherent small anomalies due to short geomagnetic polarity events. In general, the shape of a profile depends not only on the sequence of normally and reversely magnetized magnetic prisms that constitute the source, but also on the following five angles: the declination and inclination of the regional field; the declination and inclination of the magnetization; and the azimuth of the magnetic stripes. Our method transforms a measured anomaly to a second anomaly which would have been produced by an identical source with vertical magnetization and vertical regional field. The method uses Fourier transform and linear systems theory and employs filters with a flat spectral response, so that no information is lost at short wavelengths. No assumptions are made about the true depth or shape of the source: this may be any two-dimensional body or bodies in which the direction of magnetization is constant and the intensity varies arbitrarily. A preliminary application of the method to marine profiles from the East Pacific rise has yielded encouraging results.

Binary model for two-dimensional magnetic anomalies

As a tool to search for short geomagnetic polarity events recorded in the oceanic crust, a new computer algorithm has been devised to model oceanic magnetic anomalies. The main departure of the present approach from earlier analyses is to impose an additional constraint on the model: The blocks representing the magnetic layer all have the same intensity of magnetization except for a binary multiplier of plus or minus unity. This constraint permits the use of blocks with very small widths. Different magnetic profiles can be correlated by simple stacking of models so that real events will reinforce each other. The technique has been applied to the East Pacific Rise with reasonable results.

Paleomagnetic Chronology of Pliocene-Early Pleistocene Climates and the Plio-Pleistocene Boundary in New Zealand

The paleomagnetic chronology established for a Pliocene-Early Pleistocene sequence of marine sediments in New Zealand reveals that marked climatic coolings based on Foraminifera and oxygen isotope ratios in the late Cenozoic preceded the Pliocene-Pleistocene boundary, which is taken to be at the base of the Gilsá geomagnetic polarity event (1.79 million years ago). Major temperature fluctuations occur from the upper Gauss (Middle Pliocene) to middle Matuyama (Early Pleistocene). The first major Pliocene cooling spans the Gauss-Matuyama boundary (2.43 million years ago). A uniform and rapid (40 centimeters per 1000 years) rate of deposition is shown for the moderately shallow marine environment.

Geomagnetic polarity epochs: age and duration of the olduvai normal polarity event

New data show that the Olduvai normal geomagnetic polarity event is represented in Olduvai Gorge, Tanzania, by rocks covering a time span of roughly from 0.1 to 0.2 my and is no older than 2.0 my. Hence the long normal polarity event of this age that is seen in deep-sea sediment cores and in magnetic profiles over oceanic ridges should be called the Olduvai event. The lava from which the Gilsàevent was defined may have been erupted during the Olduvai event and, if so, the term Gilsàshould now be abandoned. Many dated lavas that were originally assigned to the Olduvai event represent one or two much shorter normal polarity events that preceded the Olduvai event; these are herein named the Réunion normal polarity events. This revision brings the geomagnetic reversal time scale into conformity with the one implied by assumptions of uniform sedimentation rates on the ocean floor and uniform rates of sea-floor spreading.

Detection of the Gilsa Geomagnetic Polarity Event on the Island of Madeira

The paleomagnetism and potassium-argon ages of ten basaltic lavas from the island of Madeira have been investigated. Eight of the lavas are in a single sequence, which also contains three distinct sedimentary horizons up to 4 m thick. The section ranges in age from 0.74 ± 0.4 m.y. to 1.76 ± 0.07 m.y. Both the Jaramillo and Gilsa geomagnetic polarity events are represented. Of the two other lavas examined, a reversed polarity basalt with an age of 3.05 ± 0.15 m.y. records the Mammoth event, and a normal polarity basalt may represent the Jaramillo, or a previously suggested short event at t = 1.07 m.y. These data provide strong confirmation of the reality of the Gilsa event, and show that the lower boundary of the event can be no older than 1.83 m.y., could be younger than 1.69 m.y., but not younger than 1.61 m.y.