Geomagnetic Reversal Sequence Research Articles

Chaos in geomagnetic reversal records: A comparison between Earth's magnetic field data and model disk dynamo data

The Earth's geomagnetic field reverses its polarity at irregular time intervals. However, it is not clear whether a reversal is a deterministic (low‐dimensional) or a random (high‐dimensional) process; the duration‐frequency distribution of the polarity time intervals resembles those generated by random processes, but many models suggest that a geomagnetic field reversal can be the outcome of a deterministic dynamics, that of the convection in the Earth's outer core. The latter, in turn, is only a part of an extremely complex system, made up of both terrestrial and extraterrestrial subsystems nonlinearly interacting with each other over a wide range of time scales. We studied the geomagnetic field reversal patterns by means of several techniques of nonlinear dynamics and compared the results obtained on actual geomagnetic reversal data with synthetic reversal sequences generated by the Rikitake and Chillingworth‐Holmes models of the Earth's magnetic field. We analyzed both the geomagnetic and the synthetic reversal scales by nonlinear forecasting and found that we cannot predict the geomagnetic reversal sequence with nonlinear forecasting. Predictability of the synthetic data varies widely depending on the model parameters. Phase portraits of data obtained from the magnetic field models show fractal structures similar to those associated with the Lorenz attractor. We measured the correlation dimension DC of the synthetic and geomagnetic data by means of the Grassberger‐Procaccia method and found that DC always has a value of about one for the synthetic data. The correlation integrals for the geomagnetic reversal sequence behave very differently from those of randomized reversal sequences and suggest that the Earth's geomagnetic field reversal dynamics is not random. However, the limited size of the magnetic reversal data set (282 points) and the poor convergence of the correlation integrals make a quantitative assessment of low‐dimensional chaos impossible. Our analysis sets a lower limit to the correlation dimension of the geomagnetic reversal dynamics: DC > 3.

Crustal magnetization in the South Atlantic from inversion of magnetic anomalies

A method for inverting magnetic anomalies in the presence of topography (Parker and Huestis, 1974) is used to analyze and describe crustal magnetization along 12 profiles in the South Atlantic. The profiles are approximately 2000 km long and are almost perpendicular to the trend of the magnetic anomalies. A bandpass filter is used to remove long‐wavelength anomalies unassociated with seafloor‐spreading processes. A filter that passes only wavelengths between 200 and 3 km produces a magnetization that approximates the positive and negative aspects of the geomagnetic reversal sequence, the positive aspects originating from normally magnetized portions of the crustal source and negative aspects originating from reversely magnetized portions of the crustal source. A 6‐km thick crust is assumed in order to produce magnetizations consistent with dredged oceanic rocks. A long‐wavelength pattern is evident in the magnetization solutions. The highest magnetizations are associated with the ridge crest (1.35–1.74 A/m). Within the western section of the study area, a sharp decrease in magnetization occurs within 100 km of the ridge crest (4.5 Ma). A minimum in magnetization is reached at 400 km from the axis or at 18 Ma (0.51–0.57 A/m). The magnetization then gradually increases out to 1100 km from the ridge axis or 50 Ma (0.91–0.93 A/m) and dramatically decreases to 1900 km or 86 Ma (0.34–0.43 A/m). Within the eastern section of the study area, a sharp decrease in magnetization occurs within the first 100 km of the ridge crest (5 Ma). A minimum in the magnetization occurs at approximately 400 km from the ridge axis or at 20 Ma (0.54–0.56 A/m). The magnetization then gradually increases out to 1200 km from the ridge axis or to 60 Ma (1.15–1.24 A/m). An overall pronounced decrease occurs to 1900 km from the ridge axis or to 95 Ma (0.36–0.40 A/m). In order to explain this behavior, the field intensity may have changed with a time scale of several millions of years. Alternatively, the higher magnetizations at 50–60 Ma could be explained by a uniform intensity of magnetization in a thicker crust.

The relative stabilities of the reverse and normal polarity states of the earth's magnetic field

Recent analyses of the geomagnetic reversal sequence have led to different conclusions regarding the important question of whether there is a discernible difference between the properties of the two polarity states. The main differences between the two most recent studies are the statistical analyses and the possibility of an additional 57 reversal events in the Cenozoic. These additional events occur predominantly during reverse polarity time, but it is unlikely that all of them represent true reversal events. Nevertheless the question of the relative stabilities of the polarity states is examined in detail, both for the case when all 57 “events” are included in the reversal chronology and when they are all excluded. It is found that there is not a discernible difference between the stabilities of the two polarity states in either case. Inclusion of these short events does, however, change the structure of the non-stationarity in reversal rate, but still allows a smooth non-stationarity. Only 7 of the 57 short events are pre-38 Ma, but the evidence suggests that this is a real geomagnetic phenomenon rather than degradation of the magnetic recording or a bias in observation. This could be tested by detailed magnetostratigraphic and oceanic magnetic surveys of the Paleogene and Late Cretaceous. Overall it would appear that the present geomagnetic polarity timescale for 0–160 Ma is probably a very good representation of the actual history, and that different timescales and additional events now represent only changes in detail.

Geodynamo energy source constraints from palaeomagnetic data

Analyses of (palaeo)geomagnetic data can provide surprisingly strong constraints upon models of the geodynamo. In particular, it is shown here from an analysis of the geomagnetic reversal sequence that long-term variation in reversal rate does not occur as a result of change in any inherent inhibition or encouragement in the reversal process. Instead, there must be some physical process that actually triggers reversals, and this process must derive from an energy source effectively independent of the energy source powering the main geomagnetic field. From this, two very simple generalised models, referred to as the ‘hot-blob’ and ‘cold-blob’ models, are suggested. Data from individual reversal transitions are used to discriminate between these two models. The preferred model is that the geodynamo is driven primarily by convection related to freezing of the outer-core at the inner-core-outer-core boundary; cold blobs descending from the core-mantle interface are identified as the instabilities that trigger reversals.

Invariant and changing transitional field configurations in a sequence of geomagnetic reversals

In the past few years, the increasing number of detailed records of geomagnetic transitions has provided new and important information about the behaviour of the Earth's magnetic field during a reversal. Surprisingly, however, no attempt has been made to obtain information about the evolution of the transitional field characteristics over a long period of time. We report here the results of a palaeomagnetic study of four different geomagnetic reversals, three of which are successive, recorded at different stratigraphical levels of two Upper Tortonian marine clay sections in western Crete (Greece). This sequence represents ∼1.5 Myr of geomagnetic field history. The results obtained suggest that the process of reversal from either starting polarity may remain invariant for a long period of time, then change rather abruptly between successive reversals.

Late Jurassic/Early Cretaceous magnetic stratigraphy from the sümeg section, Hungary

The pelagic limestones exposed at Sümeg appear to represent continuous deposition from Kimmeridgian through Berriasian. Detrital magnetite and haematite pigment are the carriers of remanence in the red and non-red limestones in the lower part, while magnetite becomes predominant in the upper half of the section. Thermal demagnetization has succesfully removed overprint magnetizations, and a well-defined magnetic stratigraphy has been obtained. The Late Jurassic/Early Cretaceous mixed polarity interval is correlated with the sequence of geomagnetic reversals derived from oceanic magnetic anomalies.

Paleogene magnetic stratigraphy in Umbrian pelagic carbonate rocks: The Contessa sections, Gubbio

The Umbrian sequence of pelagic carbonate rocks provides an opportunity for precise correlation between Paleogene biostratigraphy and geomagnetic polarity history. The red-to-pink, Paleocene to middle Eocene Scaglia Rossa limestone, the varicolored upper Eocene Scaglia Variegata limestone, and the gray-green Oligocene Scaglia Cinerea marlstone form a 250-m-thick, continuous exposure in the Contessa Valley near Gubbio, Italy. Magnetostratigraphic, lithostratigraphic, and biostratigraphic investigations in three sections covering the entire Paleogene have confirmed and dated the geomagnetic reversal sequence for most of this period. The ferromagnetic mineral in the Scaglia Cinerea is magnetite and AF demagnetization to peak fields of 20 mT is sufficient to define the characteristic remanent magnetization. The Scaglia Variegata and pink Scaglia Rossa samples contain an additional hematite component which is very pronounced in dark red Paleocene Scaglia Rossa samples. Changes of magnetic mineralogy take place in these limestones during heating, especially above 500 °C. However, thermal demagnetization is effective in isolating the characteristic remanence vectors, which form almost antipodal clusters of directions representing normal and reversed polarities. The directions are rotated counterclockwise, partly due to post-Oligocene tectonism. Paleontological zonations of planktic foraminifera and calcareous nannofossils were hampered by poor preservation, but the major epoch and stage 29, as in the Gubbio Bottaccione section; Paleocene-Eocene, just above anomaly 25; Eocene-Oligocene, between anomalies 13 and 15; Oligocene-Miocene, just below anomaly 6C. These correlations require slight modifications to previous conclusions on Paleogene sea-floor spreading rates.

One hundred million years of geomagnetic polarity history

Research Article| September 01, 1981 One hundred million years of geomagnetic polarity history William Lowrie; William Lowrie 1Institut für Geophysik, ETH-Hönggerberg, CH-8093 Zürich, Switzerland Search for other works by this author on: GSW Google Scholar Walter Alvarez Walter Alvarez 2Department of Geology and Geophysics, University of California, Berkeley, California 94720 Search for other works by this author on: GSW Google Scholar Author and Article Information William Lowrie 1Institut für Geophysik, ETH-Hönggerberg, CH-8093 Zürich, Switzerland Walter Alvarez 2Department of Geology and Geophysics, University of California, Berkeley, California 94720 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1981) 9 (9): 392–397. https://doi.org/10.1130/0091-7613(1981)9<392:OHMYOG>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 William Lowrie, Walter Alvarez; One hundred million years of geomagnetic polarity history. Geology 1981;; 9 (9): 392–397. doi: https://doi.org/10.1130/0091-7613(1981)9<392:OHMYOG>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 Since 1968, absolute ages have been assigned to the Late Cretaceous–Cenozoic geomagnetic reversal time scale by fixing the ages of two or more calibration points in a composite marine magnetic-anomaly profile and interpolating between or extrapolating beyond these points, assuming constant spreading rates in each interval. Previously, no more than 4 calibration points were used, but it is now possible to specify 11 calibration points, in addition to the 0 m.y. datum. This improvement is based on magnetostratigraphic studies in Italian pelagic limestones; these studies closely tie the geomagnetic reversal sequence to the foraminiferal and coccolith zonations. Absolute ages of calibration points are provided by the best available dates on stage boundaries, which are located from the biostratigraphic zonation. The greatest changes from previous scales come in the late Paleocene–early Eocene, where the new ages are as much as 3 m.y. younger than in the 1977 scale of LaBrecque and others, and as much as 1.8 m.y. younger than in the 1980 scale of Ness and others. 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.

Paleomagnetic Studies of Central North Pacific Sediment Cores: Stratigraphy, Sedimentation Rates, and the Origin of Magnetic Instability

Magnetic studies of five North Pacific piston cores taken from around a giant piston core at 30°209N, 157°499W show three major intervals with distinctly different remanent magnetizations. The sediments deposited during the last 2.43 m.y. have a stable remanence and reversal pattern that corresponds to the standard geomagnetic reversal sequence. Core to core variations in sedimentation rates reach as much as a factor of two, but the variations are very consistent for each of the magnetic units of the dated interval. From 2.43 to about 55 m.y. ago, the sediments were deposited an order of magnitude more slowly than the younger deposits, and are magnetically unstable. The transition from unstable to stable magnetic properties is attributed to the dilution of a magnetically unstable authigenic sediment component by a massive influx of eolian debris carrying magnetically stable minerals. The oldest sediments in the giant piston core (55 to about 65 m.y. old) are more like the surface deposits, and carry a stable remanent magnetization that indicates deposition at a paleolatitude of about 11°N. This is consistent with van Andel and others9 (1975) model of rotation of the Pacific plate but not with Dalrymple and others9 (1977) model.

Paleomagnetic studies of central North Pacific sediment cores: Stratigraphy, sedimentation rates, and the origin of magnetic instability: Summary

Magnetic studies of five North Pacific piston cores taken from around a giant piston core at 30°209N, 157°499W show three major intervals with distinctly different remanent magnetizations. The sediments deposited during the last 2.43 m.y. have a stable remanence and reversal pattern that corresponds to the standard geomagnetic reversal sequence. Core to core variations in sedimentation rates reach as much as a factor of two, but the variations are very consistent for each of the magnetic units of the dated interval. From 2.43 to about 55 m.y. ago, the sediments were deposited an order of magnitude more slowly than the younger deposits, and are magnetically unstable. The transition from unstable to stable magnetic properties is attributed to the dilution of a magnetically unstable authigenic sediment component by a massive influx of eolian debris carrying magnetically stable minerals. The oldest sediments in the giant piston core (55 to about 65 m.y. old) are more like the surface deposits, and carry a stable remanent magnetization that indicates deposition at a paleolatitude of about 11°N. This is consistent with van Andel and others9 (1975) model of rotation of the Pacific plate but not with Dalrymple and others9 (1977) model.

Magnetic polarity stratigraphy

Our task of reviewing magnetic polarity stratigraphic research was made considerably easier by the publication of several recent review articles on various facets of the subject. Reviews of paleomagnetism of deep‐sea sediments have been presented by Opdyke (1977) and Harrison (1974). Larson and Helsley (1975) reviewed the Mesozoic geomagnetic reversal sequence. A rather exhaustive review of land‐based polarity stratigraphy was published by Irving and Pullaiah (1976).

Middle and early cretaceous magnetic stratigraphy from the Cismon section, northern Italy

The pelagic limestones exposed in the valley of the Cismon river (near Feltre) appear to represent continuous deposition from Valanginian to Campanian, apart from a short hiatus in the Early Albian. Detrital magnetite is the carrier of remanence in these predominantly white-grey limestones, and a well-defined magnetic stratigraphy has been obtained. The Cretaceous quiet zone at Cismon is totally normal in polarity and stretches from Early Aptian to Early Campanian. Below the Lower Aptian, the Early Cretaceous mixed polarity interval is tentatively correlated with the sequence of geomagnetic reversals derived from the oceanic magnetic anomalies.

地磁気層序と放射年代―地磁気編年にともなう問題点

This paper briefly reviews the history and present status of magnetostratigraphy, and re-examines the usefulness of magnetostratigraphy in absolute chronology. At present, two different kinds of time scale are used as standards for magnetostratigraphic age determination. The first is that time scale developed from a combination of paleomagnetic polarity data and radiometric age dates derived from volcanic rock sequences on land. The second is based on the interrelationships between the sequence of geomagnetic reversals and marine planktonic microfossils. Usually, these time scales are assumed to be equivalent.Based on the collection of radiometric dates made on magneto- and biostratigraphically-studied sedimentary sections, the relationships between the geomagnetic-radiometric and geomagnetic-microbiostratigraphic time scales were examined. A distinct discrepancy was noticed between the radiometric age and the estimated absolute age from the magnetic-microbiostratigraphic scale, as indicated in Fig. 5. This discrepancy is worthy of serious consideration by uses of the geomagnetic time scale in absolute chronology. Further examination of various methods of measurement is needed to increase the reliability of magnetostratigraphy, microbiostratigraphy and radiometric chronology.In Japanese marine sedimentary sections, there exist many pyroclastic intercalations that may provide suitable materials for such examinations.