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1.
A representative collection of Cretaceous rocks of Mongolia is used for the study of the magnetic properties of the rocks and for determination of the paleodirections and paleointensities H anc of the geomagnetic field. The characteristic NRM component in the samples is recognized in the temperature interval from 200 to 620–660°C. The values of H anc are determined by the Thellier-Coe method with observance of all present-day requirements regarding the reliability of such kind of results. Comparison of data in the literature on paleointensity in the Cretaceous superchron and in the Miocene supports the hypothesis of the inverse correlation between the average intensity of the paleofield and the frequency of geomagnetic reversals. The increase in the average intensities is accompanied by an appreciable increase in the variance of the virtual dipole moment (VDM). We suggest that the visible increase in the average VDM value in the superchron is due to the greater variability of VDM in this period compared to the Miocene.  相似文献   

2.
Summary It has been predicted that the geomagnetic field strength will be at its highest during periods of low reversal frequency. Using basaltic lavas from Israel and India, which were erupted during the 35 Ma interval of normal polarity in the mid-Cretaceous (the Cretaceous Quiet Zone), we have obtained palaeointensity estimates. The mean virtual dipole moments from the two areas are about 75% of the present value. This suggests that there is no simple relationship between the time averaged strength of the dipole and the frequency of reversals.  相似文献   

3.
The global database on the paleointensity, containing determinations of the virtual dipole moment (VDM) for a stable (normal) regime of the geomagnetic field in a time interval of up to 3.5 Ga, is supplemented by new VDM determinations and analyzed. The field generation process started no later than 3–3.5 Ga (earlier data are absent) at the stage of the Earth’s core formation. Since that time, the dipole value has differed from its present value by no more than an order of magnitude, and the deviations that have already been detected tend toward smaller values. The distribution of VDM values in the time interval 0–400 Ma is bimodal, which apparently reflects the presence of two different generation levels of the geomagnetic field distinguished by a relatively large value (close to the present field value) and a relatively small value (approximately half as large as the present value). The total duration of decreased VDM values appreciably exceeds that of increased VDM values (179.1 and 28.6 Myr, respectively). On the whole, data on the paleointensity do not contradict the hypothesis about the dipole nature of the field over the last 400 Myr; however, the number of determinations at high paleolatitudes is too small to draw decisive conclusions on the validity (or invalidity) of the dipole field approximation based solely on paleointensity data.  相似文献   

4.
Marine magnetic anomalies 33 and 34, corresponding to the first two reversals following the long normal polarity interval in the Cretaceous, are anomalously skewed by 30° to 40° throughout the North and South Atlantic. This phenomenon is most likely related to some aspect of the dipole paleomagnetic field. Specifically the magnetic field at the time of anomalies 33 and 34 appears to be characterized by the following: the dipole field gradually decreases in average intensity between reversals and/or there is an increase in the frequency or duration of undetected short polarity events toward the end of long periods (>106 years) of predominantly one polarity. Such long-period trends in the field are in conflict with the popular model for the generation of the earth's magnetic field that treats reversals as a Poisson process and assumes that the core has no memory greater than about 104 years.  相似文献   

5.
The results of comparative analysis of the behavior of paleointensity and polarity (intervals between reversals) of the geomagnetic field for the last 167 Ma are presented. Similarities and differences in the behavior of these characteristics of the geomagnetic field are discussed. It is shown that bursts of paleointensity and long intervals between reversals occurred at high mean values of paleointensity in the Cretaceous and Paleogene. However, there are differences between the paleointensity behavior and the reversal regime: (1) the characteristic times of paleointensity variations are less than the characteristic times of the frequency of geomagnetic reversals, (2) the achievement of maximum values of paleointensity at the Cretaceous–Paleogene boundary and the termination of paleointensity bursts after the boundary of 45–40 Ma are not marked by explicit features in the geomagnetic polarity behavior.  相似文献   

6.
The behavior of the main magnetic field components during a polarity transition is investigated using the α2-dynamo model for magnetic field generation in a turbulent core. It is shown that rapid reversals of the dipole field occur when the helicity, a measure of correlation between turbulent velocity and vorticity, changes sign. Two classes of polarity transitions are possible. Within the first class, termed component reversals, the dipole field reverses but the toroidal field does not. Within the second class, termed full reversals, both dipole and toroidal fields reverse. Component reversals result from long term fluctuations in core helicity; full reversals result from short term fluctuations. A set of time-evolution equations are derived which govern the dipole field behavior during an idealized transition. Solutions to these equations exhibit transitions in which the dipole remains axial while its intensity decays rapidly toward zero, and is regenerated with reversed polarity. Assuming an electrical conductivity of 3 × 105 mho m?1 for the fluid core, the time interval required to complete the reversal process can be as short as 7500 years. This time scale is consistent with paleomagnetic observations of the duration of reversals. A possible explanation of the cause of reversals is proposed, in which the core's net helicity fluctuates in response to fluctuations in the level of turbulence produced by two competing energy sources—thermal convection and segregation of the inner core. Symmetry considerations indicate that, in each hemisphere, helicity generated by heat loss at the core-mantle boundary may have the opposite sign of helicity generated by energy release at the inner core boundary. Random variations in rates of energy release can cause the net helicity and the α-effect to change sign occasionally, provoking a field reversal. In this model, energy release by inner core formation tends to destabilize stationary dynamo action, causing polarity reversals.  相似文献   

7.
Preliminary paleointensity results are presented from 36 sites with virtual geomagnetic pole (VGP) latitudes of about 30–90° normal polarity in the Quaternary West Eifel volcanic field. A strong correlation between VGP latitude and the Earth's virtual magnetic dipole moment (VDM) is observed, with low intensities for low VGP latitudes indicating possibly an emplacement during an excursion or event of the Brunhes epoch. The age distribution of the West Eifel volcanics is, as yet, poorly known. Also, the mean VDM value for sites with high VGP latitudes is considerably lower than the present day dipole moment of the Earth.  相似文献   

8.
The data on geomagnetic reversals are compared with the changes in the organic world and with the lower-mantle plumes. The times of the formation of plumes and the times of their appearance on the Earth’s surface relate to the intervals characterized by the different frequencies of geomagnetic reversals, i.e., there is no interrelation between the formation of plumes and the frequency of the changes in the geomagnetic field polarity. At the same time, a certain synchronism is observed between the frequency of the geomagnetic reversals and the boundaries of the biostratigraphic ages, i.e., the changes in the organic world in the long-period range. A hypothesis is proposed, which explains the change in the sign of the geomagnetic field by the combined effect of the irregular rotation of the internal core relative to the mantle and the changes in the slope angle of the axis of the Earth’s rotation, which, in turn, results in synchronous events on the Earth’s surface: the rates of changes in the organic world.  相似文献   

9.
We present the results of analyzing a representative collection of the middle Miocene 12.4–10.0 Ma basalts that compose the volcanic cover of the Shufan and Sovgavan plateaus, namely the Nikolo-L’vovsk (NL) and Sovetskaya Gavan (SG) volcanic fields. Preliminary data are obtained about the relicts of some volcanic edifices within the West and East Sikhote-Alin volcanic belts, namely the Shishlovskii, Malyshevo, and Truzhenik objects. It is established that the volcanic rocks from these localities are characterized by similar petrologic and magnetic properties. Thermal cleaning of the samples is carried out, and the coordinates of the paleomagnetic pole are determined as Λ = 190.2°E, Φ = 71.3°N for basalts of the Nokolo-L’vovsk area and Λ = 180.4°E, Φ = 71.9°N for rocks from the Sovgavan locality. These values are consistent with the data for coeval volcanics from other regions of Eurasia. Reliable determinations of the paleointensity H pal for a representative collection of samples were obtained using the Thellier method. The corresponding values of the virtual dipole moment (VDM) are almost half its present-day value. The analysis of the Miocene VDM values available from the world database revealed a low average field 5.06 × 1022 Am2 characterized by high variance σ = 2.13 × 1022 Am2 at that time. The similarity of VDM values for the Miocene characterized by frequent inversions and for the Cretaceous Superchron supports the hypothesis of the lack of a correlation between the VDM values and the frequency of geomagnetic inversions.  相似文献   

10.
Models of geomagnetic reversals as a stochastic or gamma renewal process have generally been tested for the Heirtzler et al. [1] magnetic polarity time scale which has subsequently been superseded. Examination of newer time scales shows that the mean reversal frequency is dominated in the Cenozoic and Late Cretaceous by a linearly increasing trend on which a rhythmic fluctuation is superposed. Subdivision into two periods of stationary behavior is no longer warranted. The distribution of polarity intervals is visibly not Poissonian but lacks short intervals. The LaBrecque et al. [2] polarity time scale shows the positions of 57 small-wavelength marine magnetic anomalies which may represent short polarity chrons. After adding these short events the distribution of all polarity intervals in the age range 0–40 Myr is stationary and does not differ significantly from a Poisson distribution. A strong asymmetry develops in which normal polarity chrons are Poisson distributed but reversed polarity chrons are gamma distributed with indexk = 2. This asymmetry is of opposite sense to previous suggestions and results from the unequal distribution of the short polarity chrons which are predominantly of positive polarity and concentrated in the Late Cenozoic. If short-wavelength anomalies arise from polarity chrons, the geomagnetic field may be more stable in one polarity than the other. Alternative explanations of the origin of short-wavelength marine magnetic anomalies cast doubt on the inclusion of them as polarity chrons, however. The observed behavior of reversal frequency suggests that core processes governing geomagnetic reversals possess a long-term memory.  相似文献   

11.
The results of determining the paleointensity of the Siberian traps sampled from the Kotui sequence, Truba ravine, Onkuchak Suite, and dated to Permian-Triassic are presented. Most of the H anc values for separate lava flows are significantly lower than the values of the present geomagnetic field at the observation point (approximately 50 μT). This is consistent with the known conclusion that the intensity of the geomagnetic field at the Permian-Triassic boundary was lower by a factor of two to three lower than its present value. We carried out the comparative analysis of the behavior of H an and VDM for two trap sequences (Kotui (the Onkuchak Suite) and Norilsk) from the standpoint of the eruptive pulse hypothesis. In both sections, the average VDM values and their dispersion are similar. For comparison, similar analysis of the VDM behavior is conducted for the Brunhes epoch and Miocene in the interval of 5–10 Ma. It is hypothesized that during the three considered periods, namely, the eruption of the Siberian traps, in the Brunhes epoch, and in Miocene (in the interval of 5–10 Ma), the time behavior of the geomagnetic field was close to a stationary stochastic process.  相似文献   

12.
Four sections in Majocian-Bathonian (Middle Jurassic) pelagic limestone with standard ammonite zonation have yielded magnetic polarity sequences. Magnetic directions in these red to white limestones were obtained by thermal demagnetization and were stable from about 300°C to in excess of 450°C. The polarity patterns indicate that the majority of the Bajocian and Bathonian is characterized by quite frequent reversals of the magnetic field. Lengthy periods of constant polarity, particularly constant normal polarity, were not observed. The average frequency of reversals is about 6 per ammonite zone, which roughly may be interpreted as a frequency of a reversal every 260,000 years, a rate comparable to that of the Miocene-Pliocene. Paleolatitudes of these sites (25–28°) are about 10° south of their present positions; variable clockwise block rotations within the Subbectic region have rotated these sites relative to stable Iberia.  相似文献   

13.
Paleomagnetic studies of the basalt samples of Mid-Atlantic Ridge recovered during DSDP Leg 45 and the FAMOUS Project have led to a revision of our view of the oceanic igneous crust as a recorder of geomagnetic field reversals. The discovery of several magnetic polarity reversals with depth in the crust has indicated that oceanic igneous basement should not necessarily be considered magnetized uniformly in direction, or even polarity, in a given vertical cross section. Statistical arguments, based on the ratio of the average time of crustal formation to the average length of a magnetic polarity interval, indicate that magnetic reversals with depth are to be expected in typical ocean crust, but also that this does not conflict with current theories of plate tectonics or exclude the upper layers of the crust from making a major contribution to the overlying linear magnetic anomalies. Certain ratios of average crustal formation time to average polarity interval do, however, result in an effective zero magnetization for the oceanic crust and these conditions may be responsible for the reduced amplitude of magnetic anomalies in some areas.  相似文献   

14.
The data on the amplitude of variations in the direction and paleointensity of the geomagnetic field and the frequency of reversals throughout the last 50 Myr near the Paleozoic/Mesozoic and Mesozoic/Cenozoic boundaries, characterized by peaks of magmatic activity of Siberian and Deccan traps, and data on the amplitude of variations in the geomagnetic field direction relative to contemporary world magnetic anomalies are generalized. The boundaries of geological eras are not fixed in recorded paleointensity, polarity, reversal frequency, and variations in the geomagnetic field direction. Against the background of the “normal” field, nearly the same tendency of an increase in the amplitude of field direction variations is observed toward epicenters of contemporary lower mantle plumes; Greenland, Deccan, and Siberian superplumes; and world magnetic anomalies. This suggests a common origin of lower mantle plumes of various formation times, world magnetic anomalies, and the rise in the amplitude of geomagnetic field variations; i.e., all these phenomena are due to a local excitation in the upper part of the liquid core. Large plumes arise in intervals of the most significant changes in the paleointensity (drops or rises), while no correlation exists between the plume generation and the reversal frequency: times of plume formation correlate with the very diverse patterns of the frequency of reversals, from their total absence to maximum frequencies, implying that world magnetic anomalies, variations in the magnetic field direction and paleointensity, and plumes, on the one hand, and field reversals, on the other, have different sources. The time interval between magmatic activity of a plume at the Earth’s surface and its origination at the core-mantle boundary (the time of the plume rise toward the surface) amounts to 20–50 Myr in all cases considered. Different rise times are apparently associated with different paths of the plume rise, “delays” in the plume upward movement, and so on. The spread in “delay” times of each plume can be attributed to uncertainties in age determinations of paleomagnetic study objects and/or the natural remanent magnetization, but it is more probable that this is a result of the formation of a series of plumes (superplumes) in approximately the same region at the core-mantle boundary in the aforementioned time interval. Such an interpretation is supported by the existence of compact clusters of higher field direction amplitudes between 300 and 200 Ma that are possible regions of formation of world magnetic anomalies and plumes.  相似文献   

15.
The ages of polarity chrons in previous M-sequence magnetic polarity time scales were interpolated using basal sediment ages in suitably drilled DSDP holes. This method is subject to several sources of error, including often large paleontological age ranges. Magnetostratigraphic results have now tied the Early Cretaceous and Late Jurassic paleontological stage boundaries to the M-sequence of magnetic polarity. The numeric ages of most of these boundaries are inadequately known and some have been determined largely by intuition. An examination of relevant data suggests that 114 Ma, 136 Ma and 146 Ma are optimum estimates for the ages of the Aptian/Barremian, Cretaceous/Jurassic and Kimmeridgian/Oxfordian stage boundaries, respectively. Each of these boundaries has a good correlation to the M-sequence of magnetic reversals. The magnetostratigraphic tie-level ages are linearly related to the spreading distance and have been used to calculate a new magnetic polarity time scale for the Early Cretaceous and Late Jurassic. All stage boundaries in this time interval were correlated by magnetic stratigraphy to the proposed new time scale which was then used to estimate their numeric ages. These are, with the approximate relative errors of placement within the M-sequence:The absolute errors of these interpolated stage boundary ages depend on the accuracy of the tie-level ages.  相似文献   

16.
Until recently, the existing data prevented the geophysicists from accurately dating the Bysy-Yuryakh stratum, which outcrops in the middle reach of the Kotuy River, constraining the time of its formation to a wide interval from the end of the Late Cambrian to the beginning of the Silurian. The obtained paleomagnetic data unambiguously correlate the Bysy-Yuryakh stratum to the Nyaian regional stage and constrain its formation, at least a considerable part of it, by the Tremadocian. This result perfectly agrees with the data on the Bysy-Yuryakh conodonts studied in this work and yields a spectacular example of the successful application of paleomagnetic studies in solving important tasks of stratigraphy and, correspondingly, petroleum geology. Within the Bysy-Yuryakh stratum, we revealed a large normal-polarity interval corresponding to the long (>1 Ma) period when the geomagnetic reversals were absent. This result, in combination with the data for the Tremadocian and Middle–Upper Cambrian sequences of the other regions, indicates that (1) the rate of occurrence of the geomagnetic reversals on the eve of the Ordovician Moyero superchron of reversed polarity was at most one reversal per Ma; (2) the superchron does not switch on instantaneously but is preceded by a certain gradual change in the operation conditions of the dynamo mechanism which, inter alia, manifests itself by the reduction of the frequency of geomagnetic reversals with the approach of the superchron. This finding supports the views according to which a process preparing the establishment of the superchrons takes place at the core–mantle boundary.  相似文献   

17.
The extreme scarcity of data on the behavior of the paleointensity H an in the geological past from rocks older than 400 Ma significantly hinders the development of our ideas of the geomagnetic field evolution and the geological history of the Earth as a planet. This work presents H an determinations for the Early Proterozoic using the Thellier method and meeting modern requirements for their reliability. The data are obtained from 1850-Ma rocks of granite intrusions sampled in the south of the Siberian platform. The rocks are virtually unaltered granites and granitoids. The paleointensity was determined on 15 samples; results from 11 samples were found to be suitable for the calculation of H an, which is good for experiments of this type. The common feature in the behavior of the natural remanent magnetization (NRM) is a very narrow interval of blocking temperatures: destruction of (60–90)% NRM often took place between 500 and 550°C. Because of the large thickness of the sampled magmatic body, the paleointensity estimates were corrected for its slow cooling rate. With regard for this correction, the probable value of the virtual dipole moment (VDM) from the given collection amounts to 5 × 1022 A m2. Analysis of all published data obtained by the Thellier method for the Precambrian and satisfying the well-known minimal criteria of reliability showed that the average VDM value is about 2 × 1022 A m2, which is four times smaller than the VDM value of the last million years. This phenomenon can be interpreted in terms of the hypothesis that the solid inner core formed only in Proterozoic and, in its absence, the generation of the geomagnetic field was relatively weak, which yielded a small intensity value of the geomagnetic field at early stages of the Earth’s evolution.  相似文献   

18.
The chaotic dynamics of the Rikitake two-disc dynamo system is studied for a wide range of parameters and results are compared with the sequence of geomagnetic polarity reversals. The chaos of the Rikitake system belongs to the Lorenz type, which is characterized by irregular travelling of an orbit between two unstable fixed points. Travelling corresponds to the polarity reversal. The frequency of the polarity reversals depends strongly on the parameter μ representing the resistive dissipation in the core. In the center of the chaotic regime, there is a parameter region in which reversals seldom occur and the dynamics is less disordered. The Markov entropy of the Lorenz map for the system has a sharp minimum in this parameter region named as the minimum entropy regime (M.E.R.). The average frequency of reversals in the M.E.R. is less than 0.1 per cycle of oscillation, but is not uniform even seen through a 10-m.y.-long window. The non-uniformity as well as the low rate of reversals in the M.E.R. markedly resemble the behavior of geomagnetic reversals over the past 153 m.y. It is suggested that the Earth selected the minimum entropy regime on the principle of the minimum entropy production, as is known to be the case for other dissipative dynamical systems.  相似文献   

19.
One of the reasons for performing paleomagnetic studies is to determine whether the geomagnetic field remains dipolar during a polarity transition. Data on 23 field reversals of Recent, Tertiary and Upper Mesozoic age are examined with regard to the longitudinal and latitudinal distribution of paleomagnetic poles during a polarity change. Both frequency distributions of the transitional pole positions are not random. The results suggest that some field reversals are characterized by the rotation of the dipole axis in the meridional plane and show that two preferential meridional bands of polarity transitions exist centered on planes through 40°E–140°W and 120°E–60°W respectively. The latitudinal distribution of transitional paleopoles shows that there is a decrease in the number of observed poles with decreasing latitude. This is interpreted as the result of an acceleration in the motion of the dipole axis when it approaches the equator. Comparison of transitional velocities and paleointensity magnitudes reveals that the dipole moment is very weak only for a short part of the transitional period when the paleopole position lies within the latitudes of 10°N and 10°S. The overall conclusion is that the geomagnetic field retains its dipolar character during polarity changes.  相似文献   

20.
We have sampled a sequence of 107 lava flows in the Waianae series on Oahu, Hawaii, in two separate stretches. The first (51 flows) at Kaena Point extends from sea level to an altitude of 190 m, and the second (56 flows) follows the Satellite Tracking Station road from 218 m to 360 m above sea level. Thermal and af demagnetization yield very similar results, and reveal only normal polarities. K/Ar dating at five different horizons combined with the paleomagnetic results indicate that the sequence was emplaced entirely in the normal polarity interval between the Upper Mammoth and the Lower Kaena polarity transitions (3.22–3.11 Ma). Some of the flows are serially correlated. Filtering does not, however, drastically affect the final results. Inclinations are shallower than those expected from a centered dipole field. Although large (13°), the inclination anomaly is consistent with results from other sites at the same latitude. The scatter of the VGPs about the geographic pole is consistent with available data from other regions at similar latitudes. The scatter of the directions is also consistent with the predictions of recent statistical models of paleosecular variation. Therefore, these results do not support the hypothesis of a Pacific dipole window.  相似文献   

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