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1.
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
2.
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
3.
D. M. Pechersky A. A. Lyubushin Z. V. Sharonova 《Izvestiya Physics of the Solid Earth》2012,48(1):42-60
The data on geomagnetic reversals, organic changes, and lower-mantle plume magmatism in the Phanerozoic are collected and
correlated. No direct relationship is revealed between the geomagnetic reversals, plumes, and biozones. However, the frequency
of geomagnetic reversals is found to correlate to the frequency of biozonal alterations. We relate this inconsistency to the
coupling of the two processes, which are mutually independent, with the long-term changes in the Earth’s rotation. The plumes
are formed at the core-mantle boundary and, thus, the reversals should have a different source. We hypothesize that the change
in the geomagnetic polarity is due to the nonuniform rotation of the inner core relative to the mantle in combination with
the changes in the axial tilt of the Earth’s rotation. 相似文献
4.
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
5.
D. M. Pechersky A. A. Lyubushin Z. V. Sharonova 《Izvestiya Physics of the Solid Earth》2010,46(7):613-623
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
6.
复杂度理论为研究地磁变化场提供了新思路和新途径.地磁变化场从物理起源上可以看作是一种复杂系统,因此,本文从军事工程应用需求出发,基于复杂度理论,提出运用样本熵、多尺度熵及滑动窗样本熵方法对不同磁扰程度下的地磁变化场时间序列进行复杂度特征分析,结果表明：（1）样本熵和多尺度熵能够很好地表征地磁扰动强度及演化特征,启发我们可设计一种新的“熵指数”来衡量地磁扰动大小;（2）滑动窗样本熵能够准确定位地磁扰动时间段,对于磁暴识别与预测、空间灾害性天气预警等有一定的参考价值;（3）Mackey-Glass时滞混沌方程随时间的演化呈现出与地磁场日变曲线非常相似的形态,因此,地磁变化场或许可用时滞混沌方程来表示,对于我们更好地认识地磁变化场物理机理、建模预测与地磁寻的等军事工程应用都有一定的意义. 相似文献
7.
The data that describe the long-term reversing behavior of the geodynamo show strong and sudden changes in magnetic reversal frequency. This concerns both the onset and the end of superchrons and most probably the occurrence of episodes characterized by extreme geomagnetic reversal frequency (>10–15 rev./Myr). To account for the complexity observed in geomagnetic reversal frequency evolution, we propose a simple scenario in which the geodynamo operates in three distinct reversing modes: i—a “normal” reversing mode generating geomagnetic polarity reversals according to a stationary random process, with on average a reversal rate of ～3 rev./Myr; ii—a non-reversing “superchron” mode characterizing long time intervals without reversal; iii—a hyper-active reversing mode characterized by an extreme geomagnetic reversal frequency. The transitions between the different reversing modes would be sudden, i.e., on the Myr time scale. Following previous studies, we suggest that in the past, the occurrence of these transitions has been modulated by thermal conditions at the core-mantle boundary governed by mantle dynamics. It might also be possible that they were more frequent during the Precambrian, before the nucleation of the inner core, because of a stronger influence on geodynamo activity of the thermal conditions at the core-mantle boundary. 相似文献
8.
A.R.T Jonkers 《Physics of the Earth and Planetary Interiors》2003,135(4):253-266
The frequency distribution of intervals between Cenozoic geomagnetic reversals approximates a power law, while their occurrence over time shows temporal clustering of short and long intervals. Application of the Aggregate Variance and Absolute Value methods suggest long-range dependence in this time series, a possible indication that the geodynamo operates as a self-organised complex system. This hypothesis may allow the Cretaceous superchron to be considered as an integral part of the ordinary reversal regime attested in the Cenozoic record. 相似文献
9.
TianShui Yang Masayuki Hyodo ZhenYu Yang ShiHong Zhang Toshiaki Mishima HuaiChun Wu HaiYan Li Yi Li XingAn Shi Kan Wang YiMing Ma 《中国科学:地球科学(英文版)》2014,57(8):1929-1943
Paleomagnetic records of the Gauss-Matuyama reversal were obtained from two loess sections at Baoji on the Chinese Loess Plateau. Stepwise thermal demagnetization shows two obvious magnetization components. A low-temperature component isolated between 100 and 200–250°C is close to the present geomagnetic field direction, and a high-temperature component isolated above 200–250°C reveals clearly normal, reversed, and transitional polarities. Magnetostratigraphic results of both sections indicated that the Gauss-Matuyama reversal consists of a high-frequency polarity fluctuation zone, but the characteristic remanent magnetization directions during the reversal are clearly inconsistent. Rock magnetic experiments demonstrated that for all the specimens with normal, reversed, and transitional polarities magnetite and hematite are the main magnetic carriers. Anisotropy of magnetic susceptibility indicates that the studied loess sediments have a primary sedimentary fabric. Based on virtual geomagnetic pole latitudes, the Gauss-Matuyama reversal records in the two sections are accompanied by 14 short-lived geomagnetic episodes (15 rapid polarity swings) and 12 short-lived geomagnetic episodes (13 rapid polarity swings), respectively. Our new records, together with previous ones from lacustrine, marine, and aeolian deposits, suggest that high-frequency polarity swings coexist with the Gauss-Matuyama reversal, and that the Gauss-Matuyama reversal may have taken more than 11 kyr to complete. However, we need more detailed analyses of sections across polarity swings during reversals as well as more high-resolution reversal records to understand geomagnetic behavior and inconsistent characteristic remanent magnetization directions during polarity reversals. 相似文献
10.
Reversals and excursions of Earth's geomagnetic field create marker horizons that are readily detected in sedimentary and volcanic rocks worldwide. An accurate and precise chronology of these geomagnetic field instabilities is fundamental to understanding several aspects of Quaternary climate, dynamo processes, and surface processes. For example, stratigraphic correlation between marine sediment and polar ice records of climate change across the cryospheres benefits from a highly resolved record of reversals and excursions. The temporal patterns of dynamo behavior may reflect physical interactions between the molten outer core and the solid inner core or lowermost mantle. These interactions may control reversal frequency and shape the weak magnetic fields that arise during successive dynamo instabilities. Moreover, weakening of the axial dipole during reversals and excursions enhances the production of cosmogenic isotopes that are used in sediment and ice core stratigraphy and surface exposure dating. The Geomagnetic Instability Time Scale (GITS) is based on the direct dating of transitional polarity states in lava flows using the 40Ar/39Ar method, in parallel with astrochronologic age models of marine sediments in which oxygen isotope and magnetic records have been obtained. A review of data from Quaternary lava flows and sediments gives rise to a GITS that comprises 10 polarity reversals and 27 excursions that occurred during the past 2.6 million years. Nine of the ten reversals bounding chrons and subchrons are associated with 40Ar/39Ar ages of transitionally-magnetized lava flows. The tenth, the Gauss-Matuyama chron boundary, is tightly bracketed by 40Ar/39Ar dated ash deposits. Of the 27 well-documented geomagnetic field instabilities manifest as short-lived excursions, 14 occurred during the Matuyama chron and 13 during the Brunhes chron. Nineteen excursions have been dated directly using the 40Ar/39Ar method on transitionally-magnetized volcanic rocks and these form the backbone of the GITS. Excursions are clearly not the rare phenomena once thought. Rather, during the Quaternary period, they occur nearly three times as often as full polarity reversals. 相似文献
11.
Polarity zones of sedimentary sections reflect a pattern of alternating polarity of the geomagnetic field recorded by the
remanent magnetization of rocks. Unfortunately, this pattern can have been modified by the variable sedimentation rate, which
complicates the identification of polarity zones against the reference geomagnetic polarity time scale. To avoid this obstacle,
the present paper suggests a transform applied to both the sequence of levels of polarity reversal horizons and the sequence
of ages of polarity reversals before computing their cross-correlation. This transform usually reduces the impact of the variable
sedimentation rate so that a sequence of more than eight polarity reversal horizons may be identified without biostratigraphic
constraints. Numerical experiments involving random processes to simulate both the duration of polarity reversals and the
sedimentation rate proved, however, that not all the parts of a hypothetical stratigraphic section spanning the past 165 Ma
would be equally suitable for dating by magnetic polarity stratigraphy.
A program performing both the compilation of polarity zones from the directions of the primary magnetization sampled along
a section and subsequent identification of these polarity zones is made available online. 相似文献
12.
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
13.
Michel Prévot 《Physics of the Earth and Planetary Interiors》1977,13(4):342-345
Directional and paleointensity data for the Steens Mountain geomagnetic polarity transition do not agree with the two simplest models of reversals: rotation of undiminished dipole or gradual diminution followed by change in polarity of the dipole moment. Instead, large and rapid changes in the intensity and direction of the field occur, probably as a result of non-dipole variations. 相似文献
14.
P. Noziéres 《Physics of the Earth and Planetary Interiors》1978,17(2):55-74
A mathematical model that might be relevant to the dynamic properties of the earth's dynamo is constructed. The model involves one mechanical variable (convection velocity at constant geometry) and two electromagnetic degrees of freedom. It represents a generalization of the Rikitake coupled dynamos. We first show that the problem involves two very different time scales, a fast MHD scale and a slow inductive scale. Using the ratio of these scales as an expansion parameter, we solve analytically the non-linear differential system which describes the dynamics of our model. We first isolate a slow motion, and we study its stability against fast MHD ocillations. For large amplitudes, we show how to construct an expansion in the drift of the fast orbit. We survey in some detail the various possible regimes. Among them, we find a typical “ratchet like” relaxation behaviour, which may perhaps explain the very sudden reversals of the earth's field. Orders of magnitude and possible generalizations are briefly sketched. 相似文献
15.
D. M. Pechersky 《Izvestiya Physics of the Solid Earth》2007,43(10):844-854
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
16.
Poorna C. Pal 《地球物理与天体物理流体动力学》2013,107(1-4):189-205
Abstract The geomagnetic field and its frequent polarity reversals are generally attributed to magnetohydrodynamic (MHD) processes in the Earth's metallic and fluid core. But it is difficult to identify convincingly any MHD timescales with that over which the reversals occur. Moreover, the geological record indicates that the intervals between the consecutive reversals have varied widely. In addition, there have been superchrons when the reversals have been frequent, and at least two, and perhaps three, 35-70 Myr long superchrons when they were almost totally absent. The evaluation of these long-term variations in the palaeogeophysical record can provide crucial constraints on theories of geomagnetism, but it has generally been limited to only the directional or polarity data. It is shown here that the correlation of the palaeogeomagnetic field strength with the field's protracted stability during a fixed polarity superchron provides such a constraint. In terms of a strong field dynamo model it leads to the speculation that the magnetic Reynolds number, and the toroidal field, increase substantially during a superchron of frequent reversals. 相似文献
17.
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
18.
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
19.
利用非线性误差增长理论,以Lorenz系统为例比较研究了初始误差和参数误差对混沌系统可预报性的影响．结果表明：在初始误差和参数误差单独存在时,系统的可预报期限随误差大小的变化规律基本上相同;对于相同的误差大小,初始误差和参数误差对系统可预报期限的影响几乎相同,这一结果基本上不随参数范围的变化而变化．当初始误差和参数误差同时存在时,两者对可预报期限影响所起的作用大小主要取决于初始误差和参数误差的相对大小．当初始误差远大于参数误差时,Lorenz系统的可预报期限主要由初始误差决定,可以不用考虑参数误差对预报模式可预报性的影响;反之,当参数误差远大于初始误差时,Lorenz系统的可预报期限主要由参数误差决定;当初始误差和参数误差大小相当时,两者都对系统的可预报期限起重要作用．在后两种情况下,在考虑初始误差对可预报性影响的同时还必须考虑参数误差的作用．这提醒我们在作实际数值天气预报的时候,不仅要重视初值的确定,也要重视数值模式控制参数的确定． 相似文献
20.
A detailed analysis of the data on the intensity of the geomagnetic dipole and frequency of its reversals presented in the world’s paleointensity databases provided the arguments in favor of the hypothesis of the negative correlation between the average virtual dipole moment (VDM) and the frequency of the reversals on the interval from 5 Ma to 100 Ma ago. However, the statistical confidence level of this hypothesis is only 60–70%, which is far below 95%, the standard required confidence level of a hypothesis to be considered statistically reliable. At a high level of confidence (above 99%), the presence of a positive correlation between the mean value and variance of VDM for a number of intervals of stable polarity in the Cenozoic and Mesozoic is confirmed. This finding means that the distribution of VDM on these time intervals is certainly non-Gaussian and is rather described by the gamma- or lognormal law. At the same time, in contrast to the earlier intervals, the histogram of VDM for the Brunhes epoch is closer to the normal distribution. Compared our conclusions with the published results on the numerical modeling of the geodynamo, we found that they are consistent in terms of a probable negative correlation between the average VDM and reversal frequency, as well as the lack of correlation between the average VDM and the length of the interval of stable polarity. 相似文献