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1.
The knowledge about past secular variations of the geomagnetic field is achieved on the basis of archaeomagnetic researches of which the Bulgarian studies form an extended data set. In Part I (Kovacheva and Toshkov, 1994), the methodology used in the Sofia palaeomagnetic laboratory was described and the secular variation curves for the last 2000 years were shown. In Part II (this paper), the basic characteristics of the prehistoric materials used in the archaeomagnetic studies are emphasised, particularly in the context of the rock magnetic studies used in connection with palaeointensity determinations. The results of magnetic anisotropy studies of the prehistoric ovens and other fired structures are summarised, including the anisotropy correction of the palaeointensity results for prehistoric materials, different from bricks and pottery. Curves of the direction and intensity of the geomagnetic field during the last 8000 years in Bulgaria are given. The available directional and intensity values have been used to calculate the variation curve of the virtual dipole moment (VDM) for the last 8000 years based on different time interval averages. The path of virtual geomagnetic pole (VGP) positions is discussed. 相似文献
2.
Compilations of historical observations, archaeomagnetic data from ancient fireplaces and palaemagetic results from short cores of sediment from lakes in southeastern Australia, particularly Lake Keilambete, provide a detailed record of the geomagnetic secular variation during the last 3000 years. The independent sets of data are in good agreement if the radiocarbon time scale for the lacustrine record is about 450 years too old. The error is attributed to systematic incorporation of ancient carbon into the lake floor sediments, mainly through erosion of sediment on the crater walls at times of low water level. A significant lag between deposition and the acquisition of stable magnetic remanence is ruled out. Inclination has been abnormally steep during the last 500 years but remained fairly close to the axial dipole field value prior to that. During the last 1000 years the predominant sense of looping of the magnetic vector corresponds to westward drift of the nondipole field. Secular variations on a time scale of ～ 100 years can be resolved by the lacustrine record. 相似文献
3.
Summary Properties of the thermoremanent magnetization of various archaeological materials (baked-clay objects, samples from old trees, bones and stones) were investigated with the purpose to assess the possibility of their use for the determination of the earth's magnetic field in the historical past and to apply the data for solving some practical problems. It turns out that most of those materials yield magnetic values that can be used for making the radiocarbon dating method more accurate, for determining the degree of burning and for direct archaeomagnetic dating. The new conclusions also contribute to the knowledge of changes in the earth's magnetic field in the past. 相似文献
4.
J. A. Jacobs 《Surveys in Geophysics》1998,19(2):139-187
The Earth's main magnetic field can be approximated by an axial, geocentric dipole. The remaining non-dipole field is much smaller and is a regional rather than a global feature – quite large changes can occur in a few ka. This review is concerned with changes in the dipole component of the geomagnetic field, and one of the problems is in separating the non-dipole from the dipole contributions to the field. Unlike the many determinations of the direction of the Earth's magnetic field in the past (which have led to fundamental contributions to our understanding of plate tectonics and shown that the field can on occasion reverse its polarity), estimates of the intensity of the field are comparatively few, especially before the Holocene. This is mainly the result of experimental difficulties in obtaining reliable measurements of the field. These problems are discussed in some detail and are followed by a short account of archaeomagnetic intensities and results from Hawaii where many of the first determinations were obtained. Measurements for the last 100 ka from both lavas and lacustrine and oceanic sediments are reviewed and results from different areas compared. An asymmetric saw-tooth pattern has been observed in some of the records over the last few Ma, and this rather controversial question is discussed. Finally an account is given of the far more limited data on palaeointensities in earlier times.A short discussion is given of the interpretation of coherent short wavelength variations which are observed in many marine magnetic profiles. Although short reversals of the field may be responsible for some of these tiny wiggles, it is more likely that in general they are the result of changes in the strength of the Earth's magnetic field. 相似文献
5.
Archaeomagnetic study and rehydroxylation dating of fired-clay ceramics from Great Britain, Spain, and the Black Sea region is carried out in order to refine the dating of the material from the archaeological monuments used in the archaeomagnetic research for determination of the elements of the main magnetic field during the past few millennia. The archaeomagnetic analysis revealed the factors responsible for deviations of the rehydroxylation dating from the true values. They include the processes of weathering (magnetite transformation into hydroxides) and secondary magnetization (e.g., magnetization in a fire). In order to bring the dating closer to the true values, corrections for the influence of the distorting factors are suggested. The data on the geomagnetic field intensity derived from the magnetization of the studied material are used as independent criteria to validate the dating of the field. When applied to the fired clay material with reliable dating, the rehydroxylation method provides the estimates of the temperature in the region of archaeological monuments during the past. 相似文献
6.
A palaeomagnetic record of geomagnetic secular variation during the last 7000 years has been obtained from the sediments of Loch Lomond, Scotland. The magnetic direction fluctuations repeat well between cores and show greater detail, especially over the last 5000 years, than other European records. A time scale has been derived from14C analyses on the Lomond sediment and comparison with other14C-dated sediments. Investigation of relative palaeointensity determination methods has shown that the widely used normalization parameter of partial ARM is insensitive to even small sediment grain size fluctuations.The new high-fidelity direction record and improved time scale show that geomagnetic field changes have not followed a simple oscillatory pattern during the last 7000 years. The record enhances the application of palaeomagnetism to dating recent sediments, as the main declination swings are now characterized by fine detail, and paired inclination data are also available. The problem of mismatching swings when correlating with other paired directional records is thus reduced.The palaeomagnetic record agrees well with some archaeomagnetic results. It confirms the period of anticlockwise motion of the geomagnetic field vector, between 1000 and 600 years B.P., which was first documented by English archaeomagnetic investigations. Clockwise motion is shown to predominate during the remainder of the last 5500 years. The VGP path does not correlate with that of Japanese archaeomagnetic results nor North American sediment data from 2000 to 0 years B.P. This suggests that the secular changes are dominated by local non-dipole sources rather than wobbling of the main geomagnetic dipole. 相似文献
7.
R. Thompson 《Physics of the Earth and Planetary Interiors》1984,36(1):61-77
Spherical harmonic coefficients of the geomagnetic field, calculated from historical observations of declination, inclination and intensity, and from archaeomagnetic inclination results, have been used to produce a film of geomagnetic change since 1600 A.D. The non-dipole geomagnetic field is found to be constantly changing: no fixed or standing non-dipole features are observed. Non-dipole foci are seen to have lifetimes of a few hundred years. The westward drift, which was an important feature of the 18th and early 19th century geomagnetic field, was less pronounced in the 17th century. The growth, evolution, decay and replacement of non-dipole foci, but not their movement are found to have been the major features producing century-long secular directional magnetic variation. Most of the low degree and order spherical harmonic coefficients have changed significantly over the last few hundred years. In particular the change in sign of the axisymmetric quadrupole around 1837 A.D. is noted. Sustained, century-long, intensity changes, however, appear to have been dominated by variations in the intensity of the centred dipole, rather than by non-dipole field fluctuations. 相似文献
8.
I. E. Nachasova O. V. Pilipenko G. P. Markov 《Izvestiya Physics of the Solid Earth》2016,52(6):889-895
The archaeomagnetic studies of ceramics from the Hermonassa multilayer archaeological monument in the Taman Peninsula provided the data on the intensity of the main geomagnetic field in the past. The data for the interval from VIII to XX centuries A.D. demonstrate a well pronounced decreasing trend in the geomagnetic field intensity during this time. Three stages, each lasting for a few centuries, are distinguished in the variations of the centennial average field which slightly varies within each stage and generally decreases from 70 to ~45 μT during the entire period from VIII to XX centuries A.D. The variations of the geomagnetic field in the interval from XII to XVII centuries A.D. have a form of quasi-harmonic oscillations with a characteristic time of about 300 years. 相似文献
9.
Fco. Javier Pavón–Carrasco Maria Luisa Osete J. Miquel Torta Luis R. Gaya–Piqué 《Pure and Applied Geophysics》2008,165(6):1209-1225
The SCHA.DI.00 directional model for the geomagnetic field in Europe for the last 2000 years (Pavón–Carrasco
et al., 2008) has been updated by modelling the palaeointensity. This model, SCHA.DI.00, was developed from available Bayesian
European Palaeosecular Variation Curves using the regional Spherical Cap Harmonic Analysis technique. The comparison of the
palaeosecular variation curves, given by the regional model, with available archaeomagnetic data not used in its development
showed an improvement with respect to the fit obtained by global archaeomagnetic models. In this paper advantage is taken
of recently published palaeointensity databases to develop a complete (direction and intensity) regional archaeomagnetic model
for the last 2000 years valid for the European region: the SCHA.DI.00–F model. Not only does this complete model provide an
improvement for example for archaeomagnetic data studies, but it is also shown that this new regional model can be used to
study the recently proposed link between the centennial secular variation of the geomagnetic field and climate change. The
pattern of the archaeosecular variation of the field intensity obtained by SCHA.DI.00–F seems to verify the hypothesis presented
by Gallet et al. (2005) about a possible (causal) connection between changes in the geomagnetic field intensity and in climate parameters,
opening the door for more discussions on this challenging subject. 相似文献
10.
This paper examines the limitations and deficiencies of the current British archaeomagnetic calibration curve and applies several mathematical approaches in an attempt to produce an improved secular variation curve for the UK for use in archaeomagnetic dating. The dataset compiled is the most complete available in the UK, incorporating published results, PhD theses and unpublished laboratory reports. It comprises 620 archaeomagnetic (directional) data and 238 direct observations of the geomagnetic field, and includes all relevant information available about the site, the archaeomagnetic direction and the archaeological age. A thorough examination of the data was performed to assess their quality and reliability. Various techniques were employed in order to use the data to construct a secular variation (SV) record: moving window with averaging and median, as well as Bayesian statistical modelling. The SV reference curve obtained for the past 4000 years is very similar to that from France, most differences occurring during the early medieval period (or Dark Ages). Two examples of dating of archaeological structures, medieval and pre-Roman, are presented based on the new SV curve for the UK and the implications for archaeomagnetic dating are discussed. 相似文献
11.
Takesi Yukutake 《Physics of the Earth and Planetary Interiors》1979,20(2-4):83-95
In view of the classification of the geomagnetic field into its axisymmetric and non-axisymmetric parts, studies of geomagnetic secular variations on the historical time-scale are reviewed. The westward drift of the geomagnetic field, which is one of the most conspicuous features of its secular variation, is examined first. The non-axisymmetric field during the past several hundred years can be well approximated by the superposition of two constant-magnitude fields, a standing and a drifting field, whose lifetimes are supposed to be longer than 1000 years. It is pointed out that the sectorial term of the non-dipole standing field is small compared with the drifting one. The lack of the n = M = 2 term of the standing field is particularly remarkable.
On the other hand, the equatorial dipole field is likely to consist of two components which are both drifting. One drifts westwards with a normal velocity and the other eastwards with a small velocity.
Besides the pronounced westward drift in an east-west direction, the poleward movements of particular foci of the secular variation are noted. This may, however, be related to the rapid growth of the axisymmetric quadrupole field.
The time variation of the dipole field is briefly examined. As far as the data on the historical time scale are concerned, an antiparallel relationship seems to exist between the variations in the dipole and the quadrupole field. As the dipole moment decreases, the magnitude of the quadrupole moment increases. Finally, characteristic oscillation periods of the dipole field are examined. Although the data are few, a 60–70-year period, a 400–600-year and a 8000-year period emerge as the dominant periods. 相似文献
12.
S. P. Burlatskaya S. F. Burlatsky A. F. Burlatsky S. A. Didenko 《Izvestiya Physics of the Solid Earth》2006,42(3):207-224
The purpose of archaeomagnetic research is to investigate the structure of the geomagnetic field and its evolution. This paper is a study of this type. In our preceding studies, we divided the geomagnetic field into dipole and nondipole components. It was then shown that the dipole component consists of the predominant 1200-yr variation and the remainder δ. The δ variation is the subject of this paper. Detailed study and comparison of the characteristic features of two 1200-yr variations (the dipole and δ variations) in both the declination and inclination indicates that, according to most of their features, they can be assigned to different types of waves, namely, traveling waves (the dipole variation) and standing waves (the δ variation). Successive averaging in time and space yielded averaged data on δI and δD not only for various parts of the world (Europe, Asia, and America) but also for the western and eastern hemispheres. 相似文献
13.
Revealing the evolution of the past geomagnetic field is of dual interest. It is crucial to estimate the conditions of the Earth's deep interiors, while it offers a reliable absolute dating tool of burned archaeological artifacts. Although a substantial effort was paid during the last decade to construct a reference archaeomagnetic curve for Mesoamerica, mostly absolute intensity variations are revealed for the previous three millennia. Here, we report an updated directional (inclination and declination) reference secular variation curve based on volcanic lava flows and archaeological artifacts carrying full thermoremanent magnetization. A bootstrap method with penalized cubic b-splines was used to develop the directional curve. Relocation error exercise and strict selection criteria allowed to use 82 well-defined palaeodirections. Both data distribution density and the firm relationship between the ages and samples, ensure the reliability of the curve for the dating purposes. The combination of directional data with available intensity variation highlights some sudden geomagnetic field fluctuations. 相似文献
14.
Ron Shaar Erez Ben-Yosef Hagai Ron Lisa Tauxe Amotz Agnon Ronit Kessel 《Earth and Planetary Science Letters》2011,301(1-2):297-306
The intensity of the geomagnetic field varies over different time scales. Yet, constraints on the maximum intensity of the field as well as for its maximum rate of change are inadequate due to poor temporal resolution and large uncertainties in the geomagnetic record. The purpose of this study is to place firm limits on these fundamental properties by constructing a high-resolution archaeointensity record of the Levant from the 11th century to the early 9th century BCE, a period over which the geomagnetic field reached its maximum intensity in Eurasia over the past 50,000 years. We investigate a 14C-dated sequence of ten layers of slag material, which accumulated within an ancient industrial waste mound of an Iron Age copper-smelting site in southern Israel. Depositional stratigraphy constrains relative ages of samples analyzed for paleointensity, and 14C dates from different horizons of the mound constrain the age of the whole sequence. The analysis yielded 35 paleointenisty data points with accuracy better than 94% and precision better than 6%, covering a period of less than 350 years, most probably 200 years. We construct a new high-resolution quasi-continuous archaeointensity curve of the Levant that displays two dramatic spikes in geomagnetic intensity, each corresponding to virtual axial dipole moment (VADM) in excess of 200 ZAm2. The geomagnetic spikes rise and fall over a period of less than 30 years and are associated with VADM fluctuations of at least 70 ZAm2. Thus, the Levantine archaeomagnetic record places new constraints on maximum geomagnetic intensity as well as for its rate of change. Yet, it is not clear whether the geomagnetic spikes are local non-dipolar features or a geomagnetic dipolar phenomenon. 相似文献
15.
Steven C. Cande 《Earth and Planetary Science Letters》1978,40(2):275-286
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
16.
S. Ghoshal 《Pure and Applied Geophysics》1970,81(1):223-229
Summary
Stewartson [1]2) has considered the inviscid flow past a sphere in the presence of a uniform magnetic field andMurray andLudford [2] have investigated a similar problem in which the magnetic field originates from an axially symmetric dipole field situated at the centre of the sphere. In connection with the study of earth's magnetic field, the toroidal part of this field plays a dominant part. This gives rise to the importance of studying the effect of a toroidal magnetic field on flows past different bodies of revolution; specially past spheres and spheroids. In the present note inviscid flows past a sphere, and a spheroid, are considered, for the case of a toroidal magnetic field originating in the fluid. In the case of the sphere the field inside the sphere consists of an electric dipole directed along the axis of symmetry together with a uniform electric field which produces a uniform current along the axis. In the case of the spheroid, the field inside it is due to an electric dipole and quadrupole directed along the axis of symmetry, together with a uniform electric field which produces a uniform current along this axis. 相似文献
17.
The aim of this study is to date by the archaeomagnetic method the last heatingcooling cycle of one Roman and two Medieval tile kilns, discovered in Belgium. The investigation demonstrates the limitations when well-documented local directional secular variation curves of the geomagnetic field in the past are used for dating and the difficulties when trying to determine field intensities from “in situ” baked clays from the kilns. The three kilns yielded very well defined ancient field directions but two possible dating solutions for each of them when no a priori time constraints are taken into account, due to field direction recurrence. As an increase of the dating accuracy and reduction of the number of dating solutions can be expected using the full field vector information, also field intensity determinations on burnt clays from the kilns were attempted. Field intensities from samples of the Roman and of one of the Medieval kilns are quite scattered. On the other hand, results obtained applying the Thellier-Thellier method and the modified method developed by Dekkers and Böhnel on sister samples from the Roman kiln agree fairly well. Rock magnetic properties reveal high variance in the kilns that point to varying spatial heating and cooling conditions in the kilns. Even well burnt material from the kilns shows irreversible changes when heated in air in the laboratory. Reliable field intensities on “in situ” baked materials from kilns themselves can therefore only be obtained when measuring sufficient number of samples from different parts of the kiln, taking into account the spatial-temporal conditions during kiln operation and cooling history. More reference intensity data is needed in our regions in order to improve dating based on directional reference data only. 相似文献
18.
We performed wavelet analysis of series of the magnetic field intensity obtained through archaeomagnetic studies in Bulgaria, Georgia, Central Asia, China, and Japan. The duration of the studied intervals is 2000, 4000, and more than 7000 yrs. It has been shown that periodic oscillations as well as those with varying characteristic times are present in this series. The distribution of oscillations with characteristic times up to 800–1000 yrs by periods, characteristic times, time, and place of their manifestation is random. 相似文献
19.
The results of wavelet-analysis of series of archaeomagnetic data on the geomagnetic field intensity
We analyze the results of wavelet analysis of series of archaeomagnetic data on the geomagnetic field’s intensity over the past 7500 yrs performed by Gurarii and Aleksyutin and presented in the paper “Wavelet Analysis of Basic Archaeomagnetic Series of Data on Geomagnetic Field Intensity over the Past 7500 Years” [Gurarii and Aleksytin, 2009]. The results in themselves are beyond question, but their interpretation given in the aforementioned paper is very controversial. Analyzing these results, we arrive at the conclusion that the periods of variations identified reliably (very strongly or identified in a long time interval) lie within certain bands with about 300-, 500-, 750-, and 1600-yr centerlines. The conclusion is drawn on the existence of general regularities in the behavior of geomagnetic field variations. 相似文献
20.
本文利用近三十年来的地面测量资料和东亚地区11个地磁台的资料,以及1965.0、1975.0国际地磁参考场等资料,对东亚大陆磁场的时、空变化特征进行了研究。内容包括:东亚大陆磁场的空间分布特征及场源深度的估算,东亚大陆磁场的中心位置及其磁场强度、磁场方向,近三十年来东亚大陆磁场的长期变化特征等。得到的主要结果是:1.东亚大陆磁场的中心位置近三十年来没有西向漂移的迹象,从而推断它的长期变化主要由场源强度的变化所引起,而不是主要由非偶场的西向漂移所引起;2.东亚大陆磁场的中心与它的长期变化的中心不相一致,同时,在青藏高原的东部还可能存在一弱中心。 相似文献