Nineteen new archeointensity results were obtained from the analysis of groups of French pottery fragments dated to the Early Middle Ages (6th to 10th centuries AD). They are from several medieval ...ceramic production sites, excavated mainly in Saran (Central France), and their precise dating was established based on typo-chronological characteristics. Intensity measurements were performed using the Triaxe protocol, which takes into account the effects on the intensity determinations of both thermoremanent magnetization anisotropy and cooling rate. Intensity analyses were also carried out on modern pottery produced at Saran during an experimental firing. The results show very good agreement with the geomagnetic field intensity directly measured inside and around the kiln, thus reasserting the reliability of the Triaxe protocol and the relevance of the quality criteria used. They further demonstrate the potential of the Saran pottery production for archeomagnetism. The new archeointensity results allow a precise and coherent description of the geomagnetic field intensity variations in Western Europe during the Early Medieval period, which was until now poorly documented. They show a significant increase in intensity during the 6th century AD, high intensity values from the 7th to the 9th century, with a minimum of small amplitude at the transition between the 7th and the 8th centuries and finally an important decrease until the beginning of the 11th century. Together with published intensity results available within a radius of 700 km around Paris, the new data were used to compute a master curve of the Western European geomagnetic intensity variations over the past 1500 years. This curve clearly exhibits five intensity maxima: at the transition between the 6th and 7th century AD, at the middle of the 9th century, during the 12th century, in the second part of the 14th century and at the very beginning of the 17th century AD. Some of these peaks are smoothed, or nearly absent when the selection of the data is extended to a 1250 km radius around Paris. The apparent regularity in the occurrence of intensity maxima, with a recurrence of similar to 250 years, is particularly intriguing and might reflect a new characteristic of the secular variation, at least in Western Europe. It clearly requires further investigation and in particular the acquisition of new data from older periods.
We investigate the origin of the so-called archeomagnetic jerks detected in the French archeomagnetic record over the past three millennia. Although only very large-scale global archeomagnetic field ...models are currently available, we show that the occurrence of archeomagnetic jerks is intimately linked to what we define as “most eccentric” events, i.e., periods of time when a simple description of the geomagnetic field in terms of an eccentric dipole reveals the center of this eccentric dipole to strongly move away from the Earth's center. From the behavior of the much better known historical field, we interpret the evolution of the center of the eccentric dipole as reflecting the production and gathering of flux patches at the core–mantle boundary within preferential hemispheres. Archeomagnetic jerks would thus correspond to episodes of maximum geomagnetic field hemispheric asymmetry. Such “most eccentric” events could also provide an explanation for some of the properties previously reported in the long-term paleomagnetic field.
Archaeomagnetic data are fundamental for our understanding of the evolution of Earth's magnetic field on centennial to millennial timescales. From the earliest studies of the Thelliers, Aitken, ...Nagata and others in the 1950s and 1960s, archaeomagnetic data have been vital for extending our knowledge of the field to times prior to observational measurements. Today, many thousands of archaeomagnetic data allow us to explore the geomagnetic field in more detail than ever before. Both regional time series of archaeomagnetic data and the inclusion of archaeomagnetic data in time-varying global spherical harmonic field models have revealed a range of newly discovered field behaviour. More sophisticated approaches to developing regional curves and global models have allowed us to resolve the field in certain regions more robustly and with greater resolution than previously possible. In this review we give an overview of the widely used global archaeomagnetic database GEOMAGIA50, discuss the methods used to obtain archaeomagnetic data, their challenges, and explore progress over the past twenty years in developing regional secular variation curves and global spherical harmonic models of the archaeomagnetic field. We end the review by covering what we see as the “grand challenges” in archaeomagnetism, including which regions of the world should be focussed on with regards to data acquisition.
•Review of all global archaeomagnetic data.•Overview of different types of data and their number.•Discussion on archaeomagnetic and dating methods.•verview of geomagnetic field models for the Holocene.•Future challenges for archaeomagnetism.
Triaxe archeointensity analysis Gallet, Yves; Le Goff, Maxime; Genevey, Agnès
Physics of the earth and planetary interiors,
11/2022, Letnik:
332
Journal Article
Recenzirano
Odprti dostop
Since 2004, numerous archeomagnetic intensity data have been obtained using the vibrating sample magnetometer Triaxe, which measures full-vector magnetization directly at high temperatures, in either ...an applied or zero field. Satisfactory comparisons have been made between Triaxe intensity data and results derived from more conventional Thellier-Thellier type techniques, indicating the reliability of Triaxe data. For each specimen analyzed, a Triaxe archeointensity value is obtained from the average of R'(Ti) data. The R'(Ti) parameter is determined every 5°C and corresponds to the ratio, multiplied by the laboratory field intensity, between the natural remanent magnetization (NRM) and laboratorythermoremanent magnetization (TRM lab) fractions that are lost between reference temperature T1 and a given temperature Ti between T1 and reference temperature T2. Here, we introduce an additional parameter, based on so-called AutoR'(Ti) data, to facilitate and improve the interpretation of Triaxe measurements. Each individual AutoR'(Ti) datum corresponds to an averaged R'(Ti) value; the AutoR'(Ti) dataset is then obtained by gradually decreasing the temperature range from T1 to T2 to a minimum temperature interval near T2. Several examples of Triaxe measurements show the value of using AutoR'(Ti) data to isolate the most appropriate temperature range for an intensity determination, as well as to characterize the cooling rate effect on TRM acquisition. In particular, these experiments confirm that the Triaxe procedure minimizes this effect because, when it is present, it appears to be largely due 2 to magnetic grains with high unblocking temperatures (>~350°C). Moreover, the AutoR'(Ti) dataset provides alternatives for estimating mean archeointensity values at both the fragment and fragment-group levels. We show that the simple approach used so far, based on the average of the R'(Ti) data determined over a single temperature interval, provides results as reliable as those derived from other options.
Twenty new intensity determinations of the ancient geomagnetic field have been obtained from groups of potsherds and brick fragments from Syria. These artifacts, archeologically well dated from ∼6000 ...B.C. to approximately A.D. 1200, have been analyzed using the Thellier and Thellier 1959 method as modified by Coe 1967. Intensity values have been corrected for the effects of anisotropy of thermal remanent magnetization and cooling rate. Our results indicate that field intensities were moderate in Syria from ∼6000 B.C. to ∼3500 B.C., with values of ∼30–40 μT. There was a significant increase in intensity by a factor of 2 from ∼3500 B.C. to ∼700 B.C., which was interrupted by a moderate decrease between ∼2550 B.C. and ∼1750 B.C. During more recent periods, our results show an intensity minimum approximately A.D. 200 and a maximum around the tenth century. Comparison with different data sets from the eastern Mediterranean and central Asia shows that geomagnetic field intensity variations were consistent at this large regional scale, at least over the last 5 millennia.
Nearly 40 years ago, Emile Thellier published an article summarizing the archeomagnetic data he had obtained during his career, which had allowed him to recover the main features of the directional ...variations of the geomagnetic field in France for the last two millennia. This database went on to be significantly completed 25 years ago by Ileana Bucur, who had taken over Thellier's work on archeomagnetism; this forms the current basis of our knowledge of the directional evolution of the geomagnetic field in France. Since then, archeomagnetic studies have been continued at Thellier's historical laboratory in Saint Maur. This article presents the directional archeomagnetic data obtained in France over the past 25 years in this same laboratory. A total of 528 new data are presented, which, together with the 170 results obtained on the French territory previously listed in Bucur (1994), constitute the French directional database (698 data in all). All but two of these data were obtained using the experimental protocol developed by E. Thellier based on the analysis of large samples and on a magnetic viscosity test that has proved its reliability on numerous occasions. The directions from the entire French database have been precisely defined, with 95% of the α95 values being less than 1.9°, and ~ 50% being less than 0.8°. The selection of 286 data with dating uncertainties of ≤50 years allowed us to compute a new reference directional variation curve for France since the first century BC using sliding windows, the variable durations and shifts of which were adapted to the time distribution of the available data, and the bivariate extension of the Fisher statistics. This shows ample and smooth variations, fairly similar to those previously determined from a much smaller database. The resulting secular variation curve is particularly well suited for use in archeomagnetic dating.
•We add 528 new data to the French archeo-direction database for the past 2 millennia•All but 2 data are obtained from domestic and artisan kilns using Thellier's protocol•The directions are accurately defined with 95% of the α95 values being less than 1.9°•Bivariate statistics and a sliding window technique are used to determine a mean curve•The French curve shows ample variations well suited for use in archeomagnetic dating
•New archeointensity data were obtained at Novgorod in North-Western Russia.•Results derive from the analysis of bricks dated to between the 12th and 17th C. AD.•The data show an overall decreasing ...trend in field intensity over the past millennium.•The data do not support large intensity variations such as observed in the Balkans.•Non-dipole field effects appear limited across Europe over the past 1000years.
Reconstructing the secular variation of Europe’s geomagnetic field over the past millennium is challenging because of the lack of recently acquired archeomagnetic data from Western Russia. In this paper, we report on nine new archeointensity values obtained from groups of brick fragments sampled in Novgorod (North-Western Russia) and its vicinities. These fragments were collected from churches whose precise ages range from the beginning of the 12th century to the end of the 17th century AD. All the archeointensity measurements were carried out using the Triaxe experimental protocol, which takes into account the thermoremanent magnetization (TRM) anisotropy effect. Intensity determinations were performed using fast and slow cooling rates for laboratory-TRM acquisition. The results confirm that the Triaxe protocol overcomes the TRM cooling rate dependence. The new data shows that geomagnetic field intensities in North-Western Russia have decreased in the past millennium. Comparisons were made with other data previously obtained in Western Europe, the Balkans and Russia, as well as with intensity values expected in Novgorod from global geomagnetic field models. These comparisons yielded three main results: 1) The new archeointensity data do not show the occurrence of large intensity variations in North-Western Russia, as those observed in the Balkan dataset. Conversely, they appear more compatible with Western European results, which suggests a limited non-dipole field effect across Europe during the past millennium; 2) Our data are weaker than the intensity values expected in Novgorod from the available global geomagnetic field models. This suggests that the field models are inaccurate for the Novgorod area; 3) A constant linear decrease of the geocentric axial dipole moment since 1600 AD does not appear compatible with our younger data.
Since the mid-19th century, direct measurements of both intensity and direction of the Earth's magnetic field have been available, allowing an accurate determination of its spatio-temporal ...variations. Prior to this time, between ~1600 and 1840, only direct directional measurements are available. Therefore, the construction of global field models over this period requires either a specific treatment of the axial dipole field component or the use of archeomagnetic intensity data. In this study, we use a regional approach based on the construction of an archeointensity variation curve in Central Asia. We analyze baked clay brick fragments sampled in Bukhara (Uzbekistan), dated between the end of the 16th century and the beginning of the 19th century. This city is of particular interest for archeomagnetism due to the well-preserved old buildings accurately dated by documentary archives. A series of archeointensity results is obtained using the Triaxe experimental protocol, which shows a decreasing trend in intensity from ~1600 to ~1750, with intensities during the 18th century lower than expected from global geomagnetic field models. These new data appear consistent with other Triaxe data previously obtained in western Europe and western Russia, when transferred to Bukhara using the field geometry of the gufm1 model. Together, these data are used to recalibrate the axial dipole moment evolution provided by this model. The resulting evolution appears non-linear, with a clear relative minimum in the magnitude of the axial dipole during the late 18th century. We illustrate the fact that at present this evolution can neither be satisfactorily confirmed nor refuted by other datasets available in western Eurasia (as well as at a wider spatial scale), mainly due to the significant dispersion of the data. Our interpretation relies on the accuracy of the field geometry of the gufm1 model, which appears less reliable prior to ~1750. Nevertheless, the minimum proposed in the 18th century seems to be a true feature of axial dipole behavior.
•We report 9 new archeointensity data from Central Asia dating from ~1560 to ~1800.•Our results agree with previous Triaxe data obtained in Western Eurasia.•Taken together, these data support a non-monotonic evolution of the axial dipole between ~1600 and ~1840.•A minimum ∣g10∣ of ~29400 nT occurred during the second half of the 18th century.•The subsequent increase of ∣g10∣ from that period to ~1840 is confirmed by Triaxe data in Brazil.
Secular variation (SV) of the Earth’s magnetic field can be used for dating purposes by comparing archeomagnetic directions of unknown ages with a well-dated reference curve. In this study, we ...propose a dating technique based on the statistics of McFadden and McElhinny Geophys. J. Int. 103 (1990) 725 for testing the hypothesis that two Fisherian distributions of individual directions share a common mean direction. The statistics are adapted to test the degree of compatibility between one individual Fisherian mean direction and a reference curve constructed using the bivariate extension of the Fisher distribution. Furthermore, as the density of the data which define the archeomagnetic reference curve varies in time, we suggest that one computes the mean directions with moving windows of varying duration, where both the window widths and the time shifts between successive mean directions are fixed when a minimum threshold density of data is reached within each time interval. In our paper, we apply this new procedure to the French archeomagnetic data set.
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