Archaeomagnetic investigation of two mediaeval brick constructions in north Belgium and the magnetic anisotropy of bricks

Archaeomagnetic dates derived from geomagnetic field direction records in baked materials are proposed for a mediaeval brick kiln (from inclination I and declination D) and for bricks from a brick wall (from I only) in northern Belgium. They are used to verify whether a brick chronology based on the format of bricks is feasible in Flanders. The brick kiln yielded a highly reliable average magnetization direction corresponding to an archaeomagnetic date around A.D. 1650, using the British and French geomagnetic field secular variation curves as a reference, at least half a century younger than expected from historical data. The fidelity of the geomagnetic records was controlled by measuring the magnetic anisotropy of the bricks. Anisotropy of magnetic susceptibility (AMS) measurements demonstrate that the bricks have a shape related magnetic fabric, which is induced during the molding process. A test to control whether AMS can substitute for the anisotropy of thermo‐remanent magnetization (ATRM) failed because of induced changes during laboratory heating. © 2003 Wiley Periodicals, Inc.     

Archaeomagnetic record from Modoc Rock Shelter,Illinois, for the time range of 6200–8900 B.P.

The Modoc Rock Shelter is located in Randolph County, Illinois at the base of the Mississippi River bluffs, and is documented as having human occupation from 9000 B.P. to as recently as 800 B.P. Two hundred twenty-eight archaeomagnetic samples were collected from 28 surface hearths in two different excavation areas. Pilot samples from each hearth were A.F. demagnetized to 100 mT to establish the optimum demagnetization level for all samples from a specific hearth. The magnetic directions for individual hearths after appropriate demagnetization have high precision with α-95 values between 2° and 4°. Independent inclination and declination records from the two excavations, which were correlated with conventional radiocarbon dates, overlap in time and show good correlation. The composite record shows a prominent eastward declination swing of 15° from 6500–7800 yr B.P., followed by a westward swing of equal magnitude from 8000–8900 yr B.P. The inclination record is less varied. This secular variation record correlates well with soft sediment records from lakes in the north-central United States and is perhaps the longest and oldest archaeomagnetic secular variation record from a single site in North America. © 1997 John Wiley & Sons, Inc.     

Over how much time should the geomagnetic field be averaged to obtain the mean-palaeomagnetic field?

Average geomagnetic field models are now available over six orders of magnitude in time. Models based on Present and historical, archaeomagnetic and palaeomagnetic data illustrate how the amplitudes of secular variation of nonaxial dipole terms strongly decrease as a function of averaging window, mostly when jumping from the 1-yr to the 1700-yr average. The mean archaeomagnetic field bridges the gap between the complex historical field and the far simpler palaeomagnetic field, and appears to be much closer to the latter. Confidences in average field coefficients based on statistical field models give a quantitative basis to the practice of averaging data over a few millenia in order to 'suppress secular variation'. In particular, the axial quadrupole emerges from noise when the available time series exceed about 6 kyr. This happens to be close to the timescales of excursions and reversals. It is now a priority to obtain detailed data spanning the last 50 kyr.     

Archaeomagnetic dating of seven archaeological fireplaces in the Netherlands

The palaeomagnetic directions of seven Dutch fireplaces are compared with the archaeological age estimates which range from the first to the 17^th century AD. A comparison with the British master curve of secular variation for archaeomagnetic dating results in a refinement of the archaeological age estimates in two cases, while four other archaeological age estimates can be confirmed. For one fireplace only one sample is reliable, resulting in a very poorly defined archaeomagnetic age of 2 to 3 centuries younger than the expected age (i.e. late Middle Ages). On the other hand, accepting the archaeological age estimates, the palaeomagnetic directions can contribute to the database that is used to construct the British secular-variation master curve. We applied the classification grades proposed by Tarling & Dobson (1995) which range from unreliable (grade 1) to reliable (grade 5). Three fireplaces have grades 5, one has grade 4, one grade 3, one grade 1 and for one case no grade was assigned.     

Quaternary magnetostratigraphy in Denmark

Magnetostratigraphical results from various historical, Holocene, Lateglacial and Glacial deposits from Central Jutland, Vendsyssel, the Baltic Sea and Schleswig-Holstein are illustrated and discussed. Although only a few archaeomagnetic case studies have yet been made in Denmark, magnetic datings with accuracies of about ±50 years have been obtained on mediaeval kilns by comparing with a dipole transformed modification of the British archaeomagnetic master curve. Furthermore, a study of the expected east–west orientation of some 330 Romanesque churches in Denmark seem to indicate that about 25% of these churches were magnetically orientated; thus some kind of a magnetic compass may have been in common use in the twelfth century in Denmark. Radiocarbon-dated Holocene lake sediments from lake Skanderborg reveal a distinct pattern of magnetic secular variation with fairly short time constants, which may be useful in magnetostratigraphical correlations of other lake sediments. Also Lateglacial and Early Postglacial sediments from the southern Baltic Sea show systematical magnetic patterns, whereas glacial boulder clay at the bottom shows disturbed directions, obviously moulded by the moving ice. Lateglacial Younger Yoldia clay from North Jutland shows well-developed short periodic swings as well as an extreme declination variation of some 80° to 90°, the so-called 'Nørre Lyngby declination excursion' around 14,000 B.P. A sequence of Older Yoldia clay at the same site furthermore shows significantly low inclination values, the so-called 'Rubjerg low inclination excursion' of an age between 23,000 B.P. and 40,000 B.P. Finally, a well-clustering palaeomagnetic direction from a Weichselian glacial boulder clay from Timmerhorn north of Hamburg is discussed from a hierarchical, statistical point of view.     

近两千年来的地磁极章动

地磁场的缓慢变化(周期在一年以上)称为地磁场的长期变。由于长期变不仅与地磁场起源的理论关系极大,而且对于古地磁学中的基本假定(轴向地心偶极子假说)有着头等重要的意义,因此,近来颇受重视,讨论相当热烈。     

Palaeomagnetism and rock magnetism of the Deccan traps

Extensive work on the palaeomagnetism of the Deccan traps by several workers has revealed only one reversal (normal-reverse-normal) of the geomagnetic field during the period of eruption of these flows. The scatter in the natural remanent magnetic directions of different flows in a sequence of these traps is rather large and it persists even after magnetic cleaning. Generally this large scatter of directions is attributed to the geomagnetic secular variation during formation of the traps. Recent studies of the magnetic properties of the trap samples indicate that the presence of multidomain magnetite in a lava sequence differs from locality to locality in the Deccan trap province and so the stability of their natural remanent magnetization. While these features can be clearly perceived, an understanding of the same seems to be quite difficult. However, it appears that part of the scatter in directions could be due to the alteration of the magnetic minerals in the Deccan traps and hence it would not be proper to relate the entire scatter to the secular variation.     

Holocene sediments and magnetic stratigraphy from Lakes Zug and Zurich, Switzerland

Piston cores 7 m and 6.5 m long were collected from Lakes Zug and Zurich respectively. Radiocarbon age determinations, pollen assemblage zones and varve counts indicate dates of 5000–6000 years B.P. (Zug), and 12,000–13,000 years B.P. (Zurich) for the base of the cores. Declination of the magnetic remanence varies through 70° and inclination varies through 30°. Although the correlation with the geomagnetic secular variation curve covering the last 15,000 years as determined from Lake Windermere, England (Creer et al., 1972) is not precise, the results suggest that Lake Zug and Lake Zurich sediments have been recording the broad outline of past secular variations of the earth's magnetic field. Since intensity of magnetization and susceptibility correlate markedly with lithology, a detailed sediment stratigraphy is presented. The amount and texture of the detrital input appears to be a controlling factor for the natural magnetic remanence. Intensity varies from 90 μG in zones of organic, sulphide pigmentation and those with ultra-fine laminations to 0.8 μG in impure lacustrine chalk. Susceptibility ranges from 9 μG/Oe in laminated, fine-grained glacially derived muds to 0.5 μG/Oe in impure lacustrine chalk. Intensity of magnetization also varies systematically within individual turbidites with lowest values in the coarse-grained, basal fractions. Slumped beds were identified on the basis of erratic anomalies in magnetic declination and inclination measurements.     

Tectonomagnetic study in the seismoactive area of Narmada-Son lineament,central India: Preliminary results on repeat field observations

Repeated measurements of the total geomagnetic field on the five profiles have revealed a picture of stress-induced tectonomagnetic effect in the form of secular variation of the total geomagnetic field in the tectonically and seismically active area of Jabalpur and adjoining areas of the Narmada-Son lineament (NSL), central India. For this experiment, a reference base station was established within the study area at Jabalpur. Using proton precession magnetometers with a sensitivity of 0.1 nT, simultaneous measurements of total geomagnetic field were made annually at the base and all field stations. Five cycles of repeated observations have been performed between 2003 and 2007. For data analysis, a difference method has been applied and the residuals have been calculated as secular variations of the total geomagnetic field with values ranging from ±0.1nT to about ±14.6nT/yr over the different stations. The anomalies in secular variation of the total geomagnetic field may be related to anomalous accumulation of tectonic stresses and tensions on the deep fault zones and crustal blocks due to recent geodynamic processes and active geological inhomogeneities in the NSL.     

Near east paleomagnetic secular variation recorded in sediments from the Sea of Galilee (Lake Kinneret)

Paleomagnetic records of declination and inclination from sediments recovered from the bed of Lake Kinneret (32.4°N, 35.7°E) have been dated by radiocarbon techniques. The sediments span the last 5000 yr. The changes in inclination down the sediment cores are more pronounced than the declination fluctuations and are repeatable between the three coring sites, which are several kilometers apart. Magnetic susceptibility logs display 13 maxima in the 5-m-long sequences, with a pronounced susceptibility minimum about 1000 yr B.P. Many of the susceptibility maxima and minima can be easily correlated between coring sites and are shown to be dominantly related to changes in sediment carbonate content. The natural remanent magnetization intensity follows a similar pattern to that of susceptibility, and the natural remanence of the Kinneret sediments is presumed to reside in detrital magnetite grains carried into the lake by the river Jordan from the basalt-rich bedrock of the rift floor and the Golan Heights. The ¹⁴C chronology is strongly supported by a pollen study in which pronounced changes in the proportion of olive pollen were interpreted as being due to extensive cultivation of olives around Galilee in the Hellenistic and Byzantine periods. The Kinneret paleosecular variation records, if accurately dated, point to a complex spatial pattern of Holocene secular variation with significant variations over distances as small as 1000–2000 km.     

Contemporary archaeomagnetic results and the accuracy of archaeomagnetic dates

When multiple archaeomagnetic samples are collected from the same site or component, it is tempting to look for evidence of temporal differences between the collected features. This paper addresses the issue of the practical limits to confidently identifying temporal differences based on archaeomagnetic sample information. Three controlled data sets suggest that when the specimen polar projections of the samples compared have substantially overlapping ranges and when sample mean projections are less than five degrees apart, statistical comparisons do not support conclusions about probable age differences. These results suggest that temporal differences less than 50 to 100 years (depending on the rate of directional change in the geomagnetic field) between features within a site or component will be difficult to confidently identify. Additional analysis leads to the hypothesis, however, that the averaged results from sets of archaeomagnetic samples might provide the basis for making fine scale temporal distinctions between sites or components from which multiple archaeomagnetic samples have been collected.     

Influence of mass-loss due to stellar wind on the orbits of binary stars to a second order theory

We examine the influence of mass-loss due to stellar wind using the celestial mechanics of variable mass, and derive first-and second-order solutions, taking into account the decreasing mass due to the stellar wind. The theoretical results show that the semi-major axis exhibits secular and periodic variations in the first-and second-order theory. The orbital eccentricity exhibits periodic, but no secular variation or changes. The longitude of periastron exhibits only periodic variations in the first-order solution, but also secular variations in the second-order solution. The theoretical results are applied to the binary star HD 698, and variability of the orbital elements of this star due to stellar-wind mass loss calculated. Published in Russian in Astronomicheskiĭ Zhurnal, 2008, Vol. 85, No. 10, pp. 896–900. The article was translated by the authors.     

The paleomagnetic record from deep-sea sediment cores

The paleomagnetic record of deep-sea sediment cores is compared with that which would be expected from our knowledge of the Earth's magnetic field. It is found that some of the scatter in directions of magnetization obtained from deep-sea cores is removed in cores with very low sedimentation rate, the cause being that the secular variation of the Earth's magnetic field is more completely averaged out over the finite size of each sample, when these samples comprise a longer time span of sedimentation. Corrections have been applied to the results from a series of cores in order to obtain the inclination of the average direction of magnetization from the average inclination and the scatter of inclination. These corrected inclination values confirm the hypothesis that the average Earth's field over the past few million years has been similar to an axial dipole displaced towards the North Pole. The amount of displacement obtained was 168 km. The record of short-period polarity intervals within the Brunhes, the Matuyama and the Gauss epochs was studied. It was shown that these intervals are very scattered in position. It is thought that some hitherto undiscovered short-period polarity intervals may be responsible for part of the scatter, but it is also highly likely that many samples give spurious reversals (i.e., ones not caused by the Earth's magnetic field).The possible correlation between climatic changes and the Earth's magnetic field is examined. It is concluded that the cores which show correlations between direction and/or intensity of magnetization and climatic indicators, thus suggesting the possible correlation between climate and the Earth's magnetic field, are not accurately recording the relevant parameters of the Earth's magnetic field. The correlation must be caused by climatic effects which have a direct influence on the magnetization of the sediments.Very little is known about the mechanisms of magnetization in deep-sea sediment cores, and there are several unexplained phenomena, such as the fact that many cores have maximum susceptibilities which are vertical, and the fact that cores differ widely in their ability to record accurately the Earth's magnetic field.     

Paleomagnetic Investigation of the Bonneville Alloformation, Lake Bonneville, Utah

Paleomagnetic secular variation in a portion of the Bonneville Alloformation is compared with secular variation in lacustrine sediments in the Mono Basin, California, and with secular variation in Lake Lahontan sediments in the northwestern Great Basin. The comparison places an age of about 18,000 yr B.P., and a span of 1000 to 3000 yr, on part of a transgressive stage of Lake Bonneville near Delta, Utah, that is coeval with a wet period in the Lahontan Basin.     

New Archaeomagnetic Data from Tunisia: Dating of Two Kilns and New Archaeointensities from Three Ceramic Artifacts

Two isolated kilns of unknown age and three well‐dated artifacts from Tunisia have been investigated using archaeomagnetic techniques. Dating has been attempted for the kilns and archaeomagnetic intensities have been obtained for the artifacts. The kilns lack datable artifacts although one of them (Sidi Zahruni) could be related to a nearby late Roman site. Comparison with the SCHA.DIF.3K regional geomagnetic model suggests for both kilns two possible periods of last use, either Roman or Medieval. Three archaeomagnetic intensities were obtained for the well‐dated ceramic artifacts adding to scarce archaeomagnetic data from Africa. The new data along with recently published archaeointensities from other Tunisian sites have been compared with the regional model SCHA.DIF.3K, its updated version, and the global ARCH3k.1 geomagnetic model. The models show relatively good agreement with the new archaeointensities and with previously published intensities. The uncertainty of the experimental data tends to increase for older artifacts, and their agreement with the geomagnetic models diminishes. The observed trend of the intensity models to underestimate values in the Tunisian area for the Roman period could be overcome by adding new data from well‐dated artifacts from North African, particularly from pre‐Roman and Roman sites.     

Palaeomagnetic results from the Hart dolerite of the Kimberley Block,Australia

A routine palaeomagnetic study of the 1800 Ma old Hart dolerite shows that sample directions of magnetism give good within-site grouping at 12 sites after magnetic cleaning. However, the site mean directions are scattered over one-quarter of the sphere, a feature that apparently does not arise from poor structural control. A baked contact test confirms that the magnetization is original. Comparison of bulk NRM, TRM, ARM and saturation IRM properties has been made with the Modipe gabbro of southern Africa from which a palaeointensity has previously been estimated. These measurements are consistent with the hypothesis that the main dipole field intensity temporarily was much reduced with respect to the non-dipole field 1800 m.y. ago resulting in increased directional scatter from secular variation. The average of the site VGPs at 29N 46E with A₉₅ = 24° thus is still a viable estimate of the palaeomagnetic pole at that time bearing in mind the larger than usual statistical error associated with it.     

Estimation of lower crust magnetization from satellite derived anomaly field

Various lines of evidence point to the lower crust as the source of the long-wavelength magnetic anomaly field measured by the POGO and Magsat satellites. Using seismically determined lower crust thicknesses and equivalent source inversion of the satellite anomaly data, magnetization for the lower crust for much of the United States has been calculated. The average magnetization for two hundred sixty-six 150 × 150 km areas is 3.5 A/m with a standard deviation of 1.1 A/m. These values are consistent with laboratory measurements of mafic-ultramafic rocks expected in the lower crust, and in agreement with previous estimates of lower crust magnetization based on long-wavelength aeromagnetic data. Average lower crust thickness for the same areas is 18.2 km (σ = 6.4 km). Thus, over large regions, it appears that variation in magnetization and variation in magnetic layer thickness contribute almost equally in causing the anomaly field variation at satellite altitude.     

Frequency dependence of the evolution of the radio emission of the supernova remnant Cas A

Many-year measurements of the radio flux of the young supernova remnant Cassiopeia A relative to the radio galaxy Cygnus A were continued at 290 and 151.5 MHz. The new data are used together with previously published observations carried out at decameter, meter, centimeter, and millimeter wavelengths to derive the frequency dependence of the secular variation of the radio flux density of Cas A: $d_\nu [\% year^{ - 1} ] = - (0.63 \pm 0.02) + (0.04 \pm 0.01)\ln \nu [GHz] + (1.51 \pm 0.16) \times 10^{ - 5} (\nu [GHz])^{ - 2.1} $ . The observed slowing of the secular variations with decreasing frequency at decameter wavelengths can be explained by a decrease in the optical depth of a remnant HII zone around Cas A with time due to recombination of hydrogen atoms. The new derived frequency dependence for the rate of the secular decrease, absolute and relative measurements of the radio flux density of Cas A carried out over the last 25 years, and the absolute spectrum of Cyg A are used to construct the spectrum of Cas A in the range 5–250 000 MHz predicted for epoch 2015.5.     

Orbital-Period Variations of the Eclipsing Binaries RY Aur,GG Cas,RS Lep,and RV Tri

Variations of the orbital periods of the eclipsing binaries RY Aur, GG Cas, RS Lep, and RV Tri are analyzed. The period variations in all of these systems can be represented as a superposition of a secular period decrease and cyclic variations that can be explained with the light-time effect due to the presence of a third body in the system. The secular period decrease could be due to magnetic braking.

     

A Double Wave in the Orbital-period Variations of the Eclipsing Binary RT Per

Orbital-period variations of the low-mass, semi-detached eclipsing binary RT Per are analyzed. In addition to the secular variation of the orbital period determined by the mass transfer between the components, cyclic variations are also present. Both the light-time effect and magnetic oscillations can describe the cyclic orbital-period variations of RT Per. The secular period increase can be explained by a uniform flow of matter from the lower-mass to the higher-mass component at a rate of 0.60 × 10⁻⁸M-/year, with the total angular momentum being conserved. The period variations can be represented equally accurately by either a superposition of two cyclic variations or a superposition of a secular period increase and two cyclic variations. Approximately the same parameters are derived for the lower-period (36.8 year) variations when the times of minima are fitted with a linear or a quadratic formula. For the longer-period variation, a period of 275 years and amplitude of 0.104 days are found using the linear formula, or 89 years and 0.045 days using the quadratic formula.