Paleomagnetic studies carried out on the bottom sediments from lake El Trebol, South Argentina are described. Two cores, of length 7.5 m and 11 m, were collected and subsampled. The pairs of magnetic susceptibility and intensity of remanent magnetization profiles show a good correlation with the sedimentological macroscopic descriptions, and also with those of shorter cores previously studied. The stability of the natural remanent magnetization was investigated by alternating field demagnetization and the stable remanent magnetization was isolated. Declination and inclination logs were consistent with the correlations. Radiocarbon dating from earlier studies have enabled us to construct a secular variation record from South Argentina for the past 17, 000 years. The declination and inclination logs show a significant anomaly at about 15, 500 yr b.p., which is accompanied by a decrease in both intensity of magnetization and modified Koenigsberger ratio, but without any change in susceptibility. These results suggest the possibility that a geomagnetic excursion or very short reversal of the geomagnetic field was recorded. Excursions at about this time have been recorded in several other parts of the world.

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Five stalagmitic speleothems from two caves in Sichuan province China, have yielded data on the palaeosecular variation (PSV) of the past 8.9 ka. The uranium-thorium disequilibrium method has been utilised in order to estimate the age of each record. Due to the high detrital load contributing some allogenic thorium, age corrections had to be applied to most sub-samples. The directional agreement between coeval central and lateral sub-samples where growth layers are horizontal and steeply dipping respectively suggests that inclination errors are absent in these speleothems. Correlation between features of declination and inclination in the individual records, even after time-averaging, are reasonable indicating a local consistency in recorded PSV and adding to the reliability of the palaeomagnetic and age data. Comparisons between our speleothem directions and the directions of the modern field drifted westward past our site at 0.13° yr^-1 suggests westward drifting of non-dipole sources during the past 2.8 ka. Using directional data from two speleothems with the longest growth periods a composite curve covering the past 8.9 ka has been constructed. The resulting VGP path again indicates predominantly westward drift. The declination and inclination features of these speleothems also compare reasonably with features of PSV in contemporaneous records from China and Japan.

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Archaeomagnetic data from “Britain” (46°N-58°N, 12°W-8°E) and S. E. Europe (37°N-49°N, 15°1-25°E) for the last 2, 300 years are selected on the basis of the reported reliability of their archaeological age as well as the precision in the determination of their mean site values. When grouped into mean values for each decade, clear trends are visible for both areas, although individual sites show somewhat anomalous values that are largely attributed to in situ site disturbances. The trend lines, when smoothed by a two-point running mean, appear to give a good representation of the geomagnetic secular variation in these two areas. Comparison between the areas, that are 9° in latitude and 26° longitude apart, suggests that an axial geocentric dipole could largely account for the average difference in inclination. The quasi-sinusoidal patterns are more complex in declination than inclination, as previously proposed on less reliable data. There are visual indications of periodicities of some 300 years in inclination and some 400 years in declination but periodicities longer than the time range represented by the data are present. The time of occurrence of maxima and minima for both declination and inclinations suggests that both westward and eastward drift of the geomagnetic field patterns have occurred.

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North American archaeomagnetic data have been compiled using a relational database management system. There are currently 1875 archaeomagnetic directions and 151 paleointensity results in the database.

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A recent (2000 BP) lava flow from central Mexico has been sampled along a vertical profile with 55 cores covering a total flow thickness of 6.6 m. A wide range of physical and magnetic parameters have been studied to characterise the samples: Curie temperature and saturation magnetisation as intrinsic properties; density, magnetic susceptibility and remanence intensity as bulk properties; Königsberger Q-factor and hysteresis parameters as coercivity parameters. All parameters vary smoothly over the profile, most probably due to grain size variation of the magnetic minerals present in the samples. Optical observations indicate that the main opaque minerals are deuterically oxidised titanomagnetites (C3-C5) and ilmenites (R2-R3), which increase in size away from the edges of the flow. Paleointensity (PI) was determined using the double heating Thellier-Thellier method with pTRM checks. According to reliability parameters (f-, g-, and q-factor) the obtained PIs are of reasonable to good quality. PI shows marked variation with vertical position in the flow, across a range of about 25 to 125 μT, with most samples having a PI between 50 and 100 μT. The flow-mean PI of 72 μT is higher than the present day field, consistent with global data for this time-period. No obvious correlation could be found between PI and any other measured parameter. The variation of PI with vertical position in the flow may show some systematic behaviour. It is important, therefore, to sample a flow both horizontally and vertically in order to obtain a reliable paleointensity.

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Rock magnetic properties of archaeomagnetic samples taken from ovens are studied. Thermal demagnetization of saturation remanences and other studies reveal the presence of: (titano)magnetite, iron oxyhydroxides, and maghemite with T_b ∼ 650°C which is stable with respect to inversion. During thermal demagnetization the soft IRM component (0.06 T) is always the strongest one, indicating the importance of the (titano)magnetite contribution. A break-up in the values of coercivity H_cr' with respect to initial mass-specific magnetic susceptibility (χ) occurs at χ ∼ 1.5 × 10^-6 m³/kg and separates poorer from better quality samples in the palaeointensity experiment. Separation of bad and good Thellier experiments is also observed in the relation between the coercivities and the concentration-independent parameter SIRM/χ. Thus, the poor quality of the palaeointensity experiments for some samples can probably be ascribed to the presence of weathering products and MD magnetite grains while the good quality of these experiments is related to “soft” hysteresis properties and a broad unblocking temperature spectrum.

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Samples with a significant fraction of multidomain magnetic grains, hard secondary components and thermally unstable magnetic phases have been shown to be unreliable for paleointensity studies. However, mineral magnetic screening is rarely performed before paleointensity determinations are made even through non-ideal magnetic properties are the main reasons for rejecting data after the work has been completed. We have conducted a detailed mineral magnetic investigation of 23 archaeological samples from Bulgaria which yielded both satisfactory and unsatisfactory paleointensity results. Our study demonstrates how the non-ideal magnetic properties lead to unacceptable paleointensity results. We have used our findings to develop a simple and practical sample selection procedure which requires only two specimens from each sample and which can be done with conventional paleomagnetic equipment. We suggest that any sample which fails to pass this screening should not be subjected to time-consuming Thellier-Thellier experiments.

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Andesite lava sequences of the Quaternary Ruapehu Volcano, New Zealand were studied to obtain paleosecular variations, both direction and intensity. K-Ar dating was performed on 7 lavas of the younger Whakapapa Formation and 22 lavas of the older Te Herenga Formation. Initial experiments by the isotope dilution method using an ³⁸Ar spike were unsuccessful on the younger lavas, while an amplitude method in which the abundance of Ar is estimated from a peak height of mass spectrum gave consistent results of 32 ± 12 ka after the correction of the Ar isotopic ratios for mass dependent fractionation. The latter method also gave a more accurate result of 229 ± 35 ka for the older group. No significant relationship was observed in age versus stratigraphy for the 250 m thick older lava section. This suggests a short time span, less than the errors of dating, for the lava accumulation. Primary paleodirections were easily obtained, after cleaning by both AF and thermal demagnetization, from all lavas except two in which a possible effect of lightning was suspected. Paleointensity experiments, using mainly the Shaw method, were made on 15 lavas which showed good stability to thermal demagnetization. In the analysis of the Shaw results, the correction method of Kono (1978) was applied, since most samples showed changes in ARM capacity after heating. The double heating technique of Tsunakawa and Shaw (1994) was also applied to some lavas, and was useful to discriminate erroneous data when inconsistent results were obtained. Serial correlation was not detected in the paleodirections of any series of the lavas. In spite of the random appearance of the data, the dipole direction was not obtained from the older section which included 20 data.

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Despite the limited number of paleointensity data available for the last 400 Ma, some general features of the magnetic field in the past can be drawn from their analysis. A mild selection applied to the set drastically reduced the number of determinations, underscoring the unequal quality of the paleointensity estimates at hand and the clear need for many more new reliable studies. However, with or without selection, the record is characterized by a succession of periods with alternatively low and high fields, but data available are yet insufficient to propose any model of transition between both regimes. For the last 400 Ma, the dipole nature of the main field is preserved. This is also true when only data from the Mesozoic Dipole Low (120-260 Ma) are considered. Moreover it is shown that the Mesozoic data are very unlikely to represent an insufficient sampling of a Neogene-type field. These last observations strengthen the reality of this long period where the intensity of the main field was roughly only one third of the present-day value. A possible relation between field strength and secular variation (approximated by standard deviation) appears to exist, although this remark is compromised by the existence of a similar relation between standard deviation and number of determinations. The distribution of all Virtual Dipole Moments is log-normal, as shown before, but when only the selected data set is considered the distribution is clearly bi-modal. An oscillatory or bimodal paleointensity behaviour rather than a monotone variation is not at all unexpected given the highly non-linear geodynamo equations.

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Expressions are derived for the magnetic field directions produced by randomly varying Gauss coefficients. This approach was pioneered by Constable and Parker (1988), and followed by Kono and Tanaka (1995) and Kono and Hiroi (1996). The present treatment is a sequel to the latter two. The basic assumption is that the magnitude of the axial dipole component is much larger than all the other terms: equatorial dipoles or nondipole components. It is also assumed that, in a sufficiently long time interval, Gauss coefficients, g_l^m and h_l^m, behave as independent random variables that have the first and second moments defined. General expressions are obtained for the field directions (inclination I and declination D). Paleosecular variation (PSV) of field directions is formulated as latitude dependences of statistical parameters (the means and variances) of inclination and declination. It is possible to derive a family of PSV models by specifying the statistical nature of individual Gauss coefficients, i.e., assigning means and variances to them. In this paper, we describe a “homogeneous background model” (HBM), which consists of a large axial dipole component (g₁⁰) and other harmonics that have zero mean and variances decreasing exponentially with the degree of the harmonic. This model is too homogeneous to reproduce observed latitudinal variation of secular variation, but is quite useful to identify the harmonics that have effect that is larger than produced by the HBM. Comparison with paleomagnetic data obtained from volcanic rocks of 0-5 Ma ages (Quidelleur et al., 1994) shows that it is necessary that the mean of g₂⁰ must be about 5% of the mean of g₁⁰, and that g₂¹ (or h₂¹) should have about three to four times as large an effect as suggested by the HBM. The former conclusion is consistent with the result of Constable and Parker (1988), while the latter conclusion substantiates the similar conclusion obtained by Kono and Tanaka (1995) by the analysis of the scatter of the virtual geomagnetic poles (VGPs).

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