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1.
The results of numerical modeling of the geomagnetic secular variation by the method of the Giant Gaussian Process (GGP) are presented and compared with the information derived from the presentday databases for paleointensity. The variances of the positions of the virtual geomagnetic pole (VGP) calculated from the synthetic and experimental data (Brunhes epoch, effusive rocks) are nearly similar, which supports the validity of the theoretical model. The average value of the virtual axial geomagnetic dipole (VADM) calculated from the PINT world database on paleointensity and the Sint-2000 model is lower than VADM calculated by the GGP model; at the same time, the estimates based on the archaeomagnetic data give the VADM value slightly above the model prediction. The largest difference is observed in the variances of VADM, which is for all the three databases noticeably higher than the value calculated from the GGP model. Most probably, this is due to the contribution of the neglected measurement errors of VADM.  相似文献   

2.
We carried out an integrated paleomagnetic, rock-magnetic and paleointensity study of Miocene volcanic succession from the trans-Mexican volcanic belt (TMVB) north of Guadalajara. A total of 37 consecutive basaltic lava flows (326 oriented standard paleomagnetic cores) were collected at Lazo locality. Continuous susceptibility measurements with temperature and hysteresis experiments yield in most cases reasonably reversible curves with Curie points close to that of pseudo-single-domain magnetite. Two geomagnetic reversals were observed in the 300 m thick composite section. Paleosecular variation was lower than the one observed in general during Miocene. It appears that the volcanic units have been emplaced during a relatively short time span of about 1 Ma. The mean paleomagnetic directions obtained from this study do not differ significantly from that expected for the middle Miocene. The mean paleomagnetic direction calculated from all data is I=31.1°, D=354.6°, k=124 and 95=2.1°, N=37. Seventy-two samples with apparently preserved primary magnetic mineralogy and without secondary magnetization, mostly belonging to reverse polarity chron were pre-selected for Thellier paleointensity determination. The flow-mean paleointensity values are ranging from 22.4±3.4 to 53.8±6.0 μT and the corresponding virtual dipole moments (VDMs) are ranging from (5.4±0.8) to (12.0±1.4)×1022 A m2. This corresponds to mean value of (7.7±2.2)×1022 A m2, which is close to present day geomagnetic field strength. Altogether, our data suggest the existence of relatively high geomagnetic field strength undergoing low fluctuations.  相似文献   

3.
The results of paleomagnetic studies and paleointensity determinations from two Neoarchaean Shala dikes with an age of ~2504 Ma, located within the Vodlozerskii terrane of the Karelian craton, are presented. The characteristic components of primary magnetization with shallow inclinations I = ?5.7 and 1.9 are revealed; the reliability of the determinations is supported by two contact tests. High paleointensity values are obtained by the Thellier–Coe and Wilson techniques. The calculated values of the virtual dipole moment (11.5 and 13.8) × 1022 A m2 are noticeably higher than the present value of 7.8 × 1022 A m2. Our results, in combination with the previous data presented in the world database, support the hypothesized existence of a period of high paleointensity in the Late Archaean–Early Proterozoic.  相似文献   

4.
To test the reliability of the Thellier method for paleointensity determinations, we studied six historic lavas from Hawaii and two Gauss-age lava flows from Raiatea Island (French Polynesia). Our aim is to investigate the effects of the NRM fraction and concave-up behavior of NRM–thermal remanent magnetization (TRM) diagrams on paleointensity determinations. For the Hawaiian samples, the paleointensity results were investigated at both sample and site levels. For consistency and confidence in the paleointensity results, it is important to measure multiple samples from each cooling unit. The results from the Raiatea Island samples confirm that reliable paleointensities can be obtained from NRM–TRM diagrams with concave-up curvature, provided the data are accompanied by successful partial TRM (pTRM) checks and no significant chemical remanent magnetization (CRM) production. We conclude that reliable determinations of the paleofield strength require analyses of linear segments representing at least 40–50% of the total NRM. This new criterion has to be considered for future studies and for evaluating published paleointensities for calculating average geomagnetic field models. Using this condition together with other commonly employed selection criteria, the observed mean site paleointensities are typically within 10% of the Definitive Geomagnetic Reference Field (DGRF). Our new results for the Hawaii 1960 lava flow are in excellent agreement with the expected value, in contrast to significant discrepancies observed in some earlier studies.

Overestimates of paleointensity determinations can arise from cooling-rate dependence of TRM acquisition, viscous remanent magnetization (VRM) at elevated temperatures, and TRM properties of multidomain (MD) particles. These outcomes are exaggerated at lower temperature ranges. Therefore, we suggest that, provided the pTRM checks are successful and there is no significant CRM production, it is better to increase the NRM fraction used in paleointensity analyses rather than to maximize correlation coefficients of line segments on the NRM–TRM diagrams.

We introduce the factor, Q = Nq, to assess the quality of the weighted mean paleointensity, Hw, for each cooling unit.  相似文献   


5.
A detailed paleointensity study was made of a sequence of 70 successive lava flows of the Piton des Neiges volcano on the island of La Réunion (Indian Ocean). Radiometric dating brackets the age of this sequence between 130 ± 3 ka and 72 ± 3 ka. Rock magnetic investigations show that titanomagnetites in the pseudo single domain range are the main magnetic carrier of the Natural Remanent Magnetisation (NRM). Over 350 samples were used for paleointensity determinations carried out with the Thellier method in vacuum or in an argon atmosphere. Of these, 89 samples yielded reliable results, with within-flow scatter often lower than 20%. These results indicate that the geomagnetic field intensity has varied at La Réunion between 13 and 65 μT during the period of time explored. The average value, 42 μT, is higher than the present field at La Réunion (35 μT). The results from the upper part of the section are consistent with previous results obtained for the 82–98 ka period also at La Réunion [1] and document a broad low around 95 ka. not associated with large directional changes. On a larger geographic scale, the paleointensity values from La Réunion are significantly higher than those obtained from Mount Etna [2]. Precise comparison is, however, difficult because of the lack of detail in the Etna results. In the lower part of the section, a marked intensity low, coinciding with significant deviation from the dipole field direction is observed at 115 ka and could correspond to the end of the Blake event.  相似文献   

6.
We present a detailed analysis of the Sint-800 virtual axial dipole moment (VADM) data in terms of an Ω mean field model of the geodynamo that features a non-steady generation of poloidal from toroidal magnetic field. The result is a variable excitation of the dipole mode and the overtones, and there are occasional dipole reversals. The model permits a theoretical evaluation of the statistical properties of the dipole mode. We show that the model correctly predicts the distribution of the VADM and the autocorrelation function inferred from the Sint-800 data. The autocorrelation technique allows us to determine the turbulent diffusion time τd=R2/β of the geodynamo. We find that τd is about 10–15 kyr. The model is able to reproduce the observed secular variation of the dipole mode, and the mean time between successive dipole reversals. On the other hand, the duration of a reversal is a factor 2 too long. This could be due to imperfections in the model or to unknown systematics in the Sint-800 data. The use of mean field theory is shown to be selfconsistent.  相似文献   

7.
Preliminary paleointensity results are presented from 36 sites with virtual geomagnetic pole (VGP) latitudes of about 30–90° normal polarity in the Quaternary West Eifel volcanic field. A strong correlation between VGP latitude and the Earth's virtual magnetic dipole moment (VDM) is observed, with low intensities for low VGP latitudes indicating possibly an emplacement during an excursion or event of the Brunhes epoch. The age distribution of the West Eifel volcanics is, as yet, poorly known. Also, the mean VDM value for sites with high VGP latitudes is considerably lower than the present day dipole moment of the Earth.  相似文献   

8.
We present new Thellier-Thellier palaeointensity results from three cooling units (32 samples) of Inner Mongolia lava flows (91.7 Ma) emplaced during the Cretaceous Normal Superchron (CNS). Based on rock-magnetic and microscopy observations the magneto-mineralogy of all samples is determined to be primary and unaltered high-Ti titanomagnetite. Accepted palaeointensity determinations, obtained in the 80-200 °C temperature interval, are of good technical quality with positive standard partial thermoremanent magnetisation (pTRM) checks and pTRM-tail checks. Obtained palaeointensity estimates range from 14.7 to 28.0 μT, with virtual axial dipole moments (VADM) of 2.4 to 4.6 (× 1022 Am2). The data agree well with recently published results from the same region and, combining the two datasets, we obtain independent estimates from six different cooling units yielding a time-averaged VADM of 3.2 ± 1.6 (× 1022 Am2). These data suggest a relatively low dipole moment towards the end of the Cretaceous Normal Superchron.  相似文献   

9.
Absolute geomagnetic paleointensity measurements were made on 255 samples from 38 lava flows of the ～1.09 Ga Lake Shore Traps exposed on the Keweenaw Peninsula (Michigan, USA). Samples from the lava flows yield a well-defined characteristic remanent magnetization (ChRM) component within a ～375°C–590°C unblocking temperature range. Detailed rock magnetic analyses indicate that the ChRM is carried by nearly stoichiometric pseudo-single-domain magnetite and/or low-Ti titanomagnetite. Scanning electron microscopy reveals that the (titano)magnetite is present in the form of fine intergrowths with ilmenite, formed by oxyexsolution during initial cooling. Paleointensity values were determined using the Thellier double-heating method supplemented by low-temperature demagnetization in order to reduce the effect of magnetic remanence carried by large pseudosingle-domain and multidomain grains. One hundred and two samples from twenty independent cooling units meet our paleointensity reliability criteria and yield consistent paleofield values with a mean value of 26.3 ± 4.7μT, which corresponds to a virtual dipole moment of 5.9 ± 1.1×1022 Am2. The mean and range of paleofield values are similar to those of the recent Earth’s magnetic field and incompatible with a “Proterozoic dipole low”. These results are consistent with a stable compositionally-driven geodynamo operating by the end of Mesoproterozoic.  相似文献   

10.
Determination of geomagnetic paleointensity by the Thellier method compares the decay of natural remanent magnetisation with the gain of a laboratory induced thermoremanent magnetisation. If fragile samples lose some amount of their material after each heating step, the Thellier experiment will be systematically disturbed and paleointensity will be over-estimated. For a lost of 5% of the sample’s mass an over-estimate of 10% in paleointensity is observed. This can easily be corrected by a normalisation to the initial mass of the unheated sample. This is necessary for any fragile materials such as baked clays or when a specimen breaks into pieces during the Thellier experiment.  相似文献   

11.
The extreme scarcity of data on the behavior of the paleointensity H an in the geological past from rocks older than 400 Ma significantly hinders the development of our ideas of the geomagnetic field evolution and the geological history of the Earth as a planet. This work presents H an determinations for the Early Proterozoic using the Thellier method and meeting modern requirements for their reliability. The data are obtained from 1850-Ma rocks of granite intrusions sampled in the south of the Siberian platform. The rocks are virtually unaltered granites and granitoids. The paleointensity was determined on 15 samples; results from 11 samples were found to be suitable for the calculation of H an, which is good for experiments of this type. The common feature in the behavior of the natural remanent magnetization (NRM) is a very narrow interval of blocking temperatures: destruction of (60–90)% NRM often took place between 500 and 550°C. Because of the large thickness of the sampled magmatic body, the paleointensity estimates were corrected for its slow cooling rate. With regard for this correction, the probable value of the virtual dipole moment (VDM) from the given collection amounts to 5 × 1022 A m2. Analysis of all published data obtained by the Thellier method for the Precambrian and satisfying the well-known minimal criteria of reliability showed that the average VDM value is about 2 × 1022 A m2, which is four times smaller than the VDM value of the last million years. This phenomenon can be interpreted in terms of the hypothesis that the solid inner core formed only in Proterozoic and, in its absence, the generation of the geomagnetic field was relatively weak, which yielded a small intensity value of the geomagnetic field at early stages of the Earth’s evolution.  相似文献   

12.
Paleointensity measurements have been carried out on 3.5 Ga samples from the Komati Formation type locality using both the Thellier and Van Zijl methods. These samples contain a single steeply-directed negative TRM component acquired during metamorphism of the Komati lavas. Thellier experiments yielded values ranging from 12 to 37 μT but an average paleofield intensity for the four best determinations is 20 ± 3 μT. A slightly lower average paleointensity of 15 ± 3 μT was obtained using Van Zijl experiments. Preheating was used to chemically stabilize seven samples used in Van Zijl determinations and these produced nearly ideal plots with an average paleointensity of 13 ± 2 μT. A single basaltic komatiite sample gave a nearly ideal Van Zijl plot indicating about 21 μT, nearly the same paleointensity as the peridotitic komatiite samples even though its NRM intensity was several orders of magnitude lower. Since the Komati characteristic remanence was acquired during a slow cooling, the data must be reduced by a factor of 1.55 to account for the difference between laboratory and natural cooling rates. Calculation of an equivalent equatorial paleointensity using the paleolatitude implied by the steep Komati characteristic remanence then gives value of 5 μT for the intensity of the geomagnetic field at 3.5 Ga, lower than the present value of about 30 μT.  相似文献   

13.
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   

14.
The global database on the paleointensity, containing determinations of the virtual dipole moment (VDM) for a stable (normal) regime of the geomagnetic field in a time interval of up to 3.5 Ga, is supplemented by new VDM determinations and analyzed. The field generation process started no later than 3–3.5 Ga (earlier data are absent) at the stage of the Earth’s core formation. Since that time, the dipole value has differed from its present value by no more than an order of magnitude, and the deviations that have already been detected tend toward smaller values. The distribution of VDM values in the time interval 0–400 Ma is bimodal, which apparently reflects the presence of two different generation levels of the geomagnetic field distinguished by a relatively large value (close to the present field value) and a relatively small value (approximately half as large as the present value). The total duration of decreased VDM values appreciably exceeds that of increased VDM values (179.1 and 28.6 Myr, respectively). On the whole, data on the paleointensity do not contradict the hypothesis about the dipole nature of the field over the last 400 Myr; however, the number of determinations at high paleolatitudes is too small to draw decisive conclusions on the validity (or invalidity) of the dipole field approximation based solely on paleointensity data.  相似文献   

15.
Despite of the impressive cultural heritage and abundant archaeological sites, absolute geomagnetic intensity data from Mesoamerica are still sparse. Archeointensity determinations using the Coe variant of the Thellier and Thellier method have been carried out on some selected pottery fragments from the El Opeño archeological site which has the earliest funeral architecture known in western Mesoamerica. The El Opeño chronology is supported by six C14 datings performed on carbon-bearing materials. Detailed rock magnetic experiments including susceptibility vs. temperature curves, hysteresis cycles and thermal demagnetization procedures were carried out in order to estimate the magnetic carriers and their stability. Cooling rate and anisotropy remanence corrected intensity values range from 25.0 ± 2.3 to 40.2 ± 3.0 μT and corresponding virtual axial dipole moments (VADM) range from 5.6 ± 0.5 to 8.9 ± 0.7 × 1022 Am2. In addition, we present here a new compilation and analysis of existing absolute intensity data in order to try to estimate the variation of the Earth’s magnetic field over the past three millennia. The mean archeointensity obtained in the present study agrees reasonably well with the predicted absolute intensities retrieved from the CALS7K main field model. Other available Mesoamerican data, however, differ from this model. Most of available archeointensity data from Mesoamerica agree reasonably well with the ARCH3K main field model prediction. The broad peak defined at about 50 A.D. by our data is also predicted by the CALS3k.3 main field model but slightly displaced to the right while two smaller peaks are observed on the ARCH3K curve for the same time interval. The intensity value obtained at about 200 B.C. is a clear outlier and thus its geomagnetic significance should be confirmed by further investigations.  相似文献   

16.
All absolute paleointensity data published in peer-reviewed journals were recently compiled in the GEOMAGIA50 database. Based on the information in GEOMAGIA50, we reconstruct variations in the geomagnetic dipole moment over the past 50  kyr, with a focus on the Holocene period. A running-window approach is used to determine the axial dipole moment that provides the optimal least-squares fit to the paleointensity data, whereas associated error estimates are constrained using a bootstrap procedure. We subsequently compare the reconstruction from this study with previous reconstructions of the geomagnetic dipole moment, including those based on cosmogenic radionuclides (10Be and 14C). This comparison generally lends support to the axial dipole moments obtained in this study. Our reconstruction shows that the evolution of the dipole moment was highly dynamic, and the recently observed rates of change (5% per century) do not appear unique. We observe no apparent link between the occurrence of archeomagnetic jerks and changes in the geomagnetic dipole moment, suggesting that archeomagnetic jerks most likely represent drastic changes in the orientation of the geomagnetic dipole axis or periods characterized by large secular variation of the non-dipole field. This study also shows that the Holocene geomagnetic dipole moment was high compared to that of the preceding  40  kyr, and that  4 · 1022  Am2 appears to represent a critical threshold below which geomagnetic excursions and reversals occur.  相似文献   

17.
The reversed paleomagnetic direction of the Laschamp and Olby flows represents a specific feature of the geomagnetic field. This is supported by paleomagnetic evidence, showing that the same anomalous direction was recorded at several distinct sites, including scoria of the Laschamp volcano. To examine this anomalous geomagnetic fluctuation, we studied the paleointensity of the Laschamp and Olby flows, using the Thellier method. Twenty-five samples were selected for the paleointensity experiments, and from seven we obtained reliable results. Because the paleointensity results of the Olby and Laschamp flows as well as Laschamp scoria are very similar, they can be represented by a single mean paleointensity,F = 7.7 μT. Considering that this low paleointensity is less than 1/6 of the present geomagnetic field and is more characteristic of transitional behavior, our results suggest that the paleomagnetic directions of the Laschamp and Olby flows were not acquired during a stable reversed polarity interval. A more likely explanation is that the Laschamp excursion represents an unsuccessful or aborted reversal.  相似文献   

18.
The Thellier method for paleointensity determinations has been applied to prepared samples containing magnetites whose mean particle sizes range from single domain, SD, to multidomain, MD. Linear (ideal) PNRM-PTRM curves are obtained for samples containing SD and submicron magnetite particles. However, for MD particles non-linear (concave-up) PNRM-PTRM curves are observed such that a linear approximation to the lower blocking-temperature data leads to apparent paleointensities that are higher than the actual paleofield; however, the ratio of the end-points, NRM/TRM, yields the correct (laboratory) intensity. The non-linear (concave-up) PNRM-PTRM curves for the MD particles are explained in terms of the lack of symmetry of the domain-wall movements during the two heatings of the Thellier experiment. Low stabilities with respect to alternating fields and with respect to temperature cycles below magnetite's isotropic temperature are diagnostic in detecting samples most likely to exhibit non-linearities due to the MD effect.  相似文献   

19.
A representative collection of Upper Cretaceous rocks of Georgia (530 samples from 24 sites) is used for the study of magnetic properties of the rocks and the determination of the paleodirection and paleointensity (H an) of the geomagnetic field. Titanomagnetites with Curie points of 200–350°C are shown to be carriers of natural remanent magnetization (NRM) preserving primary paleomagnetic information during heatings to 300–350°C. The characteristic NRM component of the samples is identified in the interval 120–350°C. The Thellier and Thellier-Coe methods are used for the determination of H an meeting modern requirements on the reliability of such results. New paleointensity determinations are obtained and virtual dipole magnetic moment (VDM) values are calculated for four sites whose stratigraphic age is the Upper Cretaceous (Cenomanian-Campanian). It is shown that, in the interval 99.6–70.6 Ma, the VDM value was two or more times smaller than the present value, which agrees with the majority of H an data available for this time period. According to our results, the H an value did not change at the boundary of the Cretaceous normal superchron.  相似文献   

20.
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