Paleointensity Estimates Research Articles

Toward a robust normalized magnetic paleointensity method applied to meteorites

We propose a new paleointensity method based on normalization by isothermal remanent magnetization derivative vs. alternating field (REM′ method). It provides an estimate of the absolute paleointensity with an uncertainty of about a factor two. Contrary to methods using normalization by total isothermal remanent magnetization or anhysteretic magnetization, it is applicable to multicomponent magnetizations. Artificial or natural isothermal remanent magnetizations can be recognized, and relaxation of natural remanent magnetization can be taken into account. It is applicable for magnetizations carried by magnetite, FeNi alloys and pyrrhotite. The REM′ method is of particular interest to estimate paleointensities in meteorites, as these materials are often characterized by complex multicomponent magnetization of various origins (impact magnetization, thermoremanence, isothermal remanent magnetization) and metastable magnetic minerals not suitable for Thellier experiments. However, like other paleointensity methods, the REM′ method underestimates the true paleointensity for meteorite samples with magnetizations that are heterogeneous on spatial scales below the sampling scale. For L ordinary chondrites, an upper limit of 1 μT is proposed for the paleofield. Tentative paleofield estimates in the 0.05–0.5 μT range are proposed for LL ordinary chondrites, which is much lower than previous results. The first paleofield estimates for Rumuruti chondrites indicate the presence of magnetic fields around 6 μT during the last major impact on their parent body. Aubrites and HED achondrites paleofields of at least 10 μT may suggest the existence of a dynamo field during the cooling of their parent body. Results from Martian meteorites are scattered between 1 and 24 μT and may represent the crustal magnetic field of the planet after dynamo shutdown. Carbonaceous chondrites provide contrasting results, with possible evidence of strong field (mT) processes that clearly require additional studies.

The inverse problem of paleointensity determination

Analysis of paleointensity experimental data is often carried out using a special simple case of error-in-variable (EIV) formalism. Though mathematically sound, this approach is quite limited in the ways it offers to assess uncertainty and quality of the paleointensity estimates. The problem of paleointensity determination from experimental data is one of parameter estimation and it is more natural to treat it within the framework of inverse problem theory. In this paper, I present a new formulation of paleointensity estimation as an inverse problem whereby the entire paleofield vector is recovered from its measured components so that both its intensity and direction are obtained simultaneously. This formalism has several advantages including the flexibility to incorporate reliability criteria and consistency conditions objectively into the computational process as constraints and prior information. It also provides a wide range of appraisal tools to access error bounds and quality of the estimation results. Numerical experiments were carried out to validate the procedure using single- and multiple-specimen data and imposing different constraints including prior information on inclination, declination and expected paleointensity. The results indicate that inclination is the strongest constraint exerting the greatest control over the final paleointensity estimate. Small changes in inclination lead to relatively large changes in paleofield intensity estimates. Declination is a relatively weaker constraint and paleointensity is the weakest or least effective condition. Moreover, the results indicate that means and variances of paleointensity estimates computed by this procedure can differ by as much as 20% from those computed by the conventional method.

Is there a precursor to the Cretaceous normal superchron? New paleointensity and age determination from Liaoning province, northeastern China

Although the geomagnetic reversal record for the past 160 Myr is relatively well documented, whether a precursor exists before the onset of the Cretaceous normal superchron (CNS) remains a contentious topic in the study of the evolution of geomagnetic reversals. In the present study we have tried to find out if new paleointensity data can provide more information about this topic. A paleointensity, geochronologic and paleomagnetic investigation has been conducted on four andesitic basalt lava flows from Hulahada in Liaoning Province, northeastern China. Radiometric 40Ar/ 39Ar dating experiments indicate the volcanism occurred at about 122.7 ± 0.6 Myr, corresponding to marine anomaly Mln. Thermomagnetic experiments and hysteresis loops show that the main magnetic carrier of remanence are titanomagnetites with Curie temperatures of ∼580 °C. Thirty-eight samples from three lavas underwent Thellier–Thellier paleointensity experiments with systematic partial thermoremanent magnetization (pTRM) checks. Ten samples from the studied lavas yield reliable paleointensity estimates with a mean virtual dipole moment (VDM *) of (4.8 ± 0.2) × 10 22 Am 2, about half the strength of the modern-day field. Our paleointensity results in conjunction with previous data suggest that there is no precursor to the CNS.

Thellier paleointensity theory and experiments for multidomain grains

We extend theories of thermoremanent magnetization (TRM) and partial TRM (pTRM) in multidomain (MD) grains to model thermal demagnetization and pTRM acquisition steps in Thellier paleointensity determination. Because of the interleaving of zero‐field and in‐field heating‐cooling steps to increasing temperatures the initial state for any step is complex, and theoretical modeling is more intricate than for pTRM production or thermal demagnetization separately. At low to moderate temperature T, TRM lost exceeds pTRM regained, causing convex down Arai plots sagging below the ideal single‐domain (SD) line. At moderate to high T, pTRM acquisition outweighs TRM loss. As T approaches TCurie, pTRM gain exactly equals TRM loss, and the Arai plot becomes ideal. When pTRMs are produced perpendicular to the original TRM and measured directly rather than by differencing field‐on and field‐off results, there is less deviation from ideality at low to moderate T. Our theory agrees semiquantitatively with results for parallel and perpendicular pTRMs for large MD magnetites (135 μm). Smaller MD magnetites (6 and 20 μm) have less curved Arai plots, and the smallest magnetites (0.6 and 1 μm) have almost linear plots. Positive pTRM checks demonstrate that curved Arai plots of MD grains are reproducible, unlike curved plots for rocks that alter physicochemically in the Thellier experiment, while negative pTRM tail checks indicate undemagnetized pTRM residuals. Low‐temperature demagnetization improves linearity only slightly. Practical applications of this work include using the predicted threshold T below which no net pTRM is produced in a parallel Thellier experiment to screen data used for paleointensity fits. Straight line fits through low‐ and medium‐T points in Arai plots of MD (135 μm) grains overestimated the paleofield by as much as 100% and for small pseudo‐single‐domain (PSD) (0.6 and 1 μm) grains overestimated by about 25%. However, by using linear segments of medium‐ to high‐T data with f values ≥0.5 it may be possible to obtain reasonable paleointensity estimates even for larger PSD (6 and 20 μm) and MD grains. Middle‐ to high‐T fits for 0.6 μm grains gave paleointensities within 4% of the correct value, utilizing essentially the entire data set (f > 0.9). Perpendicular data always gave superior linear fits. Orienting samples with their natural remanent magnetizations perpendicular to the laboratory field is therefore recommended for rocks containing PSD and MD grains. However, double heatings are preferable to single heatings because they allow pTRM tail checks to be carried out.

Paleomagnetism and 40Ar/39Ar ages from volcanics extruded during the Matuyama and Brunhes Chrons near McMurdo Sound, Antarctica

Maps of virtual geomagnetic poles derived from international geomagnetic reference field models show large lobes with significant departures from the spin axis. These lobes persist in field models for the last few millenia. The anomalous lobes are associated with observation sites at extreme southerly latitudes. To determine whether these features persist for millions of years, paleomagnetic vector data from the continent of Antarctica are essential. We present here new paleomagnetic vector data and 40Ar/39Ar ages from lava flows spanning the Brunhes and Matuyama Chrons from the vicinity of McMurdo Sound, Antarctica. Oriented paleomagnetic samples were collected from 50 lava flows by E. Mankinen and A. Cox in the 1965–1966 austral summer season. Preliminary data based largely on the natural remanent magnetization (NRM) directions were published by Mankinen and Cox [1988]. We have performed detailed paleomagnetic investigations of 37 sites with multiple fully oriented core samples to investigate the reliability of results from this unique sample collection. Of these, only one site fails to meet our acceptance criteria for directional data. Seven sites are reversely magnetized. The mean normal and reverse directions are antipodal. The combined mean direction has = 12, = −86, α = 4, κ = 37 and is indistinguishable from that expected from a GAD field. We obtained reproducible absolute paleointensity estimates from 15 lava flows with a mean dipole moment of 49 ZAm2 and a standard deviation of 28 ZAm2. 40Ar/39Ar age determinations were successfully carried out on samples from 18 of the flows. Our new isotopic ages and paleomagnetic polarities are consistent with the currently accepted geomagnetic reversal timescales.

Long‐term variation of geomagnetic field strength: A cautionary note

Earth's magnetic field is generated through magnetohydrodynamic processes within the fluid outer core. Several factors that may influence these processes, such as variations in heat flow across the core‐mantle boundary, are believed to have varied through geologic time, triggering speculation that Earth's magnetic field may have been significantly different in the geological past. Based on painstakingly difficult paleo‐intensity experiments, it has, for example, been suggested that Earth's dipole moment was much weaker during the Mesozoic, and perhaps related to different mantle convection [Thomas and Biggin, 2003].The variable quality of published paleointensity estimates makes global compilations difficult, and extreme care must be taken not to interpret problematic data in terms of geodynamic features. In fact, recently available reliable paleo‐intensity data suggest that the paleo‐strength during early‐middle Cretaceous may have been comparable or even higher than the present intensity and not “anomalously” low as suggested in previous studies [Tauxe and Staudigel, 2004]. The identification of any long‐term features of virtual dipole moment (VDM) seems to rely heavily on the criteria for accepting paleo‐intensity data, which casts serious doubts on the quality and geomagnetic significance of the majority of existing data. In a recent analysis, Thomas and Biggin [2003]—based on so‐called self‐consistency selection criteria—concluded that the relatively low intensity during the Mesozoic is a real phenomenon.

Timing of volcanism along the northern East Pacific Rise based on paleointensity experiments on basaltic glasses

Samples from two adjacent and contrasting ridge segments along the East Pacific Rise were measured for their magnetic paleointensity in order to further explore the possibilities of dating very young volcanic samples using secular variations in the Earth's magnetic field. The ridge segment north of the Orozco transform fault (15°22′–16°20′N) is the shallowest and broadest along more than 5000 km of the East Pacific Rise, whereas the adjacent segment to the north (16°16′–18°N) has a “typical” morphology for its intermediate spreading rate. Both ridge segments were densely sampled during the PANR01MV cruise and 36 samples of axial lava flows, consisting mainly of glasses from the rims of the flows and some fragments of lobate basalts, were selected from this collection for paleointensity experiments. The Coe version of the Thellier double‐heating procedure (in air) was used. Twenty‐seven units provide internally consistent paleointensity estimates leading to precise estimates of the paleofield, which range between 8 μT and 57 μT. Comparisons with reference paleointensity curves compiled from subaerial flows, archeomagnetic data and sedimentary records projected to the sampling site coordinates show that the measured values can be used to constrain the volcanic history of the ridge segments over the past few thousand years. A good agreement was found between apparent “freshness” of the glasses, the geochemistry of the lavas, and their magnetic paleointensity values. The inflated southern segment seems characterized by recent activities as indicated by numerous flows with paleointensities clustering around today's value (39 μT) or around the high values typical of 2000–3000 years ago (∼55 μT). We interpret this distribution to indicate the flooding by effusive lava flows of the entire axial plateau some 2000–3000 years ago, followed by a volcanic phase producing smaller volume lava flows confined to the innermost 200 m of the ridge axis. The northern ridge segment is characterized by dispersed paleointensity values consistent with a series of small eruptions of diverse ages. Samples collected at the tips of both ridge segments across the 16°20′N axial discontinuity have the lowest paleointensities and are thus thought to be significantly older, consistent with models advocating reduced magmatism near ridge axis discontinuities. This study demonstrates the strong potential of paleointensity measurements as a tool to help constrain volcanic history at ridge axes.

Paleomagnetic Study of Lavas from the Popocatepetl Volcanic Region, Central Mexico

Results are presented for a paleodirectional and paleointensity study of 16 Popocatepetl lava flows. Popocatepetl is a tall, active, dacitic-andesitic stratovolcano; it forms the southern end of a N-S-trending Quaternary volcanic range at the eastern edge of the Basin of Mexico. Studied units are mainly andesites and one trachyandestite, and all possess similar trace- and rare-earth-element compositions. Rock-magnetic experiments show that remanence is carried in most cases by Ti-poor titanomagnetite, resulting from oxy-exsolution of original titanomagnetite during flow cooling. Unblocking temperature spectra and high coercivities point to "small" pseudo-single domain magnetic grains for these (titano)magnetites. Single-component, linear demagnetization plots were observed in most cases. A strong, lightning-produced magnetization overprint was detected for one site. Combining the new paleomagnetic data provides a mean paleodirection of I = 35.4°, D = 345.7°, k = 21, a95 = 8.5°, N = 15 for the Popocatepetl volcano region. All studied flows yield normal polarity magnetization, which supports a maximum age within the Brunhes chron. Twenty-five samples were selected for Thellier palaeointensity experiments based on magnetic properties, stable single-component remanent magnetizations, and reasonably reversible continuous thermomagnetic curves. Fourteen samples from four different flows yield reliable paleointensity estimates with the flow-mean virtual dipole moments (VDM) ranging from 5.9 to 9.2 x 1022 Am2. The NRM fractions used for paleointensity determination range from 35 to 96%, and the quality factors vary between 3.4 and 46.9, being normally greater than 6. Mean VDM obtained in this study is 7.2 ± 1.4 x 1022 Am2, slightly lower than the present-day dipolar value.

Toward an optimal geomagnetic field intensity determination technique

Paleointensity determinations based on double heating techniques (in‐field/zero‐field cooling, zero‐field/in‐field cooling, and two in‐field steps with opposite laboratory fields) are generally considered to be functionally interchangeable producing equally reliable paleointensity estimates. To investigate this premise, we have developed a simple mathematical model. We find that both the zero‐field first and in‐field first methods have a strong angular dependence on the laboratory field (parallel, orthogonal, and anti‐parallel) while the two in‐field steps method is independent of the direction of the laboratory‐produced field. Contrary to common practice, each method yields quite different outcomes if the condition of reciprocity of blocking and unblocking temperatures is not met, even with marginal (10%) tails of partial thermoremanence. Our calculations suggest that the zero field first method with the laboratory‐produced field anti‐parallel to the natural remanence (NRM) is the most robust paleointensity determination technique when the intensity of the lab‐induced field is smaller than ancient field. However, the zero field first method with the laboratory‐field parallel to the NRM is the optimum approach when the intensity of the lab‐induced field is larger than the ancient field. By far the best approach, however, is to alternatethe infield‐zerofield (IZ) steps with zerofield‐infield (ZI) steps.

Cretaceous geomagnetic paleointensities: Thellier experiments on Pillow lavas and Submarine basaltic glass from the Ontong Java Plateau

We present new Thellier paleointensity results on pillow lavas and pillow‐rim submarine basaltic glass (SBG) from 120 Ma basalts erupted on the Ontong Java Plateau. All pillow lavas have suffered low‐temperature alteration and are unsuitable for Thellier paleointensity experiments. In contrast, pillow‐rim SBG is virtually impermeable to seawater and therefore has suffered little or no low‐temperature alteration. Out of 21 selected SBG samples with magnetic moment larger than 10−10 Am2, 18 samples (from 12 chilled SBG margins) yielded paleointensity estimates of good technical quality. Rock magnetic measurements show the primary carrier of remanence is low‐Ti titanomagnetite occurring in a continuous range of compositions starting from nearly pure magnetite. As suggested by a recent study, magnetic hysteresis parameters were monitored during the entire paleointensity experiment and a drastic change of parameters was observed above 400–440°C, which is in accordance with the partial thermoremanent magnetization checks failing significantly above the same temperature. We suggest that paleointensity estimates obtained below this critical temperature represent true determination of the paleomagnetic field strength. For several pillow margins two to three samples were measured, and in such cases the data agree well. Altogether we obtain reliable estimates from 12 SBG pillow‐rims, yielding a mean paleointensity of 27.1 ± 13.4 μT and a corresponding virtual axial dipole moment of 5.8 ± 2.8 · 1022 Am2. This value is slightly higher than whole rock data from similar aged subaerial lava flows. The paleosecular variation of the intensity estimates is slightly larger than previously reported for paleointensity determinations for the Cretaceous Normal Superchron.

Further absolute geomagnetic paleointensities from Baja California: evaluation of Pliocene and Early/Middle Pleistocene data

Abstract From a large collection (more than 300 oriented cores) of Baja California Mio-Pliocene volcanic units, sampled for magnetostratigraphy and tectonics, 46 samples were selected for Thellier paleointensity experiments because of their low viscosity index, stable remanent magnetization and close to reversible continuous thermomagnetic curves. 19 samples, coming from 4 individual basaltic lava flows, yielded reliable paleointensity estimates with the flow-mean virtual dipole moments (VDM) ranging from 3.6 to 6.2 ×10 22 A m 2 . Our results, although not numerous, are of high technical quality and comparable to other paleointensity data recently obtained on younger lava flows. The NRM fractions used for paleointensity determination range from 38 to 79% and the quality factors vary between 4.8 and 16.7, being normally greater than 5. The combination of Baja California data with the available comparable quality Plio-Plesitocene paleointensity results yields a mean VDM of 6.3 ×10 22 A m 2 , which is almost 80% of the present geomagnetic axial dipole. Reliable paleointensity results for the last 5 Ma are still scarce and of dissimilar quality, which makes it hard to draw any firm conclusions regarding the Pliocene and Early/Middle Pleistocene evolution of the geomagnetic field. To cite this article: J. Morales et al., C. R. Geoscience 335 (2003).

Results of rock magnetic investigations and relative paleointensity determinations on lacustrine sediments from Birkat Ram, Golan Heights (Israel)

Detailed rock magnetic investigations, including the analysis of the laboratory installed magnetizations, the measurement of hysteresis parameters, and the high temperature‐dependency of the saturation magnetization, were carried out on sediments from Birkat Ram, Golan Heights, Israel. The 550 cm long composite profile, composed of three single cores, is supposed to span the Late Holocene, i.e., the last 4400 years, at least. Titanomagnetites in the pseudo single‐domain range were identified as the main magnetic carrier minerals in the homogenous sediments. The variations in concentration and grain size of the magnetic fraction are only partly reflected in the sediment lithology. The relative paleointensity was estimated by normalizing the intensity of the natural remanent magnetization (NRM) after demagnetization at 20 mT (JNRM(20 mT)) by different concentration parameters. The median destructive field of the NRM (MDFNRM) is coherent with the relative paleointensity estimates, indicating that a sedimentary effect has not sufficiently been removed by the normalization process. The morphologies of these estimates, however, correspond to the archeomagnetic records from Bulgaria and Greece. Therefore a second normalization, based on the linear relationship between the paleointensity estimates and the MDFNRM, was applied, resulting in an improvement of the records.

Absolute paleointensity of the Earth's magnetic field during Jurassic: case study of La Negra Formation (northern Chile)

We carried out a detailed rock-magnetic and paleointensity study of the ∼187-Ma volcanic succession from northern Chile. A total of 32 consecutive lava flows (about 280 oriented standard paleomagnetic cores) were collected at the Tocopilla locality. Only 26 samples with apparently preserved primary magnetic mineralogy and without secondary magnetization components were pre-selected for Thellier paleointensity determination. Eleven samples coming from four lava flows yielded reliable paleointensity estimates. The flow-mean virtual dipole moments range from 3.7±0.9 to 7.1±0.5(1022 A m2). This corresponds to a mean value of (5.0±1.8)×1022 A m2, which is in reasonably good agreement with other comparable quality paleointensity determinations from the Middle Jurassic. Given the large dispersion and the very poor distribution of reliable absolute intensity data, it is hard to draw any firm conclusions regarding the time evolution of the geomagnetic field. To cite this article: J. Morales et al., C. R. Geoscience 335 (2003).

How many paleointensity determinations are required from a single lava flow to constitute a reliable average?

Statistical analysis has been undertaken to investigate the minimum number of paleointensity estimates required to allow certainty (at the 95% confidence limit) that the mean is within 10% of the implied true mean for an entire lava flow. The data used for the analysis was provided by previous, paleointensity studies which used three different methods (Thellier‐Coe, Shaw, and microwave Thellier) on 19 or more samples from three different basalt lava flows. The results strongly suggest that future paleointensity studies need to produce many more estimates per lava flow than is currently the norm in order to ensure that the calculated mean paleointensity does not differ significantly from the true flow mean. Furthermore, they indicate that a low measured dispersion for a small number of estimates is not a safeguard against misrepresentation of the flow mean paleointensity. Finally, we provide quantitative evidence that paleointensity studies should always attempt to maximise vertical coverage of lava flows to avoid biasing of the mean.

Geomagnetic field strength during late Miocene: First paleointensity results from Baja California

Thellier paleointensity experiments were carried out on 73 selected samples from the late Miocene Baja California volcanic flows for the first time. The selected samples showed a stable remanent magnetization observed upon thermal treatment and reasonably reversible continuous susceptibility curves, compatible with Ti‐poor titanomagnetites. Judging from the ratios of hysteresis parameters, it seems that the samples fall in the small pseudosingle‐domain grain size region. Thirty‐three samples from 10 individual flows yielded acceptable paleointensity estimates. The natural remanent magnetization (NRM) fractions used for paleointensity determination range from 37 to 81% and the quality factor varies between 4.5 and 24.5, being normally >5. The mean paleointensity values per flow obtained in this study range from 18.1 ± 3.1 to 45.6 ± 4.3 μT and the corresponding virtual dipole moments (VDMs) range from 3.8 ± 0.6 to 8.8 ± 0.8 (1022 A m2). These values correspond to mean value of 6.6 ± 1.7 × 1022 A m2, which is ∼85% of the present geomagnetic axial dipole. The average VDM value for late Miocene, according to all available, selected data, is 6.1 ± 1.5 (1022 A m2), which is higher than average paleofield for the period 5–160 Ma (4.2 ± 2.3 × 1022 A m2 after Juarez and Tauxe [2000]). Given the large dispersion and the very poor distribution of reliable absolute intensity data, it is hard to draw any firm conclusions regarding the time evolution of the geomagnetic field. More high‐quality determinations are strongly needed.

Paleointensity in Hawaiian Scientific Drilling Project Hole (HSDP2): Results from submarine basaltic glass

Paleointensity estimates based on the high quality Thellier‐Thellier data from the early Brunhes (420–780 ka) are rare (only 30 in the published literature). The Second Hawaiian Scientific Drilling Project (HSDP2) drill hole recovered submarine volcanics spanning the approximate time period of 420–550 ka. These are of particular interest for absolute paleointensity studies owing to the abundance of fresh submarine basaltic glass, which can preserve an excellent record of ancient geomagnetic field intensity. We present here new results of Thellier‐Thellier paleointensity experiments that nearly double the number of reliable paleointensity data available for the early Brunhes. We also show that the magnetizations of the associated submarine basalts are dominated by viscous magnetizations and therefore do not reflect the true ancient geomagnetic field intensity at the time of extrusion. The viscous contamination is particularly severe because of a combination of low blocking temperatures in the basalts and relatively high temperatures in the deeper parts of the drill core. Our new data, when placed on the approximate timescale available for HSDP and HSDP2, are at odds with other contemporaneous paleointensity data. The discrepancy can be reconciled by adjusting the HSDP timescales to be younger by about 35 kyr.

Paleointensity study of Early Miocene lavas from Pingzhuang, Inner Mongolia, China

A paleomagnetic and geochronologic investigation has been conducted on 17 lava flows from Inner Mongolia, north China. Radiometric K‐Ar dating experiments give an age between 23 and 22 Ma for these volcanic units. Rock magnetic measurements of thermomagnetic curves and hysteresis loops show the primary carrier of remanence to be pseudo‐single domain titanomagnetite. Stepwise thermal demagnetization isolated a well‐defined direction of characteristic remanent magnetization (ChRM) in all studied lava flows with a mean direction of D/I = 359.8°/62.2° (α95 = 3.8°). Fifty samples from 11 lavas were pre‐selected on the basis of thermomagnetic analyses to carry out Thellier‐Thellier paleointensity experiments with systematic partial thermoremanent magnetization (pTRM) checks. Twenty‐three samples from 7 lavas yield reliable paleointensity estimates with a mean virtual dipole moment (VDM) of (3.9 ± 0.9) × 1022 Am2, about half the strength of the modern‐day field.

Paleosecular variation record of geomagnetic full vector during late Miocene, from the Nayarit area, Mexico

Abstract We have sampled a sequence of 45 late Miocene consecutive lava flows in the Tepic area (Nayarit State, Mexico). The age of the volcanic units lies between 8 and 9 million years (Ma) according to available radiometric data. All samples were stepwise demagnetized, partly with alternating field (AF), partly thermally with very similar results. Most of the rocks exhibited well-defined one component remanent magnetisation with high unblocking temperatures (mostly above 525 °C) and high median destructive fields (MDF) (40–50 mT). Rock-magnetic experiments combined with microscopy show that, in most cases, the main magnetic mineral is Ti-poor titanomagnetite associated with exsoluted ilmenite. Continuous susceptibility measurements with temperature and hysteresis experiments yield in most cases nearly reversible curves with Curie points close to that of magnetite and pseudo-single-domain characteristics. Characteristic remanent magnetisations (ChRM) isolated after the first steps of demagnetisation are all normal polarity. According to the dispersion of virtual geomagnetic pole (VGP) directions, paleosecular variation was abnormally lower than the one observed in general during Miocene. Considering our paleomagnetic results together with available radiometric data, it seems that the volcanic units have been emplaced during a very short time span of about 0.08 million years. The mean paleomagnetic directions obtained from this study do not differ significantly from that expected for the middle Miocene. Thirty-one samples from eight individual flows yielded acceptable paleointensity estimates. The site mean paleointensities range from 27.8±0.9 to 42.0±7.9 μT. The virtual dipole moments (VDM) range from 5.9 to 9.5×1022 Am2. This corresponds to a mean value of 7.6±1.4×1022 Am2, which is higher than the average VDM value for late Miocene. Altogether our data suggest the existence of relatively high geomagnetic field strength undergoing low fluctuations. These results support the theoretical suggestion about an inverse relationship between secular variation and local field strength as result of electromagnetic coupling between the solid inner core and liquid outer core, with the inner core tending to stabilise core convection, and hence the field, when intensity is high. Some fluctuation of absolute intensity was detected within the same directional group (DG) indicating that the intensity of the geomagnetic field varies faster than its direction.

Long-term secular variation in geomagnetic field inclination during Brunhes Chron recorded in sediment cores from Ontong-Java Plateau

A paleomagnetic study has been conducted on three gravity cores obtained from the Ontong-Java Plateau (OJP) in the western equatorial Pacific in order to investigate long-term secular variations during the Brunhes Chron. The cores consist of nanofossil foraminiferal ooze, and covers the last 500,000–800,000 years with average sedimentation rates of 6–9 m/m.y. Downcore changes of magnetic concentration represented by magnetic susceptibility are a factor of six or less, and variations in magnetic grain size and mineralogy are estimated to be small from the hysteresis parameters, ratio of anhysteretic remanent magnetization (ARM) to saturation isothermal remanent magnetization (SIRM), and S ratio. These results indicate that the sediments are rock-magnetically homogeneous in general, and are hence suitable for relative paleointensity estimation. I chose SIRM as a normalizer for relative paleointensity. The three cores could be precisely correlated with each other using relative paleointensity variations. Depth-to-age conversion was carried out by tying to the Sint-800 paleointensity stack (Guyodo and Valet, 1999). The ages are consistent with the oxygen-isotope stratigraphy available for one of the three cores. Inclination variations were obtained after alternating-field demagnetization. Large inclination anomaly, about −7° in average, was observed as has been suggested to occur in the western equatorial Pacific by some studies. The inclination records of the three cores were integrated into a single record based on the correlation by the relative paleointensity. Long-term inclination variations of the order of 10,000–100,000 years were recognized. The inclinations tend to deflect toward negative when relative paleointensity is low. The variations are similar in general to the record from the West Caroline Basin (Yamazaki and Oda, 2002), which supports their model that long-term inclination variation is controlled by relative strength of the geocentric axial dipole (GAD) and persistent non-dipole components.

A rock-magnetic and paleointensity study of some Mexican volcanic lava flows during the Latest Pleistocene to the Holocene

Eleven Late Quaternary lava flows were sampled in the Chichinautzin volcanic field of central Mexico to determine their magnetic characteristics and absolute paleointensity. The samples studied cover a geological time interval of approximately 0.39 My to 2000 years. Several rock-magnetic experiments were carried out in order to identify the magnetic carriers and to obtain information about their paleomagnetic stability. Continuous susceptibility measurements with temperature in most cases yield reasonably reversible curves with Curie points close to that of almost pure magnetite, which is compatible with low-Ti titanomagnetite resulting from oxi-exsolution. Judging from the ratios of hysteresis parameters, it seems that all samples fall within the pseudo-single domain grain size region, probably indicating a mixture of multidomain and a significant amount of single domain grains. Forty-two samples belonging to six independent cooling units yielded acceptable absolute paleointensity estimates. The NRM fractions used for paleointensity determination range from 0.34 to 0.97 and the quality factors varies between 4.5 and 97.8, being normally greater than 5. The obtained virtual dipole moment values are higher than those recently reported for the past 5 My and to the present day geomagnetic field strength. Individual paleointensity of around 2000 BP is substantially higher than the present day intensity, which is in broad agreement with worldwide archeomagnetic results.