International Geomagnetic Reference Field, 1995 revision Presented by IAGA Division V, Working Group 8

We present here new paleointensity and geochronology results from Early Cretaceous volcanic rocks of Sierra Chica de Cordoba (Argentina). The new K-Ar isotopic ages of 5 samples range from 136 to 122 Ma. Twenty five samples from 7 individual flows yielded acceptable paleointensity estimates. The mean paleointensity values per flow are ranging from 53.0 ± 1.9 to 25.4 ± 2.6 μT and the corresponding Virtual Dipole Moments (VDMs) are ranging from 9.3±1.3 to 4.6±0.5 (10 22 Am 2 ). This corresponds to the mean value of 7.3±1.7×10 22 Am 2 , which is compatible to the present geomagnetic axial dipole. Currently available selected paleointensity data from 80 to 130 Ma suggest that geomagnetic field strength frequently fluctuated before and during the Cretaceous Normal Superchron while the magnetic polarity maintained stable. The mean paleointensities derived from Córdoba lavas agree remarkably well with those obtained from the Paraná Magmatic Province (133-132 Ma). This reinforces the hypothesis about the unreliability of 'Mesozoic Dipole Low'.

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“…22 Am 2 which is basically similar to the present geomagnetic axial dipole (7.8 × 10 22 Am 2 after Barton et al, 1996).…”

Section: Resultssupporting

confidence: 85%

Early cretaceous absolute geomagnetic paleointensities from Córdoba Province (Argentina)

et al. 2006

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“…Combination of the Mexican data with the absolute intensity results currently available for the PlioPleistocene show the VDM variation ranging from 2.41 to 9.45×10 22 Am 2 (Fig. 5) yielding a mean VDM of 6.4×10 22 Am 2 , which is about 80% of the present geomagnetic axial dipole (7.8 × 10 22 Am 2 after Barton et al, 1996). Reliable absolute paleointensity results for the last 5 Ma are still scarce and of dissimilar quality.…”

Section: Discussion and Main Resultsmentioning

confidence: 99%

Further constraints for the Plio-Pleistocene geomagnetic field strength: New results from the Los Tuxtlas volcanic field (Mexico)

2014

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A rock-magnetic, paleomagnetic and paleointensity study was carried out on 13 Plio-Pleistocene volcanic flows from the Los Tuxtlas volcanic field (Trans Mexican Volcanic Belt) in order to obtain some decisive constraints for the geomagnetic field strength during the Plio-Pleistocene time. The age of the volcanic units, which yielded reliable paleointensity estimates, lies between 2.2 and 0.8 Ma according to the available K/Ar radiometric data. Thermomagnetic investigations reveal that remanence is carried in most cases by Ti-poor titanomagnetite, resulting from oxy-exsolution that probably occurred during the initial flow cooling. Unblocking temperature spectra and relatively high coercivity point to 'small' pseudo-single domain magnetic grains for these (titano)magnetites. Single-component, linear demagnetization plots were observed in most cases. Six flows yield reverse polarity magnetization, five flows are normally magnetized, and one flow shows intermediate polarity magnetization. Evidence of a strong lightning-produced magnetization overprint was detected for one site. The mean pole position obtained in this study is Plat = 83.7• , Plong = 178.1• , N = 10 and the corresponding mean paleodirection is I = 31.3• , which is not significantly different from the expected direction estimated from the North American apparent polar wander path. Thirty-nine samples were pre-selected for Thellier palaeointensity experiments because of their stable remanent magnetization and relatively weak-within-site dispersion. Only 21 samples, coming from four individual basaltic lava flows, yielded reliable paleointensity estimates with the flow-mean virtual dipole moments (VDM) ranging from 6.4 to 9.1 × 10 22 Am 2 . Combining the coeval Mexican data with the available comparable quality Pliocene paleointensity results yield a mean VDM of 6.4×10 22 Am 2 , which is almost 80% of the present geomagnetic axial dipole. Reliable paleointensity results for the last 5 Ma are still scarce and are of dissimilar quality. Additional high-quality absolute intensity determinations are needed to better constraint the geomagnetic field strength during the Plio-Pleistocene time.

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“…The Gauss coefficients of the main field are interpolated between the coefficients given by the International Geomagnetic Reference Field (IGRF) models [17] calculated every five years. The coordinates of the profiles are projected along the spreading direction (Table 1).…”

Section: Profile Analysismentioning

confidence: 99%

Geomagnetic field variations between chrons 33r and 19r (83–41 Ma) from sea-surface magnetic anomaly profiles

Bouligand

Dyment

Gallet

et al. 2006

Earth and Planetary Science Letters

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Sea-surface magnetic profiles exhibit coherent short wavelength "micro-anomalies" (or "tiny wiggles") superimposed to the main anomalies due to reversals. In this study, we investigate the nature and distribution of these tiny wiggles on oceanic crust formed during the ∼42 Myr-long period following the Cretaceous Normal Superchron. To this end, we compute stacks of anomaly profiles from different areas in the Indian and Pacific oceans.Using a simple method based on upward continuation, we demonstrate that, the tiny wiggles are consistent worldwide although their patterns exhibit different resolutions at different spreading rates. They are therefore confidently ascribed to past fluctuations of the geomagnetic dipole moment. A high resolution record of these fluctuations is obtained by selecting and stacking profiles from areas with the highest spreading rates. Modeling the micro-anomalies as short magnetic polarity intervals yields durations for these intervals generally shorter than 10 kyr, likely too short to be indeed "true" subchrons. Moreover, the number of detected tiny wiggles clearly depends on the spreading rate. These results support geomagnetic intensity fluctuations as being the cause of most tiny wiggles, as also Preprint submitted to Elsevier Science suggested by recent magnetostratigraphic data. The tiny wiggles are uniformly distributed within chrons, indicating that paleointensity fluctuations are neither inhibited after, nor enhanced before, a reversal beyond a "blind" zone of about 10 km (corresponding to 80 to 250 kyr depending on the spreading rate) for which the anomalies due to reversals prevent the detection of tiny wiggles. Most tiny wiggles probably represent a filtered record of a uniform secular variation regime, as suggested by their uniform spatial distribution over the whole investigated period.

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International Geomagnetic Reference Field, 1995 revision Presented by IAGA Division V, Working Group 8

Cited by 25 publications

References 6 publications

Early cretaceous absolute geomagnetic paleointensities from Córdoba Province (Argentina)

Early cretaceous absolute geomagnetic paleointensities from Córdoba Province (Argentina)

Further constraints for the Plio-Pleistocene geomagnetic field strength: New results from the Los Tuxtlas volcanic field (Mexico)

Geomagnetic field variations between chrons 33r and 19r (83–41 Ma) from sea-surface magnetic anomaly profiles

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