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The intensity of the time-averaged geomagnetic field: the last 5 Myr

The existing database for paleointensity estimates of the ancient geomagnetic field contains more than 1500 data points collected through decades of effort. Despite the huge amount of work put into obtaining these data, there remains a strong bias in the age and global distribution of the data toward very young results from a few locations. Also, few of the data meet strict criteria for reliability and most are of unknown quality. In order to improve the age and spatial distribution of the paleointensity database, we have carried out paleointensity experiments on submarine basaltic glasses from a number of DSDP sites. Of particular interest are the sites that provide paleointensity data spanning the time period 0.3-5 Ma, a time of relatively few high quality published data points. Our new data are concordant with contemporaneous data from the published literature that meet minimum acceptance criteria, and the combined data set yields an average dipole moment of 5.49 +/- 2.36*10^22 Am². This average value is comparable to the average paleofield for the period 5-160 Ma (4.2 +/- 2.3*10^22 Am²) [T. Juarez, L. Tauxe, J.S. Gee and T. Pick (1998) Nature 394, 878-881] and is substantially less than the value of approximately 8U1022 Am2 often quoted for the last 5 Myr (e.g. [McFadden and McElhinny (1982) J. Geomagn. Geoelectr. 34, 163-189; A.T. Goguitchaichvili, M. Preévot and P. Camps (1999) Earth Planet. Sci. Lett. 167, 15-34])

Saw-toothed pattern of sedimentary paleointensity records explained by cumulative viscous remanence

The relative paleointensity of the earth's magnetic field from ODP Site 851 has been characterized by progressive decay w x towards polarity reversals, followed by sharp recovery of pre-reversal values 1 . We resampled the Gilbert-Gaub reversal boundary of this deep-sea core, and show that during demagnetization this 'saw-toothed' pattern disappears. Further, the w x recently published Cumulative Viscous Remanence model 2 using the herewith obtained paleointensity record and w x constraints from thermal treatment replicates the saw-tooth of 1 , implying that it is of non-geomagnetic origin

A simplified statistical model for the geomagnetic field and the detection of shallow bias in paleomagnetic inclinations: Was the ancient magnetic field dipolar?

The assumption that the time-averaged geomagnetic field closely approximates that of a geocentric axial dipole (GAD) is valid for at least the last 5 million years and most paleomagnetic studies make this implicit assumption. Inclination anomalies observed in several recent studies have called the essential GAD nature of the ancient geomagnetic field into question, calling on large (up to 20%) contributions of the axial octupolar term to the geocentric axial dipole in the spherical harmonic expansion to explain shallow inclinations for even the Miocene. In this paper, we develop a simplified statistical model for paleosecular variation (PSV) of the geomagnetic field that can be used to predict paleomagnetic observables. The model predicts that virtual geomagnetic pole (VGP) distributions are circularly symmetric, implying that the associated directions are not, particularly at lower latitudes. Elongation of directions is North-South and varies smoothly as a function of latitude (and inclination). We use the model to characterize distributions expected from PSV to distinguish between directional anomalies resulting from sedimentary inclination error and from non-zero non-dipole terms, in particular a persistent axial octupole term. We develop methodologies to correct the shallow bias resulting from sedimentary inclination error. Application to a study of Oligo-Miocene redbeds in central Asia confirms that the reported discrepancies from a GAD field in this region are most probably due to sedimentary inclination error rather than a non-GAD field geometry or undetected crustal shortening. Although non-GAD fields can be imagined that explain the data equally well, the principle of least astonishment requires us to consider plausible mechanisms such as sedimentary inclination error as the cause of persistent shallow bias before resorting to the very "expensive" option of throwing out the GAD hypothesis

Properties of a detrital remanence carried by hematite from study of modern river deposits and laboratory redeposition experiments

Although detrital haematite is often observed in red sedimentary rocks, its contribution to the magnetization is usually a matter of debate. Part of the problem is that the properties of magnetic remanence carried by detrital haematite are not well known. Studies on both naturally and experimentally deposited modern river sediments whose remanence is carried by detrital haematite lead to the following observations: (1) The declinations of river-laid sediments deposited under known field conditions average to that of the Earth's field. (2) A substantial inclination error is observed in both river-laid and experimentally deposited sediments which varies as: tan (Io) =f x tan (If) where Io and If are the remanent and applied inclinations respectively and f is about 0.55 in these experiments. (3) The intensity of remanence is a function of both the magnitude and the orientation of the applied magnetic field, increasing with field strength and decreasing with field inclination. This observation is consistent with models involving contributions to the remanence by plates (constrained to lie nearly horizontally) and spheres (aligned with the applied field). (4) Sediments deposited in zero field and then subjected to an applied field acquired a p-DRM by grain rotation. The intensity of p-DRM increased with time according to a power law, P-DRM is acquired parallel to the applied field but, unless the sediment is disturbed, has an intensity an order of magnitude lower than the DRM acquired in the same field. (5) If generally valid, the inclination error for a haematite DRM presents the paradox that while both the age and the polarity of the DRM may be determined, the direction of the DRM magnetization will tend to underestimate palaeolatitude and give palaeopole positions that are far-sided

Ivory Coast microtektite strewn field: description and relation to the Jaramillo geomagnetic event

During the present study the Ivory Coast microtektite layer was found in cores from five equatorial Atlantic sites, bringing the total number of Ivory Coast microtektite-bearing cores to eleven. The strewn field appears to be restricted to between 9°N and 12°S latitude. There is a general increase in the concentration of microtektites towards the Bosumtwi crater, which is generally thought to be the source of the Ivory Coast tektites. The relationship between the onset of the Jaramillo subchron and the Ivory Coast microtektite layer has been investigated in six cores. A plot of the difference in depth between the base of the Jaramillo subchron and the microtektite layer versus sediment accumulation rate was used to determine the average post-depositional remanent magnetization (PDRM) acquisition depth and the age difference between the onset of the Jaramillo subchron and the deposition of the microtektites. Assuming that the PDRM acquisition depth does not vary with sediment accumulation rate, we find that the average PDRM acquisition depth is 7 cm and that the microtektites were deposited approximately 8 ky after the onset of the Jaramillo subchron. This indicates that the impact responsible for the Ivory Coast tektites and microtektites could not be causally related to the geomagnetic reversal at the base of the Jaramillo subchron

Corrected Late Triassic Latitudes for Continents Adjacent to the North Atlantic

We use a method based on a statistical geomagnetic field model to recognize and correct for inclination error in sedimentary rocks from early Mesozoic rift basins in North America, Greenland, and Europe. The congruence of the corrected sedimentary results and independent data from igneous rocks on a regional scale indicates that a geocentric axial dipole field operated in the Late Triassic. The corrected paleolatitudes indicate a faster poleward drift of ~0.6 degrees per million years for this part of Pangea and suggest that the equatorial humid belt in the Late Triassic was about as wide as it is today