Paleointensity of the Earth's Magnetic Field for the Past 400 Ma: Evidence for a Dipole Structure during the Mesozoic Low.

Abstract

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.

Circular asymmetry of the paleomagnetic directions observed at low latitude volcanic sites

“…However, it is probably possible to interpret this feature of more elongation in field directions as a general indication of dipole nature which the paleomagnetic field contains. This observation is very natural from the fact that the paleomagnetic field is well approximated by a dipole field which has been established by various evidences such as success of the geocentric axial dipole hypothesis in paleomagnetism applied to plate movement (ex., McElhinny, 1973;Gordon, 1995), paleoinclination versus latitude (ex., Opdyke, 1972;Schneider and Kent, 1990), mean paleointensity versus paleolatitude (Tanaka et al, 1995;Perrin and Shcherbakov, 1997), and low-degree spherical harmonic analysis (ex., Creer et al, 1973;Johnson and Constable, 1997).…”

Paleomagnetism, paleointensity and geochronology of Miocene basalts and baked sediments from Velay Oriental, French Massif Central

“…Among 200 published paleointensity determinations for Miocene, very few fulfill basic minimum reliability criteria [Perrin and Shcherbakov, 1997]. Hence, on the basis of the available data set it is hardly possible to draw any conclusions about the strength of the Earth's magnetic field in Miocene times.…”

Eos, Transactions, American Geophysical Union Volume 91, Number 2, 12 January 2010

“…The database, fi rst compiled by Tanaka and Kono [1994], has received numerous updates [Perrin and Shcherbakov, 1997;Perrin et al, 1998;Perrin and Schnepp, 2004;Tauxe and Yamazaki, 2007;Biggin et al, 2009]. It now contains 3764 paleointensity measurements from 291 references published between 1938 and 2009.…”