A new set of qualitative reliability criteria to aid inferences on palaeomagnetic dipole moment variations through geological time

Abstract

Records of reversal frequency support forcing of the geodynamo over geological timescales but obtaining these for earlier times (e.g., the Precambrian) is a major challenge. Changes in the measured virtual (axial) dipole moment of the Earth, averaged over several millions of years or longer, also have the potential to constrain core and mantle evolution through deep time. There have been a wealth of recent innovations in palaeointensity methods, but there is, as yet, no comprehensive means for assessing the reliability of new and existing dipole moment data. Here we present a new set of largely qualitative reliability criteria for palaeointensity results at the site mean level, which we term Q PI in reference to the long-standing Q criteria used for assessing palaeomagnetic poles. These represent the first attempt to capture the range of biasing agents applicable to palaeointensity measurements and to recognize the various approaches employed to obviate them. A total of 8 criteria are proposed and applied to 312 dipole moment estimates recently incorporated into the PINT global database. The number of these criteria fulfilled by a single dipole moment estimate (the Q PI value) varies between 1 and 6 in the examined dataset and has a median of 3. Success rates for each of the criteria are highly variable, but each criterion was met by at least a few results. The new criteria will be useful for future studies as a means of gauging the reliability of new and published dipole moment estimates.

Testing the geocentric axial dipole hypothesis using regional paleomagnetic intensity records from 0 to 300 ka

“…The scaling involves one multiplicative constant per time series so that the ratio of mean to variance is preserved within each time series. This is to be distinguished from separate scaling of the mean and variance sometimes used (e.g., Guyodo and Valet, 1999;Channell et al, 2009). Next, a PML model is made from the joint absolute and scaled RPI data set, in which the misfit criterion is a maximum likelihood (L1) norm and 2nd derivative regularization is used to minimize temporal complexity while allowing a reasonable level of misfit.…”

“…However, because of limited data availability, the GAD approximation has been invoked on these time scales to produce time-varying reconstructions of axial dipole field strength based only on paleointensity data (Valet et al, 2005;Channell et al, 2009;. These studies have yielded significant insights into long-term axial dipole field variations.…”

Palaeomagnetic field intensity variations suggest Mesoproterozoic inner-core nucleation

“…8) which stipulates that the number of sample palaeointensity measurements comprising the mean is $5 and that the associated standard deviation is # 25% of the mean value.…”

“…Previous studies searching for this signature have been hampered by the paucity of palaeomagnetic intensity measurements, by the lack of an effective means of assessing their reliability, and by shorter-timescale geomagnetic variations. Here we examine results from an expanded Precambrian database of palaeomagnetic intensity measurements 7 selected using a new set of reliability criteria 8 . Our analysis provides intensity-based support for the dominant dipolarity of the time-averaged Precambrian field, a crucial requirement for palaeomagnetic reconstructions of continents.…”

“…An additional criterion 'MAG' was added to the original 8 criteria outlined in ref. 8, which stipulates that the associated raw measurement data must be publicly available for scrutiny (the MagIC database http://earthref.org/MAGIC/ provides the ideal venue for these). It was recently observed that thermoremanent magnetization (TRM) preserved in non-single domain grains can be meta-stable, producing an additional potential source of bias to palaeointensity estimates 23 .…”