A PALEOINTENSITY PERSPECTIVE ON THE LONG-TERM HISTORY OF THE GEODYNAMO

Data on the long-term evolution of Earth’s magnetic field are crucial for understanding the mechanisms of the geodynamo and planetary evolution. However, there has been a controversy regarding whether the long-term variations of geomagnetic polarity reversal frequency, secular variation and field strength have been intrinsically coupled, or they have varied independently throughout the geological history. Many geodynamo models predict an anti-correlation between reversal frequency and field strength but the majority of the paleointensity data fails to confirm this relationship. The paleointensity record in rocks, however, may be adversely affected by a plethora of physical processes which must be taken into account when analyzing the paleointensity database. If not accounted for, these processes may lead to significant under- or overestimates of paleointensity. These effects have not been fully understood yet and require extensive experimental and theoretical rock magnetic, mineralogical and petrological studies. However, failure to recognize them may lead to some apparent paradoxes in the large-scale picture of geodynamo evolution derived from statistical treatments of the global paleointensity database. In particular, the presence of non-ideal carriers of magnetic remanence in rocks may have resulted in a pervasive low-field bias affecting most paleointensity data in the global database. Such a bias offers a feasible explanation for the discrepancy between the experimental data and numerical models, and thus supports an inverse relationship between the reversal rate and field strength as a fundamental property of the geodynamo. Other fundamental challenges and perspectives of paleointensity determination will be discussed in the context of current models of the long-term geomagnetic and thermal evolution of our planet.