Assessing the Robustness of Long-term Field Variations in the Paleomagnetic Record

Abstract:

Deciphering long-term variations in the magnetic field at Earth’s surface informs us of the deep interior processes powering our planet. However, long-term field variations are difficult to discern because they require large amounts of data well distributed in both space and time. As part of an international effort to address this issue, here, we synthesize the results of a major collaboration in paleomagnetic field behavior taking place before, during and after the 2017 Nordic Paleomagnetic Workshop in Leirubakki, Iceland. Specifically, we employ numerical geodynamo models to ascertain the robustness of our previous observations of long-term variations in Mesozoic paleomagnetic field behavior. We assess the effects of spatial and temporal data distributions on paleosecular variation behavior (PSV; as determined by Model G) and the time-averaged field (for field strength only) by downsampling 79 geodynamo simulations, mimicking the spatial sampling for the Cretaceous Normal Superchron (CNS), the Middle Jurassic, and the last 10 Myr, and comparing these results to those of a uniform grid. We find that different spatial sampling yield similar distributions of the Model G a (characterizing dispersion at the equator) and field strength (virtual dipole moment, VDM), but Model G b (latitude-dependent dispersion) is less well-determined, especially when high-latitude sites are sparse. Our results add support to the, recently reassessed, inverse relationship between field strength and reversal rate, and additionally suggest that the minimal variation observed between Model G a between the CNS, Mid-Jurassic, and past 10 Myr is robust. Overall, we find that the most reliable parameters for determining changes in field behavior where data are limited are VDM and Model G a. It is important to note that for the three time periods assessed here, we cannot presently distinguish between PSV regimes, unlike for median VDM. This suggests that VDM may be a more powerful proxy for reversal frequency than PSV, with implications for studies of Precambrian field behavior where precise estimates of reversal frequency may be extremely difficult to obtain.