Mid-Proterozoic geomagnetic field was more consistent with a dipole than a quadrupole

Abstract

Abstract The current morphology of Earth’s time-averaged magnetic field can be approximated to a geocentric axial dipole (GAD), but whether such an approximation remains valid in deep time needs to be investigated. Studies have used paleomagnetic data to reconstruct the ancient field and generally support a GAD morphology since 2 Ga. Recently, the GAD model for mid-Proterozoic time has been challenged, and an alternative model was proposed wherein the mid-Proterozoic field was dominated by a normal-tesseral quadrupole (NTQ) with spherical harmonics of degree l = 2 and order m = 1. We performed forward modeling to quantitatively compare whether a GAD or an NTQ could provide a better fit to mid-Proterozoic paleomagnetic directions. To deal with the ambiguity in plate reconstruction, we first considered data only from Laurentia, and then we expanded the analysis to Baltica by reconstructing its position relative to Laurentia using the geologically based Northern Europe–North America (NENA) configuration. Finally, we included data from Siberia using two reconstruction models. Results showed that in three mid-Proterozoic intervals (1790–1740 Ma, 1485–1425 Ma, 1095–1080 Ma), a GAD morphology gives better, or equally good, fits compared to the NTQ morphology. In addition, a stable NTQ that persisted for hundreds of millions of years is disfavored from a geodynamic perspective. Overall, mid-Proterozoic paleomagnetic directions are more consistent with a dipolar field. We suggest that the GAD remains the most parsimonious model to describe the morphology of the mid-Proterozoic magnetic field.