EVOLUTION OF THE PALEOMAGNETOSPHERE SINCE 4 GA: MODELING VARIATIONS IN DIPOLE FIELD STRENGTH AND MAGNETIC SHIELDING ON MILLION-TO-BILLION YEAR TIMESCALES

The geomagnetic field is a long-lived phenomenon that shields Earth’s atmosphere from erosion by charged solar wind. Changes in atmospheric shielding may have had profound implications on the evolution of life. The geomagnetic field is generated in the liquid core through a convective process termed the geodynamo. Characterizations of how the paleomagnetic field changes on million-to-billion year timescales afford a unique opportunity for insights into core processes and conditions. Quantifying the strength of the magnetic field can be made through the collection of paleointensity estimates extracted from geologic materials. However, constructing a coherent timeline of the average dipole field is challenging, in part due to the sparsity of the paleointensity record. Recently, the site-mean absolute paleointensity database PINT (www.pintdb.org; Bono et al., GJI, 2022) received a major update to include data published up through 2019 and fully integrates the Quality of Paleointensity (Q_PI) assessments for over 90% of the database. Here, I present a minor update to the PINT database to include paleointensity records published through 2022, which is used to develop a continuous paleomagnetic axial dipole moment model spanning 50 ka to ~4 Ga, MCADAM (Monte Carlo Axial Dipole Average Model). This model yields posterior predictions of axial dipole field strength and allows for estimation of the median field with associated confidence bounds; the model is able reproduce the recent (0-2 Ma) field presented in PADM2M (Ziegler et al., GJI, 2011) and agrees with prior descriptions of salient field intervals (e.g., observations of high fields associated with superchrons). Using the MCADAM model, the paleomagnetospheric standoff distance can be estimated going back to the early Archean. Magnetic standoff evolution reveals that the strength of the magnetic atmospheric shield during the Precambrian was lower than the present day by about a factor of 2, reaching a protracted (~100 myr) minimum during the Ediacaran, before steadily climbing towards present day standoff distances. This suggests that for most of the Precambrian, atmospheric protections were weaker and perhaps more tenuous than during the Phanerozoic.