EVOLUTION OF PROTEROZOIC EARTH'S MAGNETIC FIELD IN THE CONTEXT OF PLATE TECTONICS

A detailed, geology-based, Siberia-west Laurentia paleotectonic connection may have implications for evolution of the geodynamo and growth of the Earth's inner core through Proterozoic time. The connection agrees with published Proterozoic paleomagnetic data if the Proterozoic planet had a quadrupolar magnetic field, rather than a dipolar field. A quadrupolar field is favored by a small (or absent) solid inner core and a density-stratified liquid outer core. The present dipolar field is driven by convection of the liquid outer core. The convection is driven by crystallization and growth of the solid inner core, a process which generates buoyancy through the selective accretion of dense components and the concomitant release of their heat of fusion. The inner core had to grow through a threshold diameter before inner core-tangent convective cylinders focused the dipolar field near the spin axis. If inner core growth is needed to sustain the dipolar geodynamo, it stands to reason that, looking backward in time, the inner core would have been smaller, possibly non-existent. Recent numerical models propose that the inner core did not nucleate until Neoproterozoic. In the absence of a dipole, a quadrupole is favored. A quadrupole may have dominated Earth’s geomagnetic field until early Paleozoic time, when the field transitioned to a geocentric axial dipole. The dipole would have overwhelmed the weaker quadrupole to established a strong magnetosphere that effectively shielded Earth from UV radiation, helping to make the planet habitable for Early Paleozoic fauna. Earth's Proterozoic magnetic field may have resembled that of modern Neptune, where, in the absence of a large, solid inner core, the liquid outer core is density-stratified, and the quadrupole is generated within a thin spherical shell near the core-mantle boundary.