Was the early geodynamo driven by exsolution near the CMB?

Paleomagnetic data show that the Earth has owned a dynamo magnetic field for at least 3.5 Ga 1. The geodynamo thus appeared well before the inner core nucleation 2, but its origin remains puzzling. Indeed, given the contradictory estimates of the thermal conductivity of liquid iron at core conditions 3, secular cooling may have not been strong enough to sustain it. Finding mechanisms capable of sustaining the geodynamo is thus crucial to understand the early Earth evolution. In particular, it has been proposed that the geodynamo could have been sustained by exsolution (or precipitation) of light elements near the core-mantle boundary (CMB) 4,5. This mechanism could be powerful enough to sustain dynamo action (from an energetic viewpoint 6), but its relevance has to be assessed using fluid dynamics models. Using global simulations, we explore the flow dynamics driven by exsolution of light elements in the early Earth. When the thermal conductivity is assumed to be large, we report and characterize two different flow regimes 7, which depend on the strength of thermal stratification in the core. Next, we assess the dynamo capability of these two regimes for the first time. We show that they are associated with a dipolar-multipolar transition for dynamo action. Our simulations thus constrain the thermal stratification of the core to be likely weak in the early Earth (to be compatible with the large-scale magnetic field evidenced by paleomagnetic data). 1 Tarduno et al., 2020, PNAS, 117(5), 2309-23182 Zhou et al., 2022, Nat. Comm., 13(1), 41613 Pozzo et al., 2022. EPSL, 584, 117466.4 Badro et al., 2016, Nature, 536(7616), 326-3285 Hirose et al., 2017, Nature, 543(7643), 99-102.6 Landeau et al., 2022, Nat. Rev. Earth Environ., 3(4), 255-2697 Monville, Vidal et al., 2019, GJI, 219(S1), S195-S218