Geomagnetic field behaviour and geodynamo characterisation at the end of the Cretaceous Normal Superchron

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The geomagnetic field, generated by a self-sustaining dynamo in the Earth’s fluid outer core, is modelled in first approximation by a geocentric axial dipole, yet experiencing fluctuations on centennial-to-millennial timescales—termed palaeosecular variation—and being able to stochastically reverse its polarity. Over geological timescales, the geomagnetic field can surprisingly also produce prolonged (> 20 Myr) periods with stable polarity, the most recent one being the Cretaceous Normal Superchron (CNS, 84–121 Ma). How the transition between ‘superchron’ and ‘reversing’ states of the geodynamo occurs remains a controversial issue, a gradual transition being the potential signature of changes in the thermal boundary conditions imposed by the mantle on the core whereas an abrupt transition would be more consistent with a spontaneous non-linear transition of the geodynamo. To better characterise the gradual or abrupt nature of the transition, we propose to investigate in this project the geomagnetic field behaviour and the geodynamo state at 60–100 Ma. To this end, we will first acquire new palaeomagnetic directions and intensities on sequences of lava flows from the East Sulawesi Ophiolite (ESO, emplaced ~90 Ma at low paleolatitude) and the Madagascan Flood Basalts (MFB, emplaced 84–92 Ma at mid palaeolatitude). Compared to previous results from the Okhotsk-Chukotka Volcanic Belt (OCVB, emplaced 84–90 Ma at high palaeolatitude), these new observations will allow us to better constrain the latitudinal profile of the palaeomagnetic field just before the end of the CNS. In order to produce a more global and dynamical description of the geomagnetic field behaviour across the transition from the CNS to a ‘reversing’ state of the geodynamo, we will then compile high-quality palaeomagnetic records suitable for field modelling over the 60–100 Ma interval. This will allow us to adjust the parameters of statistical field models produced by a giant gaussian process over one or two well-chosen time windows before and after the transition. With the view of inferring potential changes in the Earth’s deep dynamics, the obtained parameters will finally be compared with the outcomes of numerical dynamo simulations submitted to various boundary conditions.