Large-scale palaeoflow at the top of Earth's core

A number of recent studies have made use of statistics derived from geodynamo simulations to constrain the flow at the top of the Earth's core. Here we adapt these methods to infer possible core surface flow solutions over the past 9000 years, using a low degree geomagnetic field model, based on archaeomagnetic and sedimentary archives. Despite the low spatial resolution of the field model, we find that the largest scale aspects of the recovered flow agree well with those derived from historical field models. In particular we see the growth of an eccentric planetary gyre over the past 400 years and find that a similar feature appears on bimillennial timescales, concurrent with pulses in the zonal flow. In the flow model vigorous gyre structures are not persistent but appear as transient features with durations of a few hundred years. The zonal flow is westward over the entire period with a mean value of 0.09 degrees /yr. Finally, we consider the variations in length-of-day that are implied by the flow model under the assumption of geostrophy, and compare the total clock error with that inferred from records of historical solar eclipses. Good agreement is found between the two series of mantle rotation for at least the past 1200 years, with anomalously slow mantle rotation coupled with reduced westward zonal flow. Further back in time a rigorous assessment is hard to make, owing to uncertainty surrounding the exact locations of eclipses.