New spherical harmonic global geomagnetic field models for the Matuyama-Brunhes reversal

The geomagnetic field is created by the motion of molten iron inside the liquid outer core of the Earth. It has undergone a number of drastic changes over geological time, the most notable of which are field reversals, in which the magnetic north and south poles change locations. Because sediments can retain information about past magnetic field directional and intensity changes through depositional or post-depositional remanent magnetization acquisition mechanisms, oceanic sediment cores can be used to track reversals. Lavas also document reversals by spot readings of thermal remanent magnetization. The Matuyama-Brunhes reversal (MBR) was the last time the Earth’s poles reversed, and it is documented by the largest number of sediment records compared to earlier reversals. Investigating the MBR globally therefore can help to understand the physical processes that occur in the Earth’s core. A few global spherical harmonic (SH) models have previously been proposed for the MBR. However, the number, distribution, and timescale reliability of the input data are limitations of these models, the last one of which has been published more than 10 years ago. In this study, we take advantage of new sediments and lava data for the MBR that have been published since these models were developed, and often have better age control and higher temporal resolution than data used in previous SH models. Smoothing splines were used to examine the temporal resolution of all sediment records in our new global compilation, and the results show a median smoothing time of 350 years (±200 years). We present a new global SH geomagnetic field model for the MBR, constructed from 67 sediment cores and 93 lava sites that span the last 900-700 ka and have a reasonable geographical distribution. In addition, we investigate the robustness of model features by deriving models from sub-sets of data, e.g., using only well-dated, high-resolution sediment data that are consistent with surrounding records (if any exist). The model’s featured will be discussed, including (1) field morphology at the CMB and at the Earth’s surface; (2) axial dipole (AD) and non-axial dipole (NAD) power at the CMB; and (3) magnetic energy of AD and NAD fields at the Earth’s surface. Furthermore, we will compare the current models to the previous MBR models.