Morphology of the Iceland Basin Excursion from a spherical harmonics analysis and an iterative Bayesian inversion procedure of sedimentary records

Based on 5 published marine high-resolution sedimentary records of the Iceland Basin Excursion [IBE; Channell, J.E.T., Hodell, D.A., Lehman, B., 1997. Relative geomagnetic paleointensity and ∂18O at ODP Site 983/Gardar Drift, North Atlantic since 350ka. Earth Planet. Sci. Lett. 153, 103–118; Laj, C., Kissel, C., Roberts, A., 2006. Geomagnetic field behavior during the Iceland Basin and Laschamp geomag- netic excursions: a simple transitional field geometry? Geochem. Geophys. Geosystems. 7, Q03004, doi:10.1029/2005GC001122] dated around 186–190 kyr, we present models of the excursional geomag- netic field at the Earth’s surface using two different approaches. First a spherical harmonics analysis is performed after synchronization of the records using their paleointensity profiles. Second, we have used an iterative Bayesian inversion procedure, calibrated using the single volcanic data available so far. Both modeling approaches suffer from imperfections of the paleomagnetic signals and mostly from the still poor geographical distribution of detailed records, presently available only from the North Atlantic and the West Pacific. For these reasons, our modeling results should only be regarded as preliminary models of the geomagnetic field during the IBE, susceptible to improvements when including results from future paleomagnetic studies. Nevertheless, both approaches show distinct similarities and are stable against moderate variations of modeling parameters. The general picture is that of a dipole field undergoing a strong reduction, but remaining higher than the non-dipole field all through the excursional process, except for a very short interval of time corresponding to the dipole minimum at the center of the excur- sion. On the other hand, some differences exist between the results of the two models with each other and with the real data when the virtual geomagnetic pole (VGP) paths are considered. The non-dipole field does not appear to undergo very significant changes during the excursion except for a slight increase just at the dipole minimum. The width of mid-height of the dipole minimum, which can be considered as an approximate measure of the duration of the IBE is of the order of 3 kyr according to the SPECMAP age model, consistent with a suggestion by Gubbins [Gubbins, D., 1999. The distinction between geomagnetic excursions and reversals. Geophys. J. Int. 137, F1–F3]. These results are compared to those obtained for the Matuyama-Brunhes geomagnetic reversal [Leonhardt, R., Fabian, K., 2007. Paleomagnetic reconstruction of the global geomagnetic field evolution during the Matuyama/Brunhes transition: iterative Bayesian inversion and independent verification. Earth Planet. Sci. Lett. 253, 172–195].