Early Cretaceous geomagnetic behaviour compared to a computer model

Twelve patterns of geomagnetic dipole behaviour, recurring five times, are recorded in a 88 m bore-core penetrating some 3.2 Ma of shallow water carbonates around 127 Ma (Early Cretaceous of Southern Italy). Each pattern sequence represents a complete reversal cycle that lasts for c. 660 ka, formed of c. 300 ka of predominantly normal polarity and c. 330 ka of mainly reversed polarity. Virtual Geomagnetic Pole (VGP) excursions, mostly lasting 5 -25 ka, are initially of lower amplitude; these become increasingly common and of larger amplitude until a reversal is completed. The succession of excursions, within the reversed sequences, appears to occur as harmonics of about 20, 44 and 64 ka. These are comparable to the Earthʹs orbital frequencies (precession and obliquity). Long-term orbital eccentricity cycles have been identified in the intensity of oceanic magnetic anomalies of the same age (Iorio, M., Tarling, D.H., DʹArgenio, B. 1998b, Periodicity of magnetic intensities in magnetic anomaly profiles: the Cretaceous of the Central Pacific. Geophysical Journal International 133, 233-244). Geomagnetic behaviour predicted from 3-D computer simulations by Glatzmaier et al. (Glatzmaier, G.A., Coe, R.S., Hongre, L., Roberts, P.H., 1999. The role of the Earthʹs mantle in controlling the frequency of geomagnetic reversals. Nature 401, 885-890), based on the geophysical properties of the present-day core-mantle boundary, has very close similarities to that observed in the bore-core remanence. In particular, oscillations in the location and intensity of the computed geomagnetic dipole occur on almost identical time-scales to those previously determined for the bore-core VGP and intensity.