Paleomagnetic evidence of reversals resulting from helicity fluctuations in a turbulent core

Abstract

Detailed transition records of the upper Olduvai (N‐R) and lower Jaramillo (R‐N) reversals from deep‐sea sediment cores in both the northern and southern hemispheres constrain the presence of large scale symmetries in these transitional fields. These records, obtained from core RC14‐14 (35.91°S, 59.97°E) and Deep Sea Drilling Project hole 609B (49.86°N, 335.77°E), may be modeled by specifying the geometry of the nondipole field while allowing the axial dipole to vary according to Olson's (1983) dipole time evolution equations. The resulting synthetic records of inclination, declination, and intensity account for the major features of the observed records. Good fits are obtained for all four transition records by partitioning the energy lost by the dipole field, with 40% going to −G°2 and 55% going to +G°4, and maintaining an equatorial dipole at 5% of the dipole strength. Both reversals are modeled as full (two component) reversals; the only difference between the two models is the duration of the helicity fluctuation. Thus both of these transitional fields were dominated by low, even degree terms, suggesting similarities in these N‐R and R‐N reversal processes which result in symmetries about the equator. The successful modeling of these records demonstrates that these data are consistent with Olson's hypothesis that helicity fluctuations can result in reversals of an axial dipole field generated by a homogenous, α2 dynamo.