Reversal Asymmetry after the end of the Cretaceous Superchron

Thursday, 18 December 2014: 12:05 PM
Joseph L Kirschvink, California Institute of Technology, Pasadena, CA, United States, Ross Nelson Mitchell, Yale University, New Haven, CT, United States, Christopher Thissen, Yale Univ-Geology & Geophysics, New Haven, CT, United States, Valerie Pietrasz, California Institute of Technology, Division of Geological and Planetary Sciences, Pasadena, CA, United States, Matthew E Rioux, University of California Santa Barbara, Santa Barbara, CA, United States, Alessandro Montanari, Osservatorio Geologico di Coldigioco, Apiro, Italy, Rodolfo Coccioni, Università degli Studi di Urbino “Carlo Bo”, Urbino, Italy, Peter D Ward, University of Adelaide, Sprigg Institute of Geobiology, Adelaide, Australia, Timothy D Raub, University of St Andrews, Balmullo, United Kingdom, David AD Evans, Yale Univ, New Haven, CT, United States and Samuel A Bowring, MIT, Cambridge, MA, United States
At the end of the Cretaceous superchron, two well-dated sedimentary sequences capture in high-resolution the geomagnetic reversal process. Similar to more recent reversal records at mid-latitude sites, we observe a ~10-kyr duration in both Italy (from the extraordinarily stable Scaglia Rossa Limestone) and North America (Pierre Shale). At both sites, reversal is staggered, including transitional directions and short-lived geomagnetic excursions. Unlike reversals in the past 5 Myr, however, both sites yield reversal asymmetry, whereby the normal polarity directions prior to the transition are ~10˚ more steeply inclined, and azimuthally deflected, compared to their post-reversal counterparts. Plate motions and true polar wander, although present and appreciable, are too slow to affect the time scale of the geomagnetic deflections. Lithology in both sections is homogenous across the cm-scale reversal transition, and sedimentary inclination shallowing cannot explain the canted aspect of the asymmetry. Hence, the reversal asymmetry appears to be a feature of the C33R/C33N geomagnetic reversal process. Reversal asymmetry could suggest elevated quadrapole and/or octupole components of the geomagnetic field; however, the misfit is twice as great in declination than inclination. Non-zonal harmonics, or an offset dipole, might need to be invoked to explain the large (~20˚) declination anomalies. We suggest that the end of the superchron is marked by a profound change in core heat flux that was sufficiently large and stable to sustain localized radial convection within Earth’s outer core, creating a persistent, local deflection in Earth’s surface magnetic field.