Further Evaluation of Geomagnetic Dipole Asymmetry in Growth and Decay

Abstract:

Records of past geomagnetic intensity fluctuations provide important constraints on dynamical processes in the outer core. PADM2M is a reconstruction of the 0 to 2 Ma axial dipole moment primarily based on global sediment records, but calibrated by absolute paleointensity data. Ziegler & Constable (2011) showed that for periods longer than 25 kyr the dipole spends more time decaying than growing: thus its average growth rate is greater than its decay rate. The observed asymmetry is not limited to times when the field is reversing, and may reveal a critical dynamic of the outer core. We assess the robustness of the initial result through development of improved analysis methods and explore the possibility of identifying the asymmetry in alternative recording media and at other epochs. Improved low pass filtering is used to identify asymmetry (percent time growing, p_g < 50% ) and to quantify its uncertainty via bootstrap methods. The original results are significant and do not depend on the specifics of the filtering process, although the cutoff period corresponding to the peak in asymmetry varies relative to the initial analyses.

A long-term record of geomagnetic intensity should also be preserved by the thermoremanence of oceanic crust and stacks of marine magnetic anomalies are inverted to provide an independent means of assessing the asymmetry seen in PADM2M. We first examine a 0 to 780 kyr record from the East Pacific Rise near 19ºS finding that p_g departures from 50% are not statistically significant. We believe the record is too short and noisy; comparing the power spectra of the datasets supports this. Overall coherence with PADM2M never rises above 0.4 and, despite the presence of obvious long term geomagnetic signals in the record, the primary signals at periods of 25 - 50ky are almost certainly due to variations in crustal accretion.

A better candidate for analysis is a stack of near-bottom records from chron C5 in the NE Pacific. Multiple nearby records have high coherence, corresponding to global lineations extending over a 2My interval. Here we find pg is significantly less than 50% at periods of 10-30kyr and 60-75kyr, similar to PADM2M. This result indicates that other recording media carry a record of the asymmetric behavior first found in PADM2M and that it was present during other time periods.