Paleomagnetic evidence for long‐term (10⁴ year) Memory and periodic behavior in the Earth's core dynamo process

Abstract

A new paleomagnetic secular variation (PSV) record from the late Quaternary Wilson Creek beds of Mono Lake, California contains a distinctive periodic vector waveform that follows and is almost certainly related to the Mono Lake excursion. A comparison of all published PSV results from the Wilson Creek beds suggests that the magnetic field at Mono Lake went through an interval (36,000‐28,000 ybp) of very low amplitude PSV followed by the Mono Lake excursion (28,000‐27,000 ybp) and four subsequent recurrences (27,000‐12,500 ybp) of the excursion waveform with relatively diminished amplitudes (lower than the excursion amplitude but higher than PSV amplitudes at Mono Lake since 12,500 ybp). This pattern clearly suggests that the Mono Lake excursion is related to ‘typical’ PSV; it also suggests that the core dynamo process responsible for PSV is capable of near impulse onset, persistence over a time scale of 15,000 years, and quasi‐periodic behavior that is non‐wave‐length dispersive in time.

A New View of Long-Term Geomagnetic Field Secular Variation

“…This suggests that field variability surrounding excursions is routinely more highamplitude even when individual directions are not excursional. The evidence is not certain, however, that higher amplitude dispersion really precedes excursion intervals, an observation previously noted for the Mono Lake Excursion (Liddicoat and Coe, 1979;Lund et al, 1988). Regions C, E, and F share the relationship of higher amplitude dispersion associated with excursions even though the overall pattern of directional variability in Region F is significantly different from the other two regions.…”

“…This indicates that the Holocene is not sufficient to characterize the entire space/time pattern of dynamo activity. It also suggests that there is significant regional-scale dynamo activity operating on a 10 4 years scale (as suggested by Lund et al, 1988). It may be that such PSV analyses may provide a new perspective on the actual space/time pattern of fluid flow and magnetic-flux regeneration that occurs regionally in the outer core.…”

“…There is no magical time scale over which to average the field to study dynamo activity, but previous PSV studies suggest that simple PSV directional features or waveforms (e.g., Lund, 2007) that define the pattern of PSV variability between successive extremes in inclination or declination (Figures 4, 5) operate on time scales of ∼0.7-1.5 ka (e.g., summaries of regional PSV studies in Creer et al, 1983) and repeating patterns of more complex directional waveforms have been noted by Lund et al (1988) and Lund (1996) with repeating time intervals of 2.5-3.5 ka. On that basis, we have started by averaging FIGURE 2 | Deep-sea sediment paleomagnetic record from the subtropical Pacific Ocean.…”

“…Region B (SW USA) contains records from Mono Lake, California (Lund et al, 1988), Pyramid Lake, Nevada (Lund et al, 2017a), and Summer Lake, Oregon (Negrini et al, 2000;Zic et al, 2002). Region C (subtropical North Atlantic Ocean) contains records from the Bermuda Rise (89-9P, ODP Site 1062) (Keigwin et al, 1998;Schwartz et al, 1998;Lund S.P.…”

“…Merrill et al (1998) noted that "the interpretation of paleomagnetic results has always depended upon the fundamental hypothesis that the time-averaged geomagnetic field is that of a geocentric axial dipole." But, a large number of statistical PSV studies have made it clear that there are systematic (but usually subtle) differences in local field directions from the GAD when averaged over millions of years (e.g., Johnson and Constable, 1996;McElhinny et al, 1996;Merrill et al, 1998;Johnson et al, 2008) or shorter intervals (e.g., Lund et al, 1988Lund et al, , 2016a. The unresolved problem is what causes these non-GAD field directions and how do they vary over time-scales less than a million years.…”