Dipole strength and variation of the time‐averaged reversing and nonreversing geodynamo based on Thellier analyses of single plagioclase crystals

Abstract

[1] Single plagioclase crystals separated from lavas can contain minute magnetic inclusions that faithfully record the geomagnetic field, while the silicate host limits natural and experimentally induced alteration. Here we use paleointensity analyses of plagioclase crystals to examine three characteristic geodynamo regimes spanning the last 200 Myr: an interval of moderate reversal frequency during the Paleogene, the nonreversing field of the Cretaceous Normal Polarity Superchron, and a period of very high reversal occurrence during the Late Jurassic. An inverse relationship between reversal rate and intensity is supported. Furthermore, the strength of the time-averaged reversing field is more variable than that of the nonreversing field. These observations are consistent with an active lower mantle, shaping core-mantle boundary heat flux and efficiency of the geodynamo on timescales of tens of millions of years.

Paleointensity results from the Jurassic: New constraints from submarine basaltic glasses of ODP Site 801C

“…[49] The nearly 50 Myr duration of the Mesozoic Dipole Low supported by the data compiled here contrasts with the rather short duration called for by, e.g., Tarduno and Cottrell [2005], who sought to tie average reversal frequency with average field strength. It is worth pointing out that Tauxe and Yamazaki [2007] took a slightly different approach by tying average field strength within an individual polarity interval with the duration of that interval.…”

“…They observed a gradual increase in anomaly amplitude from at least as early as anomaly M29 extending to around M20 (158-146 Ma, according to the timescale of Gradstein et al [2004]) which they argue explains the ''Jurassic Quiet Zone'' observed in marine magnetic anomalies in the western Pacific Ocean. This behavior is in stark contrast to the ''Cretaceous Quiet Zone'' which was thought to be a period with no polarity reversals spanning millions of years [Helsley and Steiner, 1969] possibly associated with high magnetic field intensities [Tauxe and Staudigel, 2004;Tarduno and Cottrell, 2005;Tauxe and Yamazaki, 2007].…”

“…The approach taken here of plotting median values of five million year intervals, as done in Figure 1, however, is less influenced by extreme outliers and does not strongly support the notion of a very high field average value during the CNS. Apart for a brief interval represented by the single crystal data of Tarduno and Cottrell [2005], the median values throughout the CNS are indistinguishable from the data observed since. So according to this reanalysis, there is no strong connection between reversal frequency and median field strength.…”

“…As a result, there are competing views as to whether there was a period of unusually low geomagnetic field strength in the Jurassic and early Cretaceous, a period known as the ''Mesozoic Dipole Low'' (MDL) [Pr evot et al, 1990]. Recently, champions for the existence of the MDL have sought to tie it to changes in whole mantle convection processes [e.g., Tarduno and Cottrell, 2005;Biggin et al, 2012]. However, there remains much debate in the literature on the related problems of what the long-term average value of the field is [Juarez et al, 1998;Selkin andTauxe, 2000 versus Biggin and, and the duration Cottrell, 2005 versus Perrin andShcherbakov, 1997] or even existence [e.g., Goguitchaitchvili et al, 2002a versus McElhinny andSager, 2003] of the MDL.…”

Mantle superplumes induce geomagnetic superchrons

“…Controversy surrounds the overall significance of the MDL, with some studies finding a general inverse correlation between paleointensity and reversal frequency (Channell et al, 1982;Tarduno and Cottrell, 2005), while other studies questioning whether such a correlation exists (Ingham et al, 2014). Nevertheless, the best available paleointensity data indicate a Jurassic median dipole moment of 29 ZAm 2 , compared to 78 ZAm 2 at present-day, ∼72 ZAm 2 at CNS onset, and ∼42 ZAm 2 for the long-term (0-140 Ma) time average (Tauxe et al, 2013).…”