Variations in geomagnetic reversal frequency during the Earth's middle age

Abstract

[1] We have obtained new magnetostratigraphic results from two Precambrian sedimentary sections from eastern Siberia and the southern Urals dated between $1100 Ma and $800 Ma. Sample magnetizations from the uppermost Mesoproterozoic Talakh-Khaya section in the Siberian Uchur-Maya region appear to be mostly carried by a mixture of magnetite and hematite. A sequence of 33 magnetic polarity intervals is recorded within the section. All reversals occur in the first $24 m, while the upper $160 m are characterized by a single magnetic interval of normal polarity assuming a Northern Hemisphere position of Siberia around 1000 Ma. The Lower Neoproterozoic Uralian Minyar section also possesses an ancient magnetization carried by magnetite and hematite. The high-temperature magnetization component obtained from the Minyar section establishes a sequence of 43 magnetic polarity intervals. Positive reversal tests obtained from the data sets presented here and from other previously analyzed Proterozoic sections provide no convincing evidence for a long-standing asymmetric geomagnetic field during the Proterozoic that would make the Precambrian field markedly less dipolar than during the Phanerozoic. The new data further reveal the occurrence of sharp transitions and alternations between long periods without any reversal (one superchron is observed from the Talakh-Khaya section) and periods with high reversal frequencies, possibly larger than 5-10 reversals per Myr. This characteristic may well be an important property of the Precambrian field, although rather sudden transitions between reversing and nonreversing states of the geodynamo may not be unique to this period.

Mantle superplumes induce geomagnetic superchrons

“…Paleomagnetic data from ancient rocks indicate that the geodynamo has shifted between frequently reversing and superchron states on multiple occasions, back to at least 1.1 Ga (Pavlov and Gallet, 2010;Biggin et al, 2012) and possibly longer. Both the mantle and core have evolved since then, including secular cooling, supercontinent aggregation and dispersal, probable nucleation of the solid inner core (Labrosse, 2003), and major reductions in the rate of Earth's rotation due to tidal friction (Williams, 2000).…”

“…The geomagnetic field has reversed polarity ∼1 thousand times within the Phanerozoic (Gradstein et al, 2012) and there is evidence for polarity reversals throughout the Proterozoic (Pavlov and Gallet, 2010) as well as in the late Archean (Layer et al, 1996). Individual polarity reversals tend to be irregularly spaced in time, so that short sequences of reversals often conform to Poisson statistics (Lhuillier et al, 2013).…”

The Magnetic Field of Planet Earth

“…In contrast, the transition to the Cretaceous Normal Superchron seems to have been sudden and, although little is known about the reversal rate before the Kiaman reverse superchron, there also are clear indications that the transition towards the Moyero reverse superchron could have been as sudden (Pavlov and Gallet 2001). Finally, and as noted by Pavlov and Gallet (2010), both of the two previous superchrons strongly suggest even more sudden transitions between periods of frequent reversals and superchrons. Such sudden transitions seem to favor a spontaneous origin of superchrons, as proposed by Hulot and Gallet (2003).…”

“…Units ZAm 2 stand for 10 21 Am 2 . After Tauxe and Yamazaki (2007) where more details can be found cause of superchrons, it is finally worth recalling that both the data used by Coe and Glatzmaier (2006) to estimate a low 0.2 My −1 reversal rate, and the more recent data published by Pavlov and Gallet (2010) primarily reveal transitions between periods with frequent reversals and superchrons. The main cause of the apparent low reversal rate observed by Coe and Glatzmaier (2006) and of the superchron type of VGP scatter found by Biggin et al (2008a) could thus be the more frequent occurrence of superchrons before 1 Ga than during the past 160 My (or even 300 My, recall Figs.…”

“…Further interesting insight into the possible cause of superchrons can be gained from inspection of more ancient magnetostratigraphic data. These data reveal that at least four other superchrons have occurred over the past 1.4 Ga: the ∼ 50 My long Kiaman reverse superchron, from ∼ 310 to ∼ 260 Ma (Opdyke and Channell 1996); the ∼ 30 My long Moyero reversed superchron, from ∼ 490 to ∼ 460 Ma (Pavlov and Gallet 2005); and two more recently recognized (and not yet baptized) even older superchrons, one dated ∼ 1 Ga and at least 16 My long (if not 30 My or more) (Pavlov and Gallet 2010), and another one dated ∼ 1.4 Ga and ∼ 30 My long (Elston et al 2002). Some information is also often available with respect to the sequence of reversals that preceded or followed these superchrons.…”