Geomagnetic reversal rates following Palaeozoic superchrons have a fast restart mechanism

Abstract

Long intervals of single geomagnetic polarity (superchrons) reflect geodynamo processes, driven by core–mantle boundary interactions; however, it is not clear what initiates the start and end of superchrons, other than superchrons probably reflect lower heat flow across the core–mantle boundary compared with adjacent intervals. Here geomagnetic polarity timescales, with confidence intervals, are constructed before and following the reverse polarity Kiaman (Carboniferous–Permian) and Moyero (Ordovician) superchrons, providing a window into the geodynamo processes. Similar to the Cretaceous, asymmetry in reversal rates is seen in the Palaeozoic superchrons, but the higher reversal rates imply higher heatflow thresholds for entering the superchron state. Similar to the Cretaceous superchron, unusually long-duration chrons characterize the ∼10 Myr interval adjacent to the superchrons, indicating a transitional reversing state to the superchrons. This may relate to a weak pattern in the clustering of chron durations superimposed on the dominant random arrangement of chron durations.

Refining the chronostratigraphy of the Karoo Basin, South Africa: magnetostratigraphic constraints support an early Permian age for the Ecca Group

“…5b). Interestingly, the work by McKay et al (2015;2016) did not identify any ages older than ca. 280 Ma in the Ecca Group, and so early Permian-aged zircons were excluded from their age calculations by reasoning of assumed inheritance.…”

“…Typically inclination shallowing expected from sedimentary rocks which have undergone compaction is recognized by an E-W directed elongation of the paleomagnetic dataset (Tauxe and Kent, 2004). This section lies within a geomagnetic superchron, and the exact behaviour of the Earth's magnetic field during these types of events is still poorly understood (Granot et al, 2012;Hounslow, 2016;Lhuillier et al, 2016). Contributions from a non-zero non-GAD (Geocentric Axial Dipole) field can change the distribution of directions, where a non-zero axial octupole of the same sign as the axial dipole will enhance the N-S elongation of the observed directional data resulting in a shallow polarity bias (Tauxe and Kent, 2004).…”

“…262-255 Ma ages from the Adelaide Subgroup of the Beaufort Group (Rubidge et al, 2013). The apparent age controversy was explored by McKay et al (2015;2016) who demonstrated that volcanic tuffs from the Ecca Group yield consistently younger ages (250 to 274 Ma) than tuffs from the conformably overlying Beaufort Group (257 to 452 Ma) along a 650 km transect across the southern margin of the Karoo Basin. This "inverted" basin stratigraphy was ascribed to an episode of zircon exhaustion and magmatic recycling (as opposed to a disturbed U-Pb system).…”

“…Beginning in the Capitanian (ca. 265 Ma), the geomagnetic field transitioned back into a frequently reversing state, and this transition was thought to occur over a short (~10 Ma) interval (Hounslow, 2016). The Late Permian through Early Triassic, therefore, marks a period of increasing reversal frequency (~2-4/Ma; Hounslow, 2016), providing a strong chronostratigraphic marker (Fig.…”

Miocene humid intervals and establishment of drainage networks by 23 Ma in the central Sahara, southern Libya

“…13). The composite magnetostratigraphy 518 is generated using the numerical optimisation techniques described in Hounslow (2016). For 519 this data, the optimisation shifts the SK3 section (without a metre-scale change), but has 520 both a shift and a scale change for the data in the TS3 section.…”