When the Earth wobbled…
A sharp global drop in subduction flow around 700 Ma ago, at the origin of a very large pole drift in the Ediacaran?
Publication date: 25/02/2019
General public, Press, Research
Related teams :
Geomagnetism, Paleomagnetism
Related themes : Origins
About 700 million years ago, during the Ediacaran period, the Earth underwent a number of environmental upheavals: diversification of animals, large-scale glaciations, but also the final break-up of the supercontinent Rodinia (one of the supercontinents that preceded Pangea) followed by the formation of another mega-continent: the Gondwana.
Using palaeomagnetic data indicating the position of the palaeomagnetic poles, geological and glacial records and subduction flow modelling, researchers from the IPGP and the University of Oslo propose, in a study published at the end of 2018, that these changes are the consequence of the Earth’s rotation on itself, linked to a change in the distribution of masses within the planet.
The Ediacaran (635-541 Ma), the last geological period of the Precambrian, is a time interval during which major environmental changes occurred, affecting the biosphere and cryosphere as well as the tectosphere. Ediacaran palaeomagnetic observations, which are crucial for reconstructing the position of the continents, are few in number and particularly complex to interpret. Paleogeographic reconstructions from this period are therefore still hotly debated.
A selection of palaeomagnetic poles determined on three different continents, Laurentia, Baltica and the West African craton, show a very rapid common oscillation (>30 cm/year) between 615 and 565 Ma (Figure 1), and seem to indicate that a global movement of the palaeomagnetic poles occurred during this period. Two main hypotheses have been put forward to explain these observations: either the magnetic field was abnormally unstable, with the dominance of a magnetic dipole aligned in the equatorial plane (such a magnetic field was once proposed for Mars), or major episodes of pole drift (True Polar Wander) occurred. The latter mechanism corresponds to the movement of all the Earth’s envelopes in relation to its axis of rotation, and is linked to changes in the distribution of masses within the Earth. On a geological timescale, convective movements in the mantle are the main source of this phenomenon.
The authors of the study tested the pole drift hypothesis by modelling convection movements within the mantle during the Ediacaran. They used a simple model of mantle dynamics that calculates the evolution, as a function of time, of the distribution of mass in the mantle associated with cold, dense subducting plates and hot, less dense upward movements. They assumed that the geometry of the large-scale mantle flow in the Ediacaran was similar to that of more recent periods (Figure 2), consisting of a subduction belt surrounding the continents (like the present-day Pacific Ring of Fire) and two antipodal updrafts located at the centre of the belt and rising from the base of the mantle (similar to the two present-day super-panaches imaged under Africa and in the Pacific by seismic tomography). These numerical experiments show that the destabilisation of this large-scale convective system explains the very rapid movements of the pole (~ tens of cm/year). More specifically, the drastic decrease in the speed of pericontinental subduction for around 100 Ma (~730-635 Ma) and then its resumption at the start of the Ediacaran (~635 Ma) produced poleward drifts comparable to those determined by palaeomagnetism during the Ediacaran (Figure 1).
Is there any geological evidence for a decrease in subduction flow between 730 and 635 Ma? The overall frequency of detrital zircon ages seems to be a good proxy for subduction flow as a function of time, since zircons are mainly produced in magmas associated with subduction zones and are well preserved from alteration because of their high hardness. Figure 3 shows a very good correlation for recent periods (410-0 Ma) between the frequency of zircon ages and the subduction flow calculated from the palaeogeographic reconstructions in the study. The phase of low subduction flow proposed in this palaeogeographic model between 730 and 635 Ma is also validated by the low age frequency of detrital zircons during this period (Figure 3), showing that these experiments show that very large amplitude tilts (~90°) of our planet in relation to its axis at speeds of the order of ~70 cm/year is a plausible phenomenon.
The low-flow episode is also correlated with the occurrence of large-scale Marinoan and Sturtian glaciations (Figure 3). In fact, the significant reduction in subduction volcanism on a global scale that would result from this scenario could lead to a significant reduction in greenhouse gas emissions into the atmosphere, and contribute to the existence of these particularly cold periods. Finally, the drastic drop in activity of the pericontinental subductions between 730 and 635 Ma corresponds to a quiescent phase in the fragmentation of the supercontinent Rodinia (Figure 3). Since subductions can play a major role in continental rifting, their drastic reduction between 730 and 635 Ma could also have influenced the tectonic evolution of the Rodinia supercontinent.
Ref: B. Robert, M. Greff-Lefftz, & J. Besse (2018). True Polar Wander: a key indicator for plate configuration and mantle convection during the late Neoproterozoic. Geochemistry, Geophysics, Geosystems, 19(9), 3478-3495. doi.org/10.1029/2018GC007490
