Xianqing Jing, and 5 co-authors, reported in December, 2022 that the Ordovician–Silurian transition experienced severe, but enigmatic, glaciation, as well as a paradoxical combination of mass extinction and species origination. They reported a large and fast true polar wander (TPW) event that occurred 450–440 million years ago based on palaeomagnetic data from South China and compiled reliable palaeopoles from all major continents. Collectively, a ~50˚ wholesale rotation with maximum continental speeds of ~55 cm yr−1 is demonstrated.
Multiple isolated continents moving rapidly, synchronously, and unidirectionally is less consistent with and plausible for relative plate motions than TPW. Palaeogeographic reconstructions constrained by TPW controlling for palaeolongitude explain the timing and migration of glacial centers across Gondwana, as well as the protracted end-Ordovician mass extinction. The global quadrature pattern of latitude change during TPW further explains why the extinction was accompanied by elevated levels of origination as some continents migrated into or remained in the amenable tropics.
The authors demonstrated an example of the undersampling problem with a simple modeled TPW excursion. In the following example, they used the simple analytical framework of Tsai and Stevenson (ref 69) to describe the TPW due to a chosen moment of inertia tensor variation. In this formulation, the Liouville equation for a viscoelastic planet is analytically solved for a given perturbation of the moment of inertia tensor, following Munk and MacDonald (ref 70), to obtain the TPW angle. For simplicity, they chose a moment of inertia variation that is sinusoidal with a period of 150 Ma, with an average viscosity of 3 × 1022 Pa s, and an amplitude of 10−5C (where C is the Earth’s moment of inertia). This variation is chosen to very roughly approximate the observed TPW described in the next section. Equation (12) of Tsai and Stevenson (ref 69) then yields the TPW angle as a function of time, φ(t), which is plotted in Fig. 4a for an arbitrary chosen initial condition.
The associated TPW speed (black line) and the PDF (shaded gray) for this TPW curve are plotted as a function of time in Fig. 4b. As shown, the maximum instantaneous TPW speed is about 2.2° Ma−1 (24 cm yr−1) and is associated with a minimum in the PDF, showing that it is the least probable value to be observed if one randomly samples the distribution. One can also compare the probability of sampling within a finite range by using Eq. (2), or by simply reading time intervals from Fig. 4a. For example, the values of φ in the range 4° < φ < 9° represent ~32% of all measurements whereas a similar 5° range 40° < φ < 45° represents only about 1.5% of all measurements. For this example, then, it is about 20 times more likely to sample within the first range of φ (low instantaneous TPW speed) compared with the second range (maximum instantaneous TPW speed).
Original URL: https://www.nature.com/articles/s41467-022-35609-3
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