During the End Ordovician (450-440 Ma), the Earth's environment underwent dramatic changes, including the oxygenization of the atmosphere and ocean, the first mass extinction, severe glaciation, and so on. Previous palaeobiological evidence pointed to two distinct mass extinction pulses. Therefore, all the known mechanisms were tried to fit these two pulses. However, recently reported high-temporal resolution palaeobiological data demonstrated a protracted extinction (Fig. 1.c, d), which contradicts all the previous proposed mechanisms for the mass extinction. Another problem is that we could explain the development of the glaciation by using the published palaeogeographic reconstructions, which indicate an invariable distribution of ocean and land during the period.
Cold and misty road to the end Ordovician True Polar Wander
A better palaeogeographic reconstruction will be required to gain a better understanding of the global changes that occurred during the end Ordovician. A requirement is a large amount of reliable palaeomagnetic data, which is completely lacking for the early-middle Palaeozoic Era (542-358 Ma).But palaeomagnetic data is the only quantitative method we can use to reconstruct the palaeogeography, then the work must be done.
We discover a new field section for palaeomagnetic research in south China's montane zone. We carried on our work in December, which unfortunately was very cold and wet. What is worse is that the rock is crushed (Fig. 2). We dug the crushed rock all morning every day in the cold winter with no sun but drizzle (Fig. 3).
Lab work is also difficult. These rock samples fragmented when they encountered the water. I tried lots of methods to protect them, and finally I had to first wrap them up with sellotape. Eventually, we obtained very nice data.
By reviewing previous studies, we realized that our new data indicate a very fast polar wander event during the end Ordovician. To double check, we scrutinize palaeomagnetic data worldwide. Surprisingly, four other continents exhibit similar fast polar wander (Fig. 4). This cannot be explained by the plate movement caused by plate tectonics. The only reasonable explanation is the True Polar Wander.
True polar wander (TPW) is the movement of the entire solid Earth (mantle and crust) relative to Earth’s spin axis in order to stabilize Earth rotation. It is different from the plate motion of plate tectonic theory, which proposes that tectonic plates, including continents or not, move over the asthenosphere relative to the underlying convecting mantle. Tectonic plates move in different directions and with different velocities (even in the case symmetric seafloor spreading, the directions of motion are opposite of each other). In contrast, TPW can induce wholesale polar motion of the plates unidirectionally and synchronously, thus changing palaeogeography rapidly and globally. Therefore, TPW potentially impacts much of Earth system evolution including changing ocean currents, air circulation, relative sea level, and depocenters of the carbon cycle.
True polar wander triggered the dramatic changes of Earth surface system
Based on the true polar wander event, we reconstruct the palaeogeography more accurately during end Ordovician-early Silurian. The rapid change of palaeogeography triggered by the true polar wander moved more land to the polar zone during 450–447 Ma, and biomass would have experienced more extinction (Fig. 1c, d). As biomass is more diverse at low latitude zone than the high latitude zone. After that, a radiation phase (Fig. 1c, d) should correspond with the continuing equatorward shift of continents until the early Hirnantian. Thus, during the fleeting Hirnantian stage, both the increased tropical weathering of arc-continent collisions triggered glaciation and the further poleward shifting of continents, caused the second severe extinction (Fig. 1c, d). Palaeogeography after the TPW event also favored plant colonization as a majority of continents became located at low-to-mid latitudes , which is supported by the positive carbon isotope signal (Fig. 1b) and oxygen rise during this time. Overall, the proposed True Polar Wander and the palaeogeographic reorganization resulting from the ~50˚ reorientation provide a simple and basic mechanism for the dramatic environmental changes of the end Ordovician–early Silurian. These connections reflect the intimate coupling between the evolution of Earth’s spheres: TPW is induced by changes of subducting slabs in the mantle; in turn, True Polar Wander resulted in palaeogeographic changes that influenced Earth’s hydrosphere, cryosphere, and biosphere.