Jean Besse

Magnetic Field Reversals, Polar Wander, and Core-Mantle Coupling

True polar wander, the shifting of the entire mantle relative to the earths spin axis, has been reanalyzed. Over the last 200 million years, true polar wander has been fast (approximately 5 centimeters per year) most of the time, except for a remarkable standstill from 170 to 110 million years ago. This standstill correlates with a decrease in the reversal frequency of the geomagnetic field and episodes of continental breakup. Conversely, true polar wander is high when reversal frequency increases. It is proposed that intermittent convection modulates the thickness of a thermal boundary layer at the base of the mantle and consequently the core-to-mantle heat flux. Emission of hot thermals from the boundary layer leads to increases in mantle convection and true polar wander. In conjunction, cold thermals released from a boundary layer at the top of the liquid core eventually lead to reversals. Changes in the locations of subduction zones may also affect true polar wander. Exceptional volcanism and mass extinctions at the Cretaceous-Tertiary and Permo-Triassic boundaries may be related to thermals released after two unusually long periods with no magnetic reversals. These environmental catastrophes may therefore be a consequence of thermal and chemical couplings in the earths multilayer heat engine rather than have an extraterrestrial cause.

Is high obliquity a plausible cause for Neoproterozoic glaciations

[1]xa0The main features of the low-latitude Neoproterozoic glaciations remain the subject of controversial debates concerning possible triggers. Here we use an AGCM coupled with a slab ocean to test one of the earliest and simplest explanation for tropical glaciations: a higher obliquity of the Earths rotation axis. We show that high obliquity may result in an extensive glaciation during the Sturtian episode (750 Ma), due to the location of continental masses in the tropical areas, but cannot produce a large glaciation in the case of mid to high latitude paleogeographies such as those thought to typify the Varangian-Vendian episodes (620–580 Ma).