PALEOMAGNETIC INSIGHTS ON LONG-TERM EVOLUTION OF THE CORE, MANTLE, AND SURFACE ENVIRONMENT

Advances in geochronology over the past 30 years have enabled the use of classical paleomagnetic methods, bolstered by field tests on the ages of magnetization, to produce well dated and reliable poles from Precambrian rocks. These data can be used to assess long-term evolution of three Earth spheres: core, mantle, and surface environment. Core evolution is manifested by long-term behavior of the geodynamo, which shows remarkable consistencies in field strength, reversal rate, and patterns of secular variation over a two-billion-year time interval. The Ediacaran Period, however, is marked by unusually rapid paleomagnetic variations that may signify the onset of inner core nucleation. Evolution of the mantle is documented by the rates and styles of lithospheric plate motions and true polar wander (TPW), and the episodic formation of supercontinents. Distinguishing individual plate motions from TPW can be difficult for the bulk of Earth history that lacks intact oceanic plates and hotspot tracks, but several possible TPW episodes are scattered across Proterozoic time. The oscillatory nature of those data suggests the existence of large low-shearwave velocity provinces (LLSVPs) anchoring global mantle structure along an equatorial axis. Some have viewed the LLSVPs as permanent fixtures of the deep Earth, but rapid shifts in the axes of oscillation at ca. 850 and 300 Ma could instead imply LLSVP dissolution and re-formation, after a 90-degree shift in longitude, during times of Rodinia and Pangea supercontinental aggregation. This "orthoversion" concept of supercontinental transitions links whole-mantle structure, plate tectonics, and TPW in a unified model of mantle dynamics spanning the latter half of Earth history. Reconstructions of the pre-Rodinia supercontinent Nuna and its possible predecessor Kenorland are only beginning to take form. Proterozoic supercontinents may also have influenced global climate, as both the Neoproterozoic and Paleoproterozoic low-latitude "Snowball Earth" ice ages closely follow emplacement of voluminous large igneous provinces (LIPs) likely coincident with continental fragmentation. However, absence of glaciation at ca. 1300 Ma, a time of similarly widespread LIP emplacement, might alternatively suggest low sensitivity of the global carbon cycle to paleogeography.