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Mantle Origin of Global Sea-Level Fluctuations and Geomagnetic Reversals: Evidence from Non-Linear Dynamics
Correlations between global 3rd-order sea-level and the pattern of geomagnetic reversals suggest that a single dominant process may be controlling them both. The most likely candidate for this is mantle convection. Mantle-driven crustal processes have been related to sea-level change, and models currently exist whereby mantle convection also influences magnetic field behavior at the core-mantle boundary (CMB). For some time periods, correlations between magnetic and sea-level data correlations have very short geological lag times. This implies that during these times, mantle convection is organized in such a way that virtually simultaneous controlling events occur beneath the crust and at the CMB. Time-series analysis, used to investigate the motions of nonlinear dynamical systems, can be applied to sea-level and paleomagnetic data to support these ideas without derivings physical models for mantle convection.
Variations in relative plate motions, changes in sea-floor spreading and subduction rates, thermal uplift and other mantle-driven mechanisms have been linked to sea-level change at craton margins. These processes affect subsidence rates at plate margins, either directly or by translation of horizontal stresses into vertical motions. Major sea-level changes are observed because the vertical motions required to drain or flood a shelf are small (approximately 200 meters) relative to total crustal thickness (30 - 50 km.). To account for 3rd-order sea-level fluctuations, these processes must operate globally at a time-scale of less than 5 million years. Relative plate motion variations have been detected at this time-scale, and we speculate that the others may also vary at similar rates. In lieu of a specific global mechanism for these changes, studies of coastal onlap patterns in craton-margin basins, from which the sea-level curve is largely derived, often demonstrate a correspondence between sequence boundaries, tectonic events and global 3rd-order sea -level fluctuations.
Mantle processes might also be able to influence the frequency and pattern of geomagnetic reversals generated in the Earth's core. In some recent portrayals, magnetic reversals have been compared to models of nonlinear electrical dynamos or nonlinear convection cells. Either system can behave in a regime where chaotic reversals in field direction occur. Assuming that the core exists in such a regime, changes in the temperature gradient across the core could alter the frequency and pattern of the reversals. The temperature changes could be provided by thermal variations at the CMB as a result of mantle convection. Other recent work suggests that mantle convection exhibits deterministic turbulent behavior. These forms of turbulence are internally organized in such a way that there is a relationship between events at different points within the turbulent system. A deterministic relationship between mantle effects below the crust and at the CMB could account for the short lag times observed in sea-level/magnetic data correlations, circumventing the lengthy times required to physically circulate mantle material. Time-series analyses of event frequency in both sets of data are consistent with the possibility that they are products of the same non-linear motion. These results support speculations that 3rd-order global sea-level fluctuations may be largely a product of mantle-driven crustal dynamics during geologic time periods when correlations between magnetic and sea-level data exist.
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