HR: 11:35h
AN: T42A-06    [Abstracts]
TI: The Role of Continents and of the Wavelength of Mantle Convection on the Earth's Cooling
AU: * Grigne, C
EM: grigne@ess.ucla.edu
AF: Department of Earth and Space Sciences. UCLA, 3806 Geology Building Box 951567 University of California, Los Angeles, CA 90024 United States
AU: Labrosse, S
EM: labrosse@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4, place Jussieu, Paris cedex 05, 75252 France
AU: Tackley, P J
EM: ptackley@ess.ucla.edu
AF: Department of Earth and Space Sciences. UCLA, 3806 Geology Building Box 951567 University of California, Los Angeles, CA 90024 United States
AB: The long standing problem of the cooling of the Earth lies in the fact that the geophysical approach, using parameterized models of convection, leads to a too fast cooling of the Earth at the beginning of its history. The geochemical estimates of concentrations in radioactive elements in the mantle then appear too low to explain the observed present mantle heat loss. This problem has been addressed recently through new approaches aiming at increasing the sensitivity to initial conditions, by lowering the exponent $\beta$ in the scaling law between the heat flux $Q$ out of the mantle and its Rayleigh number: $Q=C\ Ra^{\beta}$. In the present study, we propose an alternative solution, focusing not on the exponent $\beta$ but on the pre-factor $C$. We show that this coefficent depends on the wavelength of convection and we build up a scaling law that gives the dependence of $C$ on this wavelength for two-dimensional models. In a previous study (Grign{\'e} and Labrosse, 2001), the insulating effect of continents has been introduced in models of Earth's cooling, considering no modification of the dynamics of the mantle due to continents. The mantle circulation can however be expected to depend on the position of continents at the surface of the Earth. Long wavelengths dominate when continents are aggregated in one supercontinent, whereas shorter cells must be present when continents are more homogeneously spread at the surface. The variations of the wavelength of convection with the Wilson cycle can then be implied to induce important variations of the heat flow out of the Earth. Noting that the wavelength has decreased over the past 400 Ma after a continental breakup, it can be proposed that the present day mantle heat loss is anomalously high compared to a monotonous trend that would not take into account the variations of the length scale of convection over time, which renders simple parameterized models of thermal evolution inaccurate for quantitative predictions. The effect of continents on the length scale of mantle convection, associated with their insulating effect, can then bring a possible explanation to the problem of the Earth's cooling.
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 8125 Evolution of the Earth
DE: 8130 Heat generation and transport
DE: 5418 Heat flow
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting