STRATIFICATION OF THE OUTER CORE AT THE CORE-MANTLE BOUNDARY

The thermal regime of the core is controlled by the heat flux taken by the mantle across the core-mantle boundary (CMB) and its ratio Nu, th e Nusselt number, to the conductive heat flux along the core adiabat. If Nu > 1, then compositional convection from the inner-core boundary (ICE) will be augmented by thermal convection from the CMB, and the ou ter core will be well-mixed and unstratified. If Nu < 1, as some recen t estimates suggest, there is a competition between a stabilising ther mal buoyancy flux from the CMB and the convective compositional buoyan cy flux from the ICE. An entirely analogous competition arises if ligh t elements diffuse from the mantle into the core. A fundamental questi on is whether such a stabilising buoyancy flux can stratify a layer at the CMB and, if so, how thick the layer would be. A simple sphericall y averaged fluid-mechanical and thermodynamic model is derived and use d to examine the possibilities. Previous suggestions that compositiona l convection is sufficiently vigorous to maintain a well-mixed state r equire active entrainment downwards of buoyant fluid, which is doubtfu l if inertial effects are negligible. It is suggested instead that a s tratified layer accumulates below the CMB, which is not incorporated i nto the underlying convection zone, though it may be mixed slowly by d ouble-diffusive 'salt-fingering.' The thickness of the layer is calcul ated theoretically as a function of the controlling fluxes of heat and composition across the CMB. In the thermal case, the layer thickness adjusts rapidly to reflect the current value of Nu and is largely inde pendent of the cooling history. The layer thickness is of order 100 km for Nu = 0.9, with a buoyancy frequency less than 10(-4) s(-1). The c ompositional case typically gives a thinner but more strongly stratifi ed layer and adjusts more slowly. Seismic, geomagnetic and length-of-d ay evidence for such layers is assessed. (C) 1998 Elsevier Science B.V .