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Geodynamic modelling and seismic studies have highlighted the possibilitythat a thin layer of low seismic velocities, potentially molten, may sit atopthe core-mantle boundary but has thus far eluded detection. In this study weemploy normal modes, an independent data type to body waves, to assess thevisibility of a seismically slow layer atop the core-mantle boundary to normalmode centre frequencies. Using forward modelling and a dataset of 353 normalmode observations we find that some centre frequencies are sensitive toone-dimensional kilometre-scale structure at the core-mantle boundary.Furthermore, a global slow and dense layer 1 - 3 km thick is better-fittingthan no layer. The well-fitting parameter space is broad with a wide range ofpossible seismic parameters, which precludes inferring a possible compositionor phase. Our methodology cannot uniquely detect a layer in the Earth but oneshould be considered possible and accounted for in future studies.

Evidence for a kilometre-scale seismically slow layer atop the core-mantle boundary from normal modes