Constraining D” structures and mineralogy: from global to small scale

The so-called D” layer marks the transition between the molten iron alloy outer core and the crystalline lower mantle such that it plays a critical role in whole mantle convection processes but also in core convection, Earth’s heat budget and geodynamo processes. Since its first detection in the 80's, seismic observations have revealed the presence of more and more complexities in D" (ULVZs, anisotropy, ...). The challenge today is to build a global image of D" and to understand its origin which is still debated. This is part of the project HAADES (Heterogeneities, Anisotropy and Attenuation in the Deep Earth through the use of Seismic signals) I am conducting. I will first show that using new normal mode cross-coupling data we have been able to highlight the importance of spherical harmonic degree 3 in the D", compared to the well known strong degree 2, which results in a more complex pattern of LLSVPs. I will then zoom on a particular region, North Atlantic, and show that by combining S and P data with the use of array seismology we can detect a 290 km thick D" with small velocity perturbations (dln(Vs)=0.7% and dln(Vp)=0.6%) which likely indicate chemical heterogeneities and/or temperature effects in D".