Massentransporte und Massenverteilungen im System Erde  
    The hidden geoid signal of LLSVPs: Counteracting effects of compositional and thermal density anomaliesThe hidden geoid signal of LLSVPs: Counteracting effects of compositional and thermal density anomalies  
 

 

The hidden geoid signal of LLSVPs: Counteracting effects of compositional and thermal density anomalies

When comparing the Earth’s geoid with a shear velocity anomaly map of the lowermost mantle, a clear spatial correlation between the positive geoid anomalies over Africa and the Pacific (Figure 1, top) and the location of Large Low Shear Velocity Provinces (LLSVPs) in the lowermost mantle (Figure 1, bottom) can be established.

 

Figure 1: Comparison of observed geoid at degrees 2-6 (top) with shear velocity anomaly map of the lowermost mantle. A clear spatial correlation between large positive geoid anomalies over Africa and the Pacific and LLSVPs in the lowermost mantle can be established.

In our study, we investigated the influence of the excess density of the near-equatorial, antipodal LLSVPs on the geoid both in Cartesian and axisymmetric spherical mantle convection simulations.

 

Interestingly, temperature and compositional density contributions to the geoid from within LLSVPs cancel out at a specific amount of compositional density, rendering LLSVPs virtually invisible in the geoid signal. For a simulation at Rayleigh number Ra=10^4, this occurred between a non-dimensional buoyancy ratio Br of 0.5 to 0.75 (Br is the ratio of compositional over thermal density variations), witnessed by the zero-crossing and unit ratio in Figure 2, left and right, respectively.

Figure 2: (Top) Steady state results from convection simulations at Ra=10^4 and Br=0 - 1. (Bottom) Sum and ratio of the associated isolated geoid signals (thermal and compositional, marked by Br). At a certain Br, marked by the zero-crossing (left) and unit ratio (right), the LLSVPs become virtually invisible in the geoid signal, since thermal and compositional density differences cancel out.

This cancelation implies that the positive geoid anomalies above LLSVPs are mainly due to dynamic surface topography. Only for high Br (Br>0.75), density excess of LLSVPs itself delivers a significant contribution to the geoid signal.

 

For higher Rayleigh numbers (Ra=10^5 and 10^6), our axisymmetric spherical simulations remained in a time-dependent regime which we analyzed using a new technique and computed the time-averaged geoid signal from these simulations. We obtained a relatively good fit of observed and modelled geoid at low spherical harmonics degrees (l=2-6). Figure 3 shows a snapshot of a simulation at Ra=10^6 and Br=1, the associated geoid signal (inset) and a comparison of the observed and modelled time-averaged geoid signals.

 

Figure 3: (Left) Snapshot of a convection simulation at Ra=10^6 and Br=1 and associated geoid signal (inset; black: total geoid, colored curves: individual contributions, red: thermal, blue: surface topography, pink: compositional density excess, green: core-mantle boundary topography). (Right) Comparison of observed (top) and modelled (bottom) geoid at low degrees (l=2-6); given the simplifications of the model, we found a relatively good agreement between the two.

For information about the study, contact Dr. M. Beuchert (beuchert@geophysik.uni-frankfurt.de).

 

Submitted paper:

Beuchert MJ, Schmeling H and Shahraki M (2011) Influence of Large Low Shear Velocity Provinces in the lower mantle on the geoid. Submitted to Journal of Geophysical Research – Solid Earth.