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The origin and evolution of cratonic roots has been debated for many years. Precambrian cratons are underlain by cold lithospheric roots that are chemically depleted. Thermal and petrologic data indicate that Archean roots are colder and more chemically depleted than Proterozoic roots. This observation has led to the hypothesis that the degree of depletion in a lithospheric root depends mostly on its age. Here we test this hypothesis using gravity, thermal, petrologic, and seismic data to quantify differences in the density of cratonic roots globally. In the first step in our analysis we use a global crustal model to remove the crustal contribution to the observed gravity. The result is the mantle gravity anomaly field, which varies over cratonic areas from 3100 to +100 mGal. Positive mantle gravity anomalies are observed for cratons in the northern hemisphere: the Baltic shield, East European Platform, and the Siberian Platform. Negative anomalies are observed over cratons in the southern hemisphere: Western Australia, South America, the Indian shield, and Southern Africa. This indicates that there are significant differences in the density of cratonic roots, even for those of similar age. Root density depends on temperature and chemical depletion. In order to separate these effects we apply a lithospheric temperature correction using thermal estimates from a combination of geothermal modeling and global seismic tomography models. Gravity anomalies induced by temperature variations in the uppermost mantle range from 3200 to +300 mGal, with the strongest negative anomalies associated with mid-ocean ridges and the strongest positive anomalies associated with cratons. After correcting for thermal effects, we obtain a map of density variations due to lithospheric compositional variations. These maps indicate that the average density decrease due to the chemical depletion within cratonic roots varies from 1.1% to 1.5%, assuming the chemical boundary layer has the same thickness as the thermal boundary layer. The maximal values of the density drop are in the range 1.7^2.5%, and correspond to the Archean portion of each craton. Temperatures within cratonic roots vary strongly, and our analysis indicates that density variations in the roots due to temperature are larger than the variations due to chemical differences. ß

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Using three months of GPS satellite‐to‐satellite tracking and accelerometer data of the CHAMP satellite mission, a new long‐wavelength global gravity field model, called EIGEN‐1S, has been prepared in a joint German‐French effort. The solution is derived solely from analysis of satellite orbit perturbations, i.e. independent of oceanic and continental surface gravity data. EIGEN‐1S results in a geoid with an approximation error of about 20 cm in terms of 5 × 5 degree block mean values, which is an improvement of more than a factor of 2 compared to pre‐CHAMP satellite‐only gravity field models. This impressive progress is a result of CHAMP's tailored orbit characteristics and dedicated instrumentation, providing continuous tracking and direct on‐orbit measurements of non‐gravitational satellite accelerations.

The long-wavelength features of the external gravity field of the Earth contain the gravitational signal from deep-seated lateral mass and density inhomogeneities sustained by dynamic Earth mantle processes. To interpret the observed gravity field with respect to mantle dynamics and structures, it is essential first to remove the lithosphere-induced anomalous gravitational potential, which is generated by the topographic surface load and its isostatically compensating masses. Based upon the most recent global compilation of crustal thickness and density data and the age distribution of cooling oceanic lithosphere, residual topography and gravity are calculated by subtracting the 'known' crustal and oceanic lithosphere compensating masses and gravitational effects from the surface fields. Empirical admittances between residual topography and gravity are then computed to estimate the effective depths of the remaining compensating masses, which are not explained by the initial data and model assumptions. This additional compensation is eventually placed by adjusting the density in the uppermost mantle between the Moho and, on average, 70 km depth, with a maximum of 118 km under Tibet. The lithospheric mass distribution is used in a subsequent forward computation to create a global model of the lithosphere-induced gravitational potential. The resulting isostatic model is considered to be valid for spatial wavelengths longer than 500 km. The isostatic lithosphere model field, expressed in terms of both gravity and geoid heights, is subtracted from the observed free-air gravity field to yield a global set of 1°×1°isostatic gravity disturbances and from a satellite-derived long-wavelength geoid to yield the isostatic residual geoid. The comparison of residual (mantle) gravity, residual topography and isostatic corrected gravity allows us to identify the main characteristics of the underlying mantle; for example, dynamic support by mantle flow of the North Atlantic topographic high. Applying the isostatic correction, the overall pattern of the geoid becomes smoother and the most pronounced features, which are separated in the observed geoid, tend to get connected to larger structures. These results stress the importance of separation of the lithospheric gravitational impact for a correct interpretation of the external gravity field, even in its very long-wavelength constituents. Also, the isostatic corrected geoid spectrum reveals a stronger decrease in power from degree 3 to degree 4 and degree 5 to degree 6, which is in accordance with seismological models of deep-mantle structures.