GRACE gravity data constrain ancient ice geometries and continental dynamics over Laurentia

The free-air gravity trend over Canada, derived from the Gravity Recovery and Climate Experiment ( GRACE) satellite mission, robustly isolates the gravity signal associated with glacial isostatic adjustment (GIA) from the longer-time scale mantle convection process. This trend proves that the ancient Laurentian ice complex was composed of two large domes to the west and east of Hudson Bay, in accord with one of two classes of earlier reconstructions. Moreover, GIA models that reconcile the peak rates contribute similar to 25 to similar to 45% to the observed static gravity field, which represents an important boundary condition on the buoyancy of the continental tectosphere.