New constraints on Laurentide postglacial rebound from absolute gravity measurements

Repeated absolute gravity measurements have been made over a period of several years at six sites along a 3000 km‐long, mid‐continental, North American profile from the coast of Hudson Bay southward to Iowa. With the exception of the southern‐most site, the observed rates of change of gravity are significantly higher than rates predicted by current models, such as ICE‐3G and a laterally homogeneous, standard Earth. The observed gravity change rates suggest significant modifications, such as a 2 to 3‐fold increase in lower mantle viscosity or a 50% increase in Laurentide ice sheet thickness west of Lake Superior.