Ice age effects on the satellite-derived J˙2 datum: Mapping the sensitivity to 3D variations in mantle viscosity

• Satellite-derived gravity harmonic J2-dot is important in modern climate studies. • The harmonic must be corrected for ongoing adjustment due to the ice age (GIA). • Sensitivity of GIA signal to mantle viscosity is limited to regions below Laurentia. • Sensitivity of GIA signal to mantle viscosity largely misses the location of LLSVPs. • Previous inferences of lower mantle viscosity based on J2-dot may be biased high. Studies of glacial isostatic adjustment (GIA) based on spherically symmetric viscoelastic Earth models have argued that the rate of change of the degree 2 zonal harmonic of the Earth's geopotential, or J ˙ 2 , provides an important constraint on mean viscosity in the deep mantle ( Mitrovica and Peltier, 1993 ; Nakada et al., 2015 ; Lau et al., 2016 ). To refine this constraint, we compute Fréchet kernels using an adjoint methodology that reveal the sensitivity of the datum to 3D variations in mantle viscosity. We demonstrate that the mantle sensitivity of the datum is largely limited to the region below the ancient Laurentide ice sheet that covered Canada and significant portions of the northeastern United States at Last Glacial Maximum (LGM). In the bottom half of the lower mantle, this region of maximum sensitivity lies outside the location of Large Low Shear Velocity Provinces (LLSVPs) imaged from seismic tomographic studies. Thus, if the low shear velocity of these provinces originates from thermal effects, previous inferences of viscosity based upon the J ˙ 2 datum are likely higher than the actual mean viscosity of the lower mantle.