What Ice Age Data Can – and Cannot – Tell Us About the Deep Mantle

Abstract:

Data related to the ongoing glacial isostatic adjustment (GIA) of the Earth in response to the last ice age have yielded important constraints on the viscosity of the Earth’s mantle, but most of these data – including relative sea level changes, crustal deformation rates and site-specific gravity anomalies – are not sensitive to viscosity structure in the deep (>1800 km depth) mantle. However, beginning with the pioneering studies of Richard O’Connell in the early 1970s, a subset of GIA data, namely observations of changes in the Earth’s rotation rate and orientation, and long-wavelength gravity field, have played a key role in constraining deep mantle viscosity (most recently by Lau et al., JGR, 2016). Calculations of 1-D (depth varying) Fréchet kernels suggest a radial resolving power of order 1000 km. Here, we report on the first adjoint based calculations of the 3-D sensitivity to mantle viscosity of GIA-induced perturbations in the Earth’s rotation vector and long wavelength gravity field. These kernels reveal significant lateral variations in the sensitivity of the data sets and provide a framework for rigorously determining the non-uniqueness of viscosity inversions based on these data. That is, they reveal what these data can – and cannot – tell us about the Earth’s deep mantle viscosity.