Inferences of mantle viscosity based on ice age data sets: Radial structure

• Main Authors: Lau, HCP, Mitrovica, JX, Austermann, J, Crawford, O, Al-Attar, D, Latychev, K
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2016
• Subjects: 37 Earth Sciences; 3709 Physical Geography and Environmental Geoscience; 3705 Geology; 3706 Geophysics; 13 Climate Action; Canada; Fennoscandian; glacier*
• Online Access: https://www.repository.cam.ac.uk/handle/1810/261284; https://doi.org/10.17863/CAM.6458

We perform joint nonlinear inversions of glacial isostatic adjustment (GIA) data, including the following: postglacial decay times in Canada and Scandinavia, the Fennoscandian relaxation spectrum (FRS), late-Holocene differential sea level (DSL) highstands (based on recent compilations of Australian sea level histories), and the rate of change of the degree 2 zonal harmonic of the geopotential, $J_2$. Resolving power analyses demonstrate the following: (1) the FRS constrains mean upper mantle viscosity to be ∼3 × 10$^{20}$ Pa s, (2) postglacial decay time data require the average viscosity in the top ∼1500 km of the mantle to be 10$^{21}$ Pa s, and (3) the $J_2$ datum constrains mean lower mantle viscosity to be ∼5 × 10$^{21}$ Pa s. To reconcile (2) and (3), viscosity must increase to 10$^{22}$-10$^{23}$ Pa s in the deep mantle. Our analysis highlights the importance of accurately correcting the $J_2$ observation for modern glacier melting in order to robustly infer deep mantle viscosity. We also perform a large series of forward calculations to investigate the compatibility of the GIA data sets with a viscosity jump within the lower mantle, as suggested by geodynamic and seismic studies, and conclude that the GIA data may accommodate a sharp jump of 1-2 orders of magnitude in viscosity across a boundary placed in a depth range of 1000-1700 km but does not require such a feature. Finally, we find that no 1-D viscosity profile appears capable of simultaneously reconciling the DSL highstand data and suggest that this discord is likely due to laterally heterogeneous mantle viscosity, an issue we explore in a companion study. National Science Foundation Division of Ocean Sciences (Grant ID: OCE-0825293 “PLIOMAX”), Cooperative Studies of The Earth’s Deep Interior (Grant ID: EAR-1464024), Harvard University