Harriet C. P. Lau

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

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, J2. Resolving power analyses demonstrate the following: (1) the FRS constrains mean upper mantle viscosity to be ∼3 × 1020 Pa s, (2) postglacial decay time data require the average viscosity in the top ∼1500 km of the mantle to be 1021 Pa s, and (3) the J2 datum constrains mean lower mantle viscosity to be ∼5 × 1021 Pa s. To reconcile (2) and (3), viscosity must increase to 1022–1023 Pa s in the deep mantle. Our analysis highlights the importance of accurately correcting the J2 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.

Tidal tomography constrains Earth’s deep-mantle buoyancy

Earth’s body tide—also known as the solid Earth tide, the displacement of the solid Earth’s surface caused by gravitational forces from the Moon and the Sun—is sensitive to the density of the two Large Low Shear Velocity Provinces (LLSVPs) beneath Africa and the Pacific. These massive regions extend approximately 1,000 kilometres upward from the base of the mantle and their buoyancy remains actively debated within the geophysical community. Here we use tidal tomography to constrain Earth’s deep-mantle buoyancy derived from Global Positioning System (GPS)-based measurements of semi-diurnal body tide deformation. Using a probabilistic approach, we show that across the bottom two-thirds of the two LLSVPs the mean density is about 0.5 per cent higher than the average mantle density across this depth range (that is, its mean buoyancy is minus 0.5 per cent), although this anomaly may be concentrated towards the very base of the mantle. We conclude that the buoyancy of these structures is dominated by the enrichment of high-density chemical components, probably related to subducted oceanic plates or primordial material associated with Earth’s formation. Because the dynamics of the mantle is driven by density variations, our result has important dynamical implications for the stability of the LLSVPs and the long-term evolution of the Earth system.

Global Tidal Impacts of Large‐Scale Ice Sheet Collapses

Recent studies show that the glaciers draining both the West Antarctic and the Greenland ice sheets are experiencing an accelerated ice loss, highlighting the possibility of large-scale ice-sheet retreat and sea-level rise in the coming centuries and millennia. These sea-level changes would vary spatially, and could significantly alter global tides as the latter are highly dependent on bathymetry (or water column thickness under ice shelves) and basin shape. This paper investigates how the principal semi-diurnal (M2) tidal amplitudes and energy dissipation respond to the non-uniform sea-level changes induced by complete ice-sheet collapses. The sea-level changes are calculated using gravitationally self-consistent sea-level theory, and the tides are simulated using an established tidal model. Results from the simulations show global and spatially heterogeneous changes in tidal amplitudes. In addition, pronounced changes in tidal energy dissipation occur in both the open ocean and in shelf seas, also altering the location of tidal mixing fronts. These changes have the potential to impact ocean mixing, and hence large-scale currents and climate patterns, and the contribution of shelf-sea to the global carbon cycle. The new results highlight the importance of considering changes in the tides in predictions of future climate and reconstructions of past climate phases such as the Last Interglacial.

Sea Level Fingerprints in a Region of Complex Earth Structure: The Case of WAIS

AbstractSea level fingerprints associated with rapid melting of the West Antarctic Ice Sheet (WAIS) have generally been computed under the assumption of a purely elastic response of the solid Earth. The authors investigate the impact of viscous effects on these fingerprints by computing gravitationally self-consistent sea level changes that adopt a 3D viscoelastic Earth model in the Antarctic region consistent with available geological and geophysical constraints. In West Antarctica, the model is characterized by a thin (~65 km) elastic lithosphere and sublithospheric viscosities that span three orders of magnitude, reaching values as low as approximately 4 × 1018 Pa s beneath WAIS. Calculations indicate that sea level predictions in the near field of WAIS will depart significantly from elastic fingerprints in as little as a few decades. For example, when viscous effects are included, the peak sea level fall predicted in the vicinity of WAIS during a melt event will increase by about 20% and about 50%, re...