Glacial isostatic adjustment in the static gravity field of Fennoscandia

Abstract

In the central part of Fennoscandia, the crust is currently rising, because of the delayed response of the viscous mantle to melting of the Late Pleistocene ice sheet. This process, called Glacial Isostatic Adjustment (GIA), causes a negative anomaly in the present-day static gravity field as isostatic equilibrium has not been reached yet. Several studies have tried to use this anomaly as a constraint on models of GIA, but the uncertainty in crustal and upper mantle structures has not been fully taken into account. Therefore, our aim is to revisit this using improved crustal models and compensation techniques. We find that in contrast with other studies, the effect of crustal anomalies on the gravity field cannot be effectively removed, because of uncertainties in the crustal and upper mantle density models. Our second aim is to estimate the effects on geophysical models, which assume isostatic equilibrium, after correcting the observed gravity field with numerical models for GIA. We show that correcting for GIA in geophysical modelling can give changes of several kilometer in the thickness of structural layers of modeled lithosphere, which is a small but significant correction. Correcting the gravity field for GIA prior to assuming isostatic equilibrium and inferring density anomalies might be relevant in other areas with ongoing postglacial rebound such as North America and the polar regions.

Comparing gravity-based to seismic-derived lithosphere densities: a case study of the British Isles and surrounding areas

“…This corroborates our assumption that compensated masses are situated in the region between the LAB and the Moho. The compensation is shown to represent the global gravity observations well in most of the spectral domain (Root et al 2015). The separation between the lithosphere and asthenosphere is obtained from Hamza & Vieira (2012), which is based on global databases of heat flow (Vieira & Hamza 2010) and crustal structure (Laske et al 2013).…”

“…Kaban et al 2004;Root et al 2015;Herceg et al 2016). However, due to the intrinsic non-uniqueness of gravity modelling the solutions need to be constrained, which introduces uncertainties in the final model.…”

“…Second, the gravity signal of the crust needs to be removed from the observations (Kaban et al 2004) to reveal the lithosphere. Errors in the used crustal model lead to uncertainty in the lithosphere solutions (Panasyuk & Hager 2000;Kaban et al 2004;Root et al 2015). For example, the global crustal model CRUST1.0 (Laske et al 2013) suffers from lack of data in large parts of the world.…”

The effect of sediment loading in Fennoscandia and the Barents Sea during the last glacial cycle on glacial isostatic adjustment observations

“…As a reference profile we use VM5a (Peltier and Drummond, 2008), which is an iteration of the VM2 profile that is used in the creation of ICE-5G (Peltier, 2004). As alternative profiles we select profiles that have been shown by Root et al (2015b) to provide a good fit to sea level data, GPS, and GRACE (Gravity Recovery and Climate Experiment) data in Fennoscandia. That study found two viscosity profiles: one with higher viscosities in the upper and lower mantle and one with lower viscosities.…”

“…That study found two viscosity profiles: one with higher viscosities in the upper and lower mantle and one with lower viscosities. The fact that sediment loading is not taken into account to obtain viscosity profiles in Root et al (2015b) will have a minor effect on our results given that three very different viscosity profiles are selected to account for uncertainty in viscosity. Out of those sets we select M8-128-150 and M4-16-80, where the first number denotes the upper mantle viscosity in 10 20 Pa s, the second number denotes the lower mantle viscosity in 10 20 Pa s, and the third number denotes the lithosphere thickness in kilometers.…”