Effect of Lateral and Stress‐Dependent Viscosity Variations on GIA Induced Uplift Rates in the Amundsen Sea Embayment

Abstract

Accurate GIA models are required for correcting measurements of mass change in Antarctica and for improving knowledge of the sub-surface, especially in areas of large current ice loss such as the Amundsen Sea Embayment(ASE). Regionally, seismic and gravity data suggests lateral differences in viscosity (3D). Furthermore, mantle flow laws allow for a stress-dependent effective viscosity which changes over time (3D-s). In this study we investigate whether models with 3D/3D-s have significant effects on the uplift in the region. We use a finite element model with composite rheology consisting of diffusion and dislocation creep, forced by an ice deglaciation model starting in 1900. We use its uplift predictions as synthetic observations to test the performance of 1D model inversion in the presence of viscosity variations. Stress-dependent rheology results in lower viscosity beneath the load and a more localized uplift pattern. We demonstrate that the background stress from earlier ice load changes can both increase or decrease the influence of stress-induced effective viscosity changes. For the ASE, fitting 1D models to 3D model uplift results in a best fitting model with viscosity that represents the average of a large area, while for 3D-s rheology, local viscosity is more influential. 1D models are statistically indistinguishable from 3D/3D-s viscosity with current GPS stations. However, 3D and 3D-

Solid Earth Uplift Due To Contemporary Ice Melt Above Low‐Viscosity Regions of the Upper Mantle

“…Such rheological complexity may also contribute to, or even generate, confined regions with effectively low viscosity, of the sort that we have modeled here. Regardless, the impact of rheological complexity needs more investigation and could help to reconcile models and observations of uplift (Adhikari et al., 2021; Blank et al., 2021; Lau et al., 2021).…”

“…Such rheological complexity may also contribute to, or even generate, confined regions with effectively low viscosity, of the sort that we have modeled here. Regardless, the impact of rheological complexity needs more investigation and could help to reconcile models and observations of uplift (Adhikari et al, 2021;Blank et al, 2021;Lau et al, 2021). Bevis et al (2012) found that in most of coastal Greenland the elastic response to contemporary ice mass change matches the historic viscous response or dominates the uplift signal.…”

Decadal to Centennial Timescale Mantle Viscosity Inferred From Modern Crustal Uplift Rates in Greenland

“…The viscosity for sub‐centennial timescale mantle deformation resolved by our constraining data is roughly one order of magnitude smaller than the upper mantle viscosity typically inferred in GIA analysis of relative sea‐level data. Future glacial loading studies should, therefore, consider a more comprehensive than the Maxwell mantle rheological model that captures mantle relaxation across a range of timescales (Blank et al., 2021; Caron et al., 2017; Ivins et al., 2020; Lau et al., 2020; Lau & Holtzman, 2019). Laboratory experiments of rock creep at high temperature and pressure environments (e.g., Faul & Jackson, 2015; Kohlstedt & Hansen, 2015) and recent studies of post‐seismic mantle flow (e.g., Liu et al., 2020; Muto et al., 2019; Pollitz, 2019) suggest the necessity of considering such higher‐order constitutive relations.…”

“…We therefore postulate the importance of alternative mantle relaxation processes that are either governed by inherently transient rheology (Ivins et al., 2020; Lau et al., 2020) or by non‐linear stress‐dependent rheology (Blank et al., 2021). Such enhancement of the relaxation process acts to lower the effective viscosity on timescales of decades to centuries and thus produce more rapid uplift rates for post‐MWP mass changes.…”

“…A more granular deglaciation history than is presented here might be possible as the constraining data are improved (e.g., Briner et al., 2020). Furthermore, we have hardly exhausted the full list of additional complexities to invoke in the underlying constitutive approximations governing the deforming solid Earth (e.g., Blank et al., 2021; Ivins et al., 2020; Lau & Holtzman, 2019). One result, nevertheless, is inescapable: incorporation of the post‐MWP load model coupled to an upper mantle of reduced creep strength compared to values found in previous GIA studies substantially improves the overall GNSS data fit (compare Figure 3c vs.…”