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We distinguish between two models of solid Earth's viscous response to unloading of the Laurentide ice sheet over the past 26,000 years.  The upper mantle viscosity in both models is 0.5 x 10²¹ Pa s.  The viscosity of the top 700 km of the lower mantle (670 –1370 km) in model L17 is 13 x 10²¹ Pa s, eight times larger than the value of 1.6 x 10²¹ Pa s in ICE-6G_D (VM5a).  In ICE-6G_D (VM5a), viscous relaxation of solid Earth was rapid 8,000 years ago and is slow today, with present-day uplift at the Laurentide ice center being 12 mm/yr.  In L17, solid Earth relaxed more slowly 8,000 years ago but is faster today, with present uplift of the ice center at 20 mm/yr.  The significant difference is not due to different ice histories given that total ice loss in L17 is just 12% less than in ICE-6G_D.  We determine a comprehensive set of GPS uplift rates for North America that is more accurate than in prior studies due to (1) more sites and a longer data time history, (2) removal of elastic loading produced by increase in Great Lakes water, and (3) technical advances in GPS positioning that have significantly reduced the dispersion in position estimates.  We find uplift at the ice center to be about 12 mm/yr, supporting low value of the viscosity of the top 700 km of the lower mantle in ICE-6G_D (VM5a), but ruling out the high value in L17.

How to cite: Argus, D., Peltier, W. R., Blewitt, G., Kreemer, C., and Vacchi, M.: The Viscosity of the Top 700 km of the Lower Mantle estimated using GPS, GRACE, and Relative Sea Level measurements of Glacial Isostatic Adjustment, GRACE/GRACE-FO Science Team Meeting 2020, online, 27–29 Oct 2020, GSTM2020-33, https://doi.org/10.5194/gstm2020-33, 2020.

Glacial Isostatic Adjustment (GIA) refers to the gradual response of the solid Earth to the deglaciation of historic ice sheets.  This ongoing rebound is contributing to the measurements of gravity change and land deformation, respectively, by Gravity Recovery And Climate Experiment (GRACE) and Global Positioning System (GPS).  When these space geodetic data are used to quantify the present-day ice mass change, the effect such as GIA must be accounted for.  In this study, we developed a method to estimate GIA and elastic deformation by the present-day ice mass change in the GPS time series with the example of Casey station in East Antarctica.  We determined a high-resolution, present-day ice mass change model on the outlet of Totten Glacier and calculated the elastic rebound over the area.  Our high-resolution model indicated a total mass loss of 15.7 ± 0.5 Gt/yr on the outlet of Totten Glacier from 2002 to 2017 with the accelerated loss in the last half of the period.  We estimated the viscoelastic deformation attributed to GIA by removing the predicted elastic deformation from GPS measurements.  Four different GPS position solutions for the Casey station, the continuously operating GPS station near the area, were examined.  The estimated GIA signal appears to be within 0.3 – 1.3 mm/yr which shows its contribution on the vertical deformation between 30 – 60 % among different GPS solutions.  On the other hand, the vertical elastic deformation trend is predicted to be 0.7 mm/yr from the ice mass change model.  The GPS measured seasonal variation is explained equally by atmospheric-oceanic loading and degree-1 loading with a couple mm amplitude in vertical time series.  The elastic rebound from the present-day ice mass change also perturbed the horizontal displacement by 0.13 mm/yr in west and 0.21 mm/yr in north directions.  This is in the opposite to the plate motion of the East Antarctica around the Casey station and amounts approximately up to 10 % of the measured tectonic motion.

How to cite: Razeghi, M., Han, S.-C., King, M., and Tregoning, P.: Estimation of Glacial Isostatic Adjustment uplift rate in the Totten glacier's outlet from GPS and GRACE, GRACE/GRACE-FO Science Team Meeting 2020, online, 27–29 Oct 2020, GSTM2020-40, https://doi.org/10.5194/gstm2020-40, 2020.

Convective motions in the Earth’s liquid core are known to  generate temporal variations of the magnetic field and of the length of day. Mass redistribution associated with these motions and exchange of matter with the lower mantle at the core mantle boundary (CMB) may eventually also contribute to the temporal variations of the gravity field, possibly detectable in the data of the GRACE and GRACE Follow On missions. In a pioneering work, Mandea et al., 2012 detected compelling spatio-temporal correlations at interannual time scale between the gravity and magnetic fields measured respectively by the GRACE and CHAMP satellite missions. These correlations were later interpreted by these authors as the results of physico-chemical interactions between the core and the mantle at the CMB. While such mechanisms are plausible, their mere existence, order of magnitude and  time scales remain an open question. Here we present the  GRACEFUL project, recently selected by the  "Synergy" programme of the European Research Council, which objective is to  explore in more detail the previously reported observations described above, in particular the interannual co-variations of the magnetic and gravity fields, as well as their link with deep Earth processes.  This presentation is focussed on the  gravity field component, in particular on the search for the deep Earth signal that we hope to be able to detect in the  GRACE/GRACE FO data,  after removing all other contributions due to water mass redistributions  occuring in the surface fluid evelopes, as well as  unrelated solid Earth signals associated with the Glacial Isostatic Adjustment and large earthquakes.

How to cite: Pfeffer, J., Cazenave, A., Mandea, M., Dehant, V., and Barnoud, A.: The GRACEFUL project : probing the Earth’s deep interior with satellite observations of the gravity field, magnetic field and earth’s rotation, GRACE/GRACE-FO Science Team Meeting 2020, online, 27–29 Oct 2020, GSTM2020-23, https://doi.org/10.5194/gstm2020-23, 2020.

Within the past decade, newly collected GPS data and geochronological constraints have resulted in refinement of glacial isostatic adjustment (GIA) models for Antarctica. These are critical to understanding ice mass changes at present-day. A correction needs to be made when using space gravity for ice mass balance assessments as any vertical movements of the solid Earth masquerade as changes in ice mass, and must be carefully removed. The main upshot of the new Antarctic GIA models is a downward revision of negative ice mass trends deduced from the Gravity Recovery and Climate Experiment (GRACE), resulting from a reduced GIA correction. This revision places GRACE inferred trend in mass balance within the 1-σ uncertainty of mass balance deduced by altimetry. Because uncertainties in Holocene ice history and the low viscosity rheology beneath the West Antarctic Ice Sheet (WAIS) continue to vex further improvement in predictions of present-day GIA gravity rate, more emphasis has been given to regional-scale GIA models. Here we use a Bayesian method to explore the gravimetric GIA trend over Antarctica, both with and without the impact of a late Pleistocene Antarctic ice loads, along with the contribution of oceanic loads. We call this model without loads associated with Antarctica a baseline for regional GIA models to build upon. We consider variations of the radial mantle viscosity profile and the volume of continental-scale ice sheets during the last glacial cycle. The modeled baseline GIA is mainly controlled by the lower mantle viscosity and continental levering caused by ocean loading. We find that the predicted baseline GIA correction weakly depends on the ice history. This correction averages to +28.4 [16.5–41.9, 95% confidence] Gt/yr. In contrast, with Pleistocene Antarctic-proximal ice included, the total modeled mass trend due to GIA is +73.7 [30.1–114.7] Gt/yr. A baseline GIA correction of 28.4 Gt/yr is of order 50% of the mean net mass trend measured during the period 1992-2017. The statistical analysis provides tools for synthesizing any regional Antarctic GIA model with a self-consistent far-field component. This may prove important for accounting for both global and regional 3-D variations in mantle viscosity.

© 2020 California Institute of Technology.
Government sponsorship acknowledged. This work was performed at the California Institute of Technology's Jet Propulsion Laboratory under a contract with the National Aeronautics and Space Administration's Cryosphere Science Program.

How to cite: Caron, L. and Ivins, E.: A baseline Antarctic GIA correction for space gravimetry, GRACE/GRACE-FO Science Team Meeting 2020, online, 27–29 Oct 2020, GSTM2020-17, https://doi.org/10.5194/gstm2020-17, 2020.