Temporal Gravity Variations near Shrinking Vatnajökull Ice Cap, Iceland

Abstract

Repeated gravity measurements were carried out from 1991 until 1999 at sites SE of Vatnajökull, Iceland, to estimate the mass flow and deformation accompanying the shrinking of the ice cap. Published GPS data show an uplift of about 13 ± 5 mm/a near the ice margin. A gravity decrease of -2 ± 1 lGal/a relative to the Höfn base station, was observed for the same sites. Control measurements at the Höfn station showed a gravity decrease of -2 ± 0.5 lGal/a relative to the station RVIK 5473 at Reykjavík (about 250 km from Höfn). This is compatible, as a Bouguer effect, with a 10 ± 3 mm/a uplift rate of the IGS point at Höfn and an uplift rate of *20 mm/a near the ice margin. Although the derived gravity change rates at individual sites have large uncertainties, the ensemble of the rates varies systematically and significantly with distance from the ice. The relationship between gravity and elevation changes and the shrinking ice mass is modelled as response to the loading history. The GPS data can be explained by 1-D modelling (i.e., an earth model with a 15-km thick elastic lithosphere and a 7Á10 17 PaÁs asthenosphere viscosity), but not the gravity data. Based on 2-D modelling, the gravity data favour a low-viscosity plume in the form of a cylinder of 80 km radius and 10 17 to 10 18 PaÁs viscosity below a 6 km-thick elastic lid, embedded in a layered PREM-type earth, although the elevation data are less well explained by this model. Strain-porosity-hydrology effects are likely to enhance the magnitude of the gravity changes, but need verification by drilling. More accurate data may resolve the discrepancies or suggest improved models.

Continuous subsidence in the Thingvellir rift graben, Iceland: Geodetic observations since 1967 compared to rheological models of plate spreading

“…From GIA studies on Vatnajökull, researchers also found viscosity for upper mantle beneath Iceland lower than ~5 × 10 18 Pa s [ Sjöberg et al ., ; Fleming et al ., ; Jacoby et al ., ] that essentially indicates wet rheology [ Barnhoorn et al ., ]. However, the observed style of deformation in the Thingvellir graben fits well with the model prediction where strain rate is proportional to 3.5 order of the stress in a dry olivine rheology (Figure ) by satisfying other geophysical properties (see next section).…”

Iceland rising: Solid Earth response to ice retreat inferred from satellite radar interferometry and visocelastic modeling

“…Jacoby et al . [] demonstrated the influence of lateral variations in the Earth's structure when comparing models to gravity data. However, they also showed that the results from these models do not provide a good fit to GPS data.…”

Long-Term and Inter-annual Mass Changes in the Iceland Ice Cap Determined From GRACE Gravity Using Slepian Functions

“…The GRACE data should be corrected for viscous deformation of the solid Earth due to glacial isostatic adjustment (GIA). Because of Iceland's unique tectonic setting, global models like Paulson et al (2007) (as adjusted by Geruo et al, 2013) are likely inappropriate (Fleming et al, 2007;Jacoby et al, 2009). The Vatnajoköll ice cap is highly sensitive to GIA (Ivins, 2014), and GIA modeling for Iceland is non-trivial.…”