Glacial-isostatic Adjustment and the Viscosity Structure Underlying the Vatnajökull Ice Cap, Iceland

Abstract

We examine the dependence of glacial-isostatic adjustment (GIA) due to changes in the Vatnajo¨kull Ice Cap, Iceland, on the underlying viscosity structure. Iceland offers a unique case study for GIA research, with a thinner elastic lithosphere underlain by a low-viscosity zone or asthenosphere, as opposed to regions such as Fennoscandia or North America described by a thicker lithosphere, while not necessarily featuring an asthenosphere.A laterally homogeneous spherical earth model is used consisting of an elastic lithosphere, a viscoelastic asthenosphere, a viscoelastic upper and lower mantle and a fluid core. We examine the response of the earth model to three ice models with circular plans and cross-section profiles based on the assumption of perfectly plastic material, but with different load histories. These are: (1) A history where the ice cap grows from a AD 900 minimum to a maximum at 1890, followed by a uniform decrease until 1991, continuing to the present day at an average rate based on recent mass-balance measurements, (2) a history that is the same as the first, except for constant ice volumes prior to 1890, and (3) a history that is again the same as the first model, except that the post-1991 changes correspond to the measured mass-balance values. We first compare the response to each ice model using typical earth-model parameters for Iceland presented in the literature. We then undertake a parameter-space search, where we assess the importance of lithosphere thickness, asthenosphere viscosity and basal asthenosphere depth, to predicted vertical-displacement rates, and compare them to rates determined from GPS measurements obtained from campaigns conducted between 1991 and 1999.The earth-viscosity structure that provides the optimum predictions with respect to the GPS-derived vertical-displacement rates consists of an elastic lithosphere with a thickness of between 20 and 30 km, an asthenosphere viscosity between 1 and 2 · 10 18 Pa s, and a basal asthenosphere depth between 250 km and possibly greater than 400 km. We find that the very low asthenosphere viscosity values of ca. 10 17 Pa s sometimes suggested in the literature are not necessary to account for the rapid vertical-displacement rates observed, which are the result of the contemporary decrease in the mass of the ice cap not considered previously.

Water in Hawaiian peridotite minerals: A case for a dry metasomatized oceanic mantle lithosphere

“…Effective viscosities calculated for individual Hawaiian xenoliths at their temperature of equilibration are > 10 −21 Pa.s (Figure ). These viscosities are at least 100 time higher than those estimated for the asthenosphere (2.7 × 10 −17 −2 × 10 −19 Pa.s, average ∼6 × 10 −18 Pa.s [ Craig and McKenzie , ; Hager , ; Pollitz et al ., ; Larsen et al ., ; Fleming et al ., ]. This result confirms that the oceanic mantle lithosphere is stronger than the asthenosphere, and that this is independent of its degree of metasomatism.…”

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).…”

The effect of signal leakage and glacial isostatic rebound on GRACE-derived ice mass changes in Iceland.

“…Since pre-LIA unloading is of little importance for modelling uplift rates in Iceland (Fleming et al 2007), we drive the Earth model by three different estimated ice unloading histories for Iceland, spanning from the LIA to 2012: (1) an Iceland-wide average annual mass balance is computed by tuning a simple degree-day mass balance model (Marzeion et al 2012) to existing records ) and forcing the model with weather station data from Stykkishólmur (1890-2010); (2) a local study for South-Iceland (A lgeirsdóttir et al 2011) is upscaled to reflect total, countrywide mass changes; (3) the known mass changes since the LIA of Langjökull ice cap ) are upscaled to reflect total, countrywide mass changes. All three unloading histories are calibrated to reproduce an estimate of the total ice mass loss of 500 Gt in Iceland between 1890 and 2012 (Björnsson & Pálsson 2008;Björnsson et al 2013).…”