Detlef Wolf

The changing role of the lithosphere in models of glacial isostasy: a historical review

During the last 125 years, the role of the lithosphere in models of glacial-isostatic adjustment experienced several changes. Following the postulation of glacial isostasy by Jamieson in 1865, the lithosphere was generally regarded as comparable in importance for the adjustment process to the fluid substratum. This changed with the initiation of quantitative modelling by Van Bemmelen and Berlage and by Haskell in 1935, whereupon effects due to the lithosphere were commonly neglected in interpretations of postglacial uplift for 30 years. After the development of a layered viscous earth model with an elastic surface layer by McConnell in 1965, the lithosphere was eventually reintroduced into models of glacial isostasy. Subsequent studies largely confirmed the original ideas regarding the importance of the lithosphere for the adjustment process, although the effects are pronounced only for short-wavelength deformations. Using this response characteristic of the lithosphere, estimates of its thickness have recently become available for several tectonic provinces.

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

We examine the dependence of glacial-isostatic adjustment (GIA) due to changes in the Vatnajokull 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.

Inverting land uplift near Vatnajökull, Iceland, in terms of lithosphere thickness and viscosity stratification

The episode of climate warming during the 20th century is documented by an ablation of the Vatnajokull ice cap, south-eastern Iceland. Due to the small thickness of the elastic lithosphere and the low viscosity of the asthenosphere below Iceland, the ablation resulted in a retarded land uplift of about 3–8 mm/a in the vicinity of the ice cap. This is confirmed by GPS campaigns carried out south of the ice cap in 1992 and 1996 (Sjoberg et al, 1999). Furthermore, a relative uplift of 12.4 cm along Lake Langisjor south-west of the ice cap was observed between 1959 and 1991 (Sigmundsson & Einarsson, 1992). In the present study, we use a compressible, self-gravitating, spherical earth model with Maxwell viscoelasticity and a load model parabolic in cross section and elliptic in plan view to interpret the observations in terms of viscosity stratification. Our results show that the lithosphere is 10 to 20 km in thickness and the asthenosphere is 7 × 1016 to 3 × 1018 Pas in viscosity.

DynaQlim – Upper Mantle Dynamics and Quaternary Climate in Cratonic Areas

The isostatic adjustment of the solid Earth to the glacial loading (GIA, Glacial Isostatic Adjustment) with its temporal signature offers a great opportunity to retrieve information of Earth’s upper mantle to the changing mass of glaciers and ice sheets, which in turn is driven by variations in Quaternary climate. DynaQlim (Upper Mantle Dynamics and Quaternary Climate in Cratonic Areas) has its focus to study the relations between upper mantle dynamics, its composition and physical properties, temperature, rheology, and Quaternary climate. Its regional focus lies on the cratonic areas of northern Canada and Scandinavia.

Contemporary Changes in the Geoid About Greenland: Predictions Relevant to Gravity Space Missions

We have examined contemporary changes in the geoid about Greenland that result from glacial-isostatic adjustment. These may be divided into contributions from ice-load changes that occurred outside of Greenland following the Last Glacial Maximum and changes in the Greenland Ice Sheet (GIS). The GISs contribution may itself be divided into past and current parts. For past ice-load changes, the resulting geoid displacement is more dependent upon the recent history of the GIS than on the earth model used. Considering an estimated accuracy for the GRACE temporal geoid signal, regional variability in the present-day mass balance of the GIS may be resolved. This variability significantly affects the geoid power spectrum, giving a signal that may be detected by measurements from gravity space missions more easily than has been proposed by other authors.

Determination of atmospheric influence on high-accuracy gravity measurements with elastic earth models

The investigation is concerned with the calculation of the total atmospheric contribution to variations of the gravity field. The relevant components are the free-air, the deformation and the air-mass contributions. The free-air and deformation contributions are calculated by means of a plane, compressible, elastic earth model. For the calculation of the air-mass contribution, a simple atmosphere model developed from the U.S. Standard Atmosphere 1976 (NOAA, 1976) and synoptic meteorological data are used. The objective is the reduction of high-accuracy gravity measurements recorded by the super-conducting gravimeter formerly located at the GeoForschungsZentrum Potsdam. A comparison between the residual after reduction with the theoretical model considered here and the residual after reduction with an alternative method is used for an assessment of the quality of the reduction methods.