Separation of the Geodetic Consequences of Past and Present Ice-Mass Change: Influence of Topography with Application to Svalbard (Norway)

Abstract

Polar regions such as Greenland, Svalbard and Antarctica are deforming today because of both the present-day ice-mass (PDIM) change of glaciers and the glacial isostatic adjustment (GIA) following the Pleistocene deglaciation. Observations handled in these areas contain both the contributions from the PDIM change and GIA. This study aims at separating them by considering two specific gravity variation-to-vertical displacement ratios. We first review the case of the viscoelastic rebound (GIA) subsequent to the Pleistocene deglaciation leading to a ratio C v . The outcome of previous studies is that C v is approximately equal to -0.15 lGal/mm and almost independent of the deglaciation history, ice geometry and viscosity profile of the mantle. Similarly we consider the elastic deformation resulting from PDIM change which leads to a second ratio C e,N . Several studies have shown that C e;N % À0:26 lGal/mm if one assumes that the changing glaciers are thin layers over the surface of a spherical Earth model. In this case, we show that the separation between the contributions from PDIM change and GIA is unique if both gravity and height changes observations are available at the same station. Next, we focus on C e,N and show that according to the deglaciation/glaciation context and from colocated gravity variation and ground vertical velocity measurements one can deduce a range of possible values for C e,N . Studying the influence of the topography on C e,N we first show that it tends to positive values if most of surrounding ice-mass changes above the altitude of the observation site and to values lower than -0.26 lGal/mm if changes are below. We next apply our general formalism to the case of the past and PDIM changes in Svalbard, Norway. We compute the ratio C e,N at the geodetic observatory at Ny-Å lesund and show the influence of the topography of the surrounding glaciers on the measured gravity and uplift rates. We show that if the ice-mass change is spatially uniform, C e, N does not depend on the speed of ice-mass change, and hence the separation of the contributions from PDIM changes and GIA can still be done univocally. However, if the ice-mass change is not spatially uniform, C e, N depends on both the speed of ice-mass change and the volume of ice-change rate.

Research Article. A new gravity laboratory in Ny-Ålesund, Svalbard

“…The gravitational effects of PDIMC for the five icemodels are listed in Table 6. Firstly, we remark that the PDIMC signal at Kongepunktet calculated for a uniform thinning rate of −1 mweq/yr (Model 0) is 0.12 µGal/yr larger than the result of a similar study by [14]. We attribute the difference to the glacier outlines, which in the present study are more complete and have higher spatial resolution.…”

“…[6]). The method has been previously used to as- sess the effect of PDIMC on geodetic observations in NyÅlesund [11,14,16,31]. Standard Green's functions presuppose loads and observation points located on a spherical Earth.…”

“…Figure 5 illustrates the situation at Brøgger-halvøya. In this region, the ice model in [14] does not include Mørebreen, Røysbreen, Trongskarbreen, and Vestre Lovenbreen. These glaciers have a total area of approximately 8 km 2 and they contribute by 0.15 µGal/yr for a uniform thinning rate of −1 mweq/yr, i.e.…”

“…This is not a criticism of [14], but illustrates the urgency of complete and accurate Figure 5: Glacier outlines at Brøggerhalvøya, i.e. the glacier outlines in the S100 map database of the NPI (blue) are overlain by reconstructed outlines used by [14] (orange). Notice that all features in the orange set are also included in the blue set.…”

Superconducting Gravimetry

“…More recently, Mémin et al (2011Mémin et al ( , 2012 cautioned that in some circumstances, the ratio can be more difficult to interpret unambiguously unless certain approximations are made. Figure 35 shows the seasonal and long-term component of surface gravity as seen by AG episodic measurements and a permanent SG at the Ny-Å lesund station in Svalbard.…”