Abstract
The geodetic rates for the gravity variation and vertical uplift in polar regions subject to past and present-day ice-mass changes (PDIMCs) provide important insight into the rheological structure of the Earth. We provide an update of the rates observed at Ny-Alesund, Svalbard. To do so, we extract and remove the significant seasonal content from the observations. The rate of gravity variations, derived from absolute and relative gravity measurements, is −1.39 ± 0.11 μGal yr −1 . The rate of vertical displacements is estimated using GPS and tide gauge measurements. We obtain 7.94 ± 0.21 and 8.29 ± 1.60 mm yr −1 , respectively. We compare the extracted signal with that predicted by GLDAS/Noah and ERA-interim hydrology models. We find that the seasonal gravity variations are well-represented by local hydrology changes contained in the ERA-interim model. The phase of seasonal vertical displacements are due to non-local continental hydrology and non-tidal ocean loading. However, a large part of the amplitude of the seasonal vertical displacements remains unexplained. The geodetic rates are used to investigate the asthenosphere viscosity and lithosphere/asthenosphere thicknesses. We first correct the updated geodetic rates for those induced by PDIMCs in Svalbard, using published results, and the sea level change due to the melting of the major ice reservoirs. We show that the latter are at the level of the geodetic rate uncertainties and are responsible for rates of gravity variations and vertical displacements of −0.29 ± 0.03 μGal yr −1 and 1.11 ± 0.10 mm yr −1 , respectively. To account for the late Pleistocene deglaciation, we use the global ice evolution model ICE-3G. The Little Ice Age (LIA) deglaciation in Svalbard is modelled using a disc load model with a simple linear temporal evolution. The geodetic rates at Ny-Alesund induced by the past deglaciations depend on the viscosity structure of the Earth. We find that viscous relaxation time due to the LIA deglaciation in Svalbard is more than 60 times shorter than that due to the Pleistocene deglaciation. We also find that the response to past and PDIMCs of an Earth model with asthenosphere viscosities ranging between 1.0 and 5.5 × 10 18 Pa s and lithosphere (resp. asthenosphere) thicknesses ranging between 50 and 100 km (resp. 120 and 170 km) can explain the rates derived from geodetic observations.
Highlights
The Svalbard archipelago is located in the Arctic between 11◦E and 26◦E of longitude and 76◦N and 81◦N of latitude, close to a continental margin (Fig. 1)
It confirms that the phases of the seasonal variations obtained by combining the GLDAS/Noah model and Toulouse Unstructured Grid Ocean model (TUGO-m) better agree with those of the seasonal vertical movements estimated from the two global positioning system (GPS) stations
To account for the deformation induced by the present-day ice-mass changes (PDIMCs) at Ny-Alesund, we use the rates of vertical displacements, updimc, and gravity variations, gpdimc, computed by Memin et al (2012) for an elastic Earth model
Summary
The Svalbard archipelago is located in the Arctic between 11◦E and 26◦E of longitude and 76◦N and 81◦N of latitude, close to a continental margin (Fig. 1). The existence of an asthenosphere with a low viscosity was confirmed later by Hagedoorn & Wolf (2003) who compared vertical displacement rates estimated using geodetic techniques to that predicted using models of past and present-day ice-mass changes (PDIMCs). This paper investigates the geodetic consequences of the past and PDIMCs on geodetic rates observed at Ny-Alesund It focuses on the geodetic contribution of the LIA deglaciation and its implication for determining the asthenosphere viscosity and lithosphere thickness. We compute the geodetic effects due to the Pleistocene and LIA deglaciations (Section 4.2) and compare them to the residuals (observed rates minus PDIMC and SLC contributions) to investigate the asthenosphere viscosity and lithosphere thickness beneath Ny-Alesund (Section 4.3). To estimate decadal rates of gravity variations and surface velocities we must remove the hydrology signal from SG and GPS time-series
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