A geophysical interpretation of the secular displacement and gravity rates observed at Ny-Ålesund, Svalbard in the Arctic-effects of post-glacial rebound and present-day ice melting

Abstract

S U M M A R YWe have analysed the Ny-Ålesund very long baseline interferometry (VLBI) data over the period 1994 August to 2004 May, and we obtain secular displacement rates relative to a NNR-NUVEL-1A reference frame of 0.2 ± 0.5 mm yr −1 , −1.7 ± 0.5 mm yr −1 and 4.8 ± 1.1 mm yr −1 for the north, east and vertical directions, respectively. The corresponding global positioning system (GPS) station displacement rates relative to the same reference frame for the north, east, and vertical directions are 0.2 ± 0.6 mm yr −1 , −2.3 ± 0.6 mm yr −1 , and 6.4 ± 1.5 mm yr −1 at NYA1 and -−0.1 ± 0.5 mm yr −1 , −1.6 ± 0.5 mm yr −1 , and 6.9 ± 0.9 mm yr −1 at NALL, where these GPS rates were derived from the ITRF2000 velocity solution of Heflin. From the comparison at 25 globally distributed collocated sites, we found that the difference in uplift rate between VLBI and GPS at Ny-Ålesund is mainly due to a GPS reference frame scale rate error corresponding to 1.6 mm yr −1 in the GPS vertical rates. The uplift rate was estimated to be 5.2 ± 0.3 mm yr −1 from the analysis of the tide gauge data at Ny-Ålesund. Hence the uplift rates obtained from three different kinds of data are very consistent each other. The absolute gravity (AG) measurements at Ny-Ålesund, which were carried out four times (period: 1998-2002) by three different FG5 absolute gravimeters, lead to a decreasing secular rate of −2.5 ± 0.9 μGal yr −1 (1 μGal = 10 −8 m s −2 ). In this analysis, the actual data obtained from a superconducting gravimeter at Ny-Ålesund were used in the corrections for the gravity tide (including the ocean tide effect) and for the air pressure effect. We have estimated three geophysical contributions to examine the observed rates: (1) the effect of the sea-level (SL) change on a timescale of a few decades, (2) the effect of the present-day ice melting (PDIM) in Svalbard and (3) the sensitivity of the computed post-glacial rebound (PGR) effects to different choices of the models of past ice history and Earth's viscosity parameters. Our analysis indicates that the effect of SL change can be neglected as the main source of the discrepancy. On the other hand, the effect of PDIM cannot be ignored in explaining the mutual relation between the observed horizontal and vertical rates and the predicted ones. A large melting rate of the order of −75 cm yr 11 (i.e. roughly 1.6 times larger than the mean rate derived from glaciology over Svalbard) would explain the observed uplift but only half of the gravity changes. Our comparison results clearly point out the importance of both the estimation accuracy of the elastic deformations and better observation accuracy to constrain the size of PGR effects in the northwestern Svalbard more tightly.

Acceleration in thinning rate on western Svalbard glaciers

“…The relatively large ''footprint'' of the GRACE data means that mass loss in NE Greenland cannot be considered alone, since mass loss Svalbard has the potential to influence results for the larger area. Surface deformation data from Ny-Å lesund, NW Svalbard as determined from GPS and VLBI data [Sato et al, 2006] indicate a contemporary uplift rate which, if it were attributed solely to elastic uplift in response to mass loss from Svalbard glaciers, would be equivalent to À0.75 m a À1 , in meters water equivalent. Again, this is a greater mass loss than prior estimates [Dowdeswell et al, 1997;Van de Wal and Wild, 2001;Hagen et al, 2003aHagen et al, , 2003b, which are based on assorted mass balance data averaged over variable periods, but roughly from the mid-1960s to 2000.…”

Horizontal and vertical velocities derived from the IDS contribution to ITRF2014, and comparisons with geophysical models

“…Furthermore, as pointed out by King & Watson (2014), the comparison of the DORIS and GNSS vertical velocities with vertical motion over much longer timescales may also be biased by non-linear rapid motion of the rotation pole since 2005. Note that earlier studies have pointed out the inconsistency between the different observing techniques (GNSS and VLBI) in Ny-Ålesund (Sato et al 2006;Kierulf et al 2009a). As no monument instability has been identified and since the ULR velocities are similar to other GPS solutions, and since the GPS and DORIS stations are only separated by 1.6 km, the inconsistency may be due to non-optimum estimation of the solar radiation pressure which induces systematic errors in the heights of high latitude stations.…”

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

“…The effect of atmospheric loading was computed by multiplying the difference between nominal pressure and observed pressure at each drop with an admittance factor. At Kongepunktet the admittance factor was set to −0.422 µGal/hPa, adopted from [31]. This is an empirical parameter estimated from SG-observations simultaneously with tidal parameters.…”

“…On the other hand, GIA from past ice-mass changes is usually modelled as a long spatial wavelength process. For instance, [31] modelled the effect over Svalbard up to the degree 180 in terms of spherical harmonics, which corresponds to a spatial resolution of approximately 222 km at the Earth's surface. Hence, we expect negligible differences at the two stations in the signal from past ice-mass changes.…”

“…A small wooden shed covered with a tarpaulin provided shelter from precipitation and winds for both the 1991 and 1998 AG-campaigns. Also, the 1991 campaign was measured at a separate pillar that was later iGrav-12 NMA † Adopted from [29], but transferred to h = 1.20 m ‡ Adopted from [31], but transferred to h = 1.20 m * Adopted from [16], but transferred to h = 1.20 m ** Processed from raw data.…”