Glacial Isostatic Adjustment as a Source of Noise for the Interpretation of GRACE Data
Viscoelastic relaxation in the Earth's mantle caused by wide-spread deglaciation following the last glacial maximum (LGM), can appear as a secular trend in measurements of the Earth's time-variable gravity field. The presence of this trend can provide an opportunity to use gravity observations to constrain models of the glacial isostatic adjustment (GIA) process. But it can also be a nuisance for people who are using the gravity observations to learn about other things. Gravity observations, whether from satellites or from ground-based gravimeters, can not distinguish between the gravitational effects of water/snow/ice variations on or near the surface, and those caused by density variations deep within the mantle. Unmodeled or mismodeled GIA signals can sometimes make it difficult to use gravity observations to learn about secular changes in water/snow/ice from such places as northern Canada, Scandinavia, Antarctica, and Greenland: places where there was considerable long-term deglaciation following the LGM. These issues have become particularly important since the 2002 launch of the GRACE gravity satellite mission. GIA signals in northern Canada and Scandinavia are clearly evident in the GRACE data. But the presence of GIA signals in these and other regions has sometimes caused problems for long-term hydrological and, especially, cryospheric studies with GRACE. GIA model errors, for example, are by far the largest source of uncertainty when using GRACE to estimate present-day thinning rates of the Antarctic ice sheet. This talk will discuss the contributions of the GIA signal to GRACE time-variable gravity measurements; partly as an opportunity to study the GIA process, but mostly as a source of uncertainty for other applications.
Accelerating Uplift in Greenland due to Rapid Glacier Melting
The Global Positioning System is a powerful precise positioning technique widely used in Earth sciences for measuring tectonic plate motion, earthquake and volcanic induced crustal deformation, and other important phenomena. Most studies primarily rely on horizontal GPS measurements, because vertical GPS observations are less accurate and difficult to interpret. Recent progress in GPS technology and processing strategies provide new opportunities to use vertical GPS observations. Our results show that high-precision GPS stations located on Greenland's rocky margin show a pattern of accelerating uplift over the last decade. We suggest that this is due to recent accelerated melting of Greenland's ice sheet, and consequent accelerating upward motion of the crust to maintain isostatic (gravitational) equilibrium.
New Appraisals of GIA Modelling and Space Gravity (GRACE) Data Treatment
The new generation of space gravity data, from GRACE and the incoming GOCE, rejuvenated our interest on some basic issues in GIA modelling, and made us to conceive new approaches in space gravity data treatment. Once cleared for hydrological effects, for those of the atmosphere and oceans, and present day mass imbalance, the signal from the deep interior is that from the GIA response to Pleistocene deglaciation. We have thus reconsidered the basic physics underlying the differential equations controlling the GIA readjustment of the mantle, following the philosophy of pushing the treatment of these differential equations as far as possible from the analytical standpoint. We obtained, among other findings, that the contribution of the denumerably infinite compressional D-modes can be dealt with accurately by normal mode approach, disclosing the very intimate nature of the compressional D-mode cluster point, which we demonstrated not contributing at all to deformation: our results definitely prove the correctness of normal mode approach, even for compressible mantle models. In parallel with these new theoretical achievements, we implemented a new procedure aimed at deriving a weighted surface mass distribution in water equivalent, starting from an initial guess, to exctract the secular gravity effects of present-day phenomena. Once these gravity effects from the various Earth's compartments except its interior are carefully removed from GRACE data, we remain with a gravity pattern where the effects of PGR are clearer, ready to be compared with viscoelastic model predictions. This cleared gravity pattern allows us to pursue a global preliminary viscosity inversion, greatly improved with respect to those based on un-cleared gravity data. Theoretical achievements in viscoelastic modelling are prerequisites to better understand many Solid Earth phenomena, GIA in the first place, and we show that our data treatments can improve interpretation of space gravity data.
Vertical Land Movements Constrained by Absolute Gravity Measurements
Repeated absolute gravity (AG) measurements have been performed across the tectonically active intraplate regions in Northwest Europe: the Ardenne and the Roer Graben. At most of the stations measurements were undertaken in 2000 and repeated twice a year. Analysis of these measurements, performed in Belgium and Germany, show that at all stations except Jülich, there is no detectable gravity variation higher than 10 nm s-2 at the 95% confidence level. This is equivalent to vertical movements of 5 mm/yr. Although not yet significant, the observed rates do not contradict the subsidence predicted by glacial isostatic adjustment models and provide an upper limit on the possible uplift of the Ardennes. In Jülich, a gravity rate of change of 36 nm -2/year, equivalent to 18 mm/yr, is at least in parts due to anthropogenic subsidence. The amplitudes of the seasonal variations range from 18±0.8 nm s-2 to 43±29 nm s-2, depending on the location. These variations should have a negligible effect on the long-term trend, but at the Membach reference station, were a longer time series is available, differences in the rates observed since 1996 and 1999 indicate that long-term environmental effects may influence the inferred trend. The observed seasonal effects also demonstrate the repeatability of AG measurements. This study indicates that, even in difficult conditions, AG measurements repeated once a year can resolve vertical land movements at a few mm level after 5 years. This also confirms the need to measure for decades, using accurate and stable geodetic techniques like AG, in order to constrain slow deformation processes.
Using GIA observables to constrain the thermal contribution to lateral variations in mantle viscosity
Lateral heterogeneities in the mantle can be caused by thermal, chemical and non-isotropic pre-stress effects. Here, observations of the glacial isostatic adjustment (GIA) process are used to constrain the thermal contribution to lateral variations in upper and lower mantle viscosities. The Coupled Laplace-Finite Element method is used to predict the GIA response on a spherical, self-gravitating, compressible, viscoelastic earth with self-gravitating oceans, induced by either the ICE-5G or ICE-4G deglaciation models. GIA observations include global historic relative sea level data, GPS uplift rates in Laurentide and Fennoscandia, altimetry together with tide-gauge data in the Great Lakes area, and GRACE data in Laurentide. The lateral viscosity perturbations are inferred from the high resolution seismic tomography model of Grand (2002) by using a conversion relation that takes into account both anelastic and anharmonic effects (Karato 2008). To determine the contribution of thermal effects in the upper and lower mantle, the scaling factor b is also inserted into the conversion relation: For b = 1, lateral velocity variations are caused by thermal effects alone; while b < 1 indicates a decreasing contribution of thermal effects; eventually when b = 0, there is no lateral viscosity variations exist and the Earth is laterally homogeneous. The value of b in the upper mantle is b1 while that in the lower mantle is b2. The lateral viscosity variations computed this way are superposed on a reference model that is able to give a reasonably good fit to the GIA observations. The parameter space for (b1, b2) is then searched to find the combination that yields the best improvement in fitting the GIA data in Laurentide, Fennoscandia or globally.
Ocean loading effects on predictions of uplift and gravity change due to glacial isostatic adjustment in Antarctica
The effect of regional ocean loading on predicted rates of crustal uplift and gravitational change due to glacial isostatic adjustment (GIA) is determined for Antarctica. The effect is found to be large (up to -8 mm/yr change in uplift rate and -3 cm/yr water equivalent gravity change) for the ICE-3G loading history but is much smaller (+1 mm/yr; +0.25 cm/yr) for the IJ05 loading history. A simple formula is derived that relates surface water thickness change to external gravitational change that is accurate to within a few percent. A fundamental difference in the definition of the load histories accounts for the differing sensitivities between IJ05 and ICE-3G. IJ05 defines its surface load history relative to the present-day surface load, rather than specifying an absolute loading history, and thus implicitly approximates the temporal and spatial mass exchange between grounded ice and open ocean. In contrast, ICE-3G specifies an absolute load history and explicit regional ocean loading substantially perturbs predicted GIA rates, including the rate of change of the long-wavelength zonal harmonics of the Earth's gravitational field. Other models that are defined in terms of an absolute load history also generate predictions that are strongly dependent on the presence or absence of ocean loading. In comparison, conclusions of previous studies that used IJ05 predictions without explicit ocean loading are relatively robust and incorporate current geological and glaciological constraints on Antarctic ice sheet evolution.
Time-variable ice mass redistribution and consequences for solid Earth geodesy
Long wavelength gravity changes associated with imbalance of the cryosphere and other interannual and secular processes are now being mapped from space using GRACE (Gravity Recovery and Climate Experiment) mission data. The gravity changes are supplemented by constraints that come from bounds on Earth rotation and the drift between the Earth's center-of-mass and center-of-figure. Although the main features of northern hemispheric post-glacial rebound are clearly recovered from GRACE, there is much to be gained from improving the modeled hydrology and ocean tides. In addition, a great deal of uncertainty exists in the forward ice models that are responsible for driving predictions of the present-day signatures arising from viscoelastic relaxation of the Earth's mantle. There is a link between the uncertainties associated with glacial isostasy and the extraction of present-day ice mass changes in Greenland, the Devon Ice Cap and land ice of Canada's Arctic archipelago from space-borne gravity observations. For example, model gravity changes associated with the long-wavelength components of the collapse of the Laurentide forebulge have far greater uncertainty in the northeastern Canadian Arctic archipelago than south of Hudson Bay. This spatially variable uncertainty can now be quantified with increasing confidence, in part, due to constraints that come from tide gauge and GPS measurements. Here we examine and discuss the implications of uncertainties in the timing and size of ice sheet collapse in eastern Canada and we quantify the errors caused in estimating ice loss in Greenland and the Canadian Arctic. The uncertainty is a sensitive function of ice dome size, location, collapse history, lithospheric thickness and mantle viscosity structure.
A Global Glacial Isostasy Filter for GRACE: Polar Ice Sheet Instabilities?
GRACE satellite measurements of gravity field time dependence have identified three primary regions from which land ice is currently disappearing, respectively Greenland, Alaska and Antarctica. In each of these regions a correction is required to the GRACE data to eliminate the "contamination" associated with residual ice-age influence. Since the water that is produced by such wastage of land ice enters the ocean basins and since GRACE also measures the rate at which mass is being added to the oceans, it is possible to address the question as to whether the budget of global sea level rise is closed with respect to its mass component. As it happens the correction to the rate at which mass is being added to the oceans due to the continuing action of the GIA process is even more important than those that must be applied to each of the regions from which land ice is disappearing. A new analysis of the global sea level budget closure issue will be presented together with comments upon the Greenland component.