Tonie van Dam

Bedrock displacements in Greenland manifest ice mass variations, climate cycles and climate change

The Greenland GPS Network (GNET) uses the Global Positioning System (GPS) to measure the displacement of bedrock exposed near the margins of the Greenland ice sheet. The entire network is uplifting in response to past and present-day changes in ice mass. Crustal displacement is largely accounted for by an annual oscillation superimposed on a sustained trend. The oscillation is driven by earth’s elastic response to seasonal variations in ice mass and air mass (i.e., atmospheric pressure). Observed vertical velocities are higher and often much higher than predicted rates of postglacial rebound (PGR), implying that uplift is usually dominated by the solid earth’s instantaneous elastic response to contemporary losses in ice mass rather than PGR. Superimposed on longer-term trends, an anomalous ‘pulse’ of uplift accumulated at many GNET stations during an approximate six-month period in 2010. This anomalous uplift is spatially correlated with the 2010 melting day anomaly.

Geodetic measurements in Greenland and their implications

We describe results from an ongoing experiment in Greenland, in which we are using absolute gravity and continuous Global Positioning System (GPS) measurements to study vertical crustal motion at two locations along the edge of the ice sheet: Kellyville, located about one third of the way up the western ice margin, and Kulusuk, located along the eastern ice margin at about the same latitude as Kellyville. The GPS measurements suggest average crustal uplift rates of −5.8±1.0 mm/yr at Kellyville and −2.1±1.5 mm/yr at Kulusuk. There have not yet been enough absolute gravity occupations to permit useful secular gravity solutions at either location. The negative uplift rate at Kellyville is consistent with independent archeological and historical evidence that the southwestern edge of the continent has been subsiding over the last 3000 years, but it is inconsistent with estimates of the Earths continuing viscoelastic response to melting ice during the early Holocene, which predict that Kellyville is likely to be uplifting, rather than subsiding, by 2.0±3.5 mm/yr. The resulting −7.8±3.6 mm/yr discrepancy between the observed and predicted uplift rates is too large to be caused by loading from present-day changes in nearby ice. However, it is consistent with independent suggestions that the western ice sheet margin in this region may have advanced by ≈50 km during the past 3000–4000 years. If this advance did occur and if the crustal subsidence it induces is not removed from altimeter measurements of Greenland ice sheet elevations, then the altimeter solutions could underestimate the true snow/ice thickness change by 5–10 mm/yr along portions of the western margin of the ice sheet.

Measuring postglacial rebound with GPS and absolute gravity

We compare vertical rates of deformation derived from continuous Global Positioning System (GPS) observations and episodic measurements of absolute gravity. We concentrate on 4 sites in a region of North America experiencing postglacial rebound. The rates of uplift from gravity and GPS agree within one standard deviation for all sites. The GPS vertical deformation rates are significantly more precise than the gravity rates, primarily because of the denser temporal spacing provided by continuous GPS tracking. We conclude that continuous GPS observations are more cost efficient and provide more precise estimates of vertical deformation rates than campaign style gravity observations where systematic errors are difficult to quantify.

Vertical crustal motion observed in the BIFROST project

Abstract This paper reports from investigations on the robustness of estimated rates of intraplate motion from the continuous GPS project BIFROST (Baseline Inferences from Fennoscandian Rebound Observations, Sealevel and Tectonics). We study loading effects due to ocean, atmosphere and hydrology and their impact on estimated rate parameters. We regularly find the admittance of a modelled perturbation at less than fifty percent of the full effect. We think that the finding relates to a difficult noise situation at all periods, and that a satisfying model for the dominating noise source has not been found yet. An additional reason for low admittance is found in the mapping process of the no-fiducial network solution into a conventional reference frame.

Two years of continuous measurements of tidal and nontidal variations of gravity in Boulder, Colorado

We report here on the results of an analysis of 2 years of data from NOAAs superconducting gravimeter located at the Table Mountain Gravity Observatory in Boulder, Colorado. Observed tidal parameters, corrected for ocean loading effects, are compared with theoretical tidal parameters predicted for a non-hydrostatic inelastic Earth model and demonstrate excellent agreement. Tidal residuals, corrected for polar motion and a linear instrument drift are highly correlated with gravity changes measured by two absolute gravimeters over the same time period. The admittance to local pressure is found to be −0.356 µGal/mbar. However, this admittance factor is found to be seasonally and frequency dependent. Correlations between rainfall events and gravity changes are observed. Attempts to model these gravity changes as exponential functions of time were unsuccessful.

Geodetic measurements reveal similarities between post-Last Glacial Maximum and present-day mass loss from the Greenland ice sheet.

Present destabilization of marine-based sectors in Greenland may increase sea level for centuries to come. Accurate quantification of the millennial-scale mass balance of the Greenland ice sheet (GrIS) and its contribution to global sea-level rise remain challenging because of sparse in situ observations in key regions. Glacial isostatic adjustment (GIA) is the ongoing response of the solid Earth to ice and ocean load changes occurring since the Last Glacial Maximum (LGM; ~21 thousand years ago) and may be used to constrain the GrIS deglaciation history. We use data from the Greenland Global Positioning System network to directly measure GIA and estimate basin-wide mass changes since the LGM. Unpredicted, large GIA uplift rates of +12 mm/year are found in southeast Greenland. These rates are due to low upper mantle viscosity in the region, from when Greenland passed over the Iceland hot spot about 40 million years ago. This region of concentrated soft rheology has a profound influence on reconstructing the deglaciation history of Greenland. We reevaluate the evolution of the GrIS since LGM and obtain a loss of 1.5-m sea-level equivalent from the northwest and southeast. These same sectors are dominating modern mass loss. We suggest that the present destabilization of these marine-based sectors may increase sea level for centuries to come. Our new deglaciation history and GIA uplift estimates suggest that studies that use the Gravity Recovery and Climate Experiment satellite mission to infer present-day changes in the GrIS may have erroneously corrected for GIA and underestimated the mass loss by about 20 gigatons/year.

GPS measurements of vertical crustal motion in Greenland

We have analyzed 5 years of continuous Global Positioning System (GPS) measurements taken at Kellyville, just off the western margin of the ice sheet in southern Greenland. A fit to the vertical component gives a negative secular uplift rate of −5.8±1.0 mm/yr. A negative rate (i.e., a subsidence) is consistent with archeological and historical evidence that the surrounding region has been subsiding over the last 3 kyr. However, it is inconsistent with estimates of the Earths continuing viscoelastic response to melting ice prior to 4 ka years ago, which predict that Kellyville should be uplifting, rather than subsiding, by 2.0±3.5 mm/yr. The resulting −7.8±3.6 mm/yr discrepancy is too large to be the result of loading from present-day changes in nearby ice. We show, instead, that it is consistent with independent suggestions that the western ice sheet margin in this region of Greenland may have advanced by ≈50 km during the past 3–4 kyr.

Results of the International Comparison of Absolute Gravimeters in Walferdange (Luxembourg) of November 2003

The results of an international comparison of absolute gravimeters held in Walferdange, Luxembourg, in November 2003 are presented here in detail. The absolute meters agreed with one another to within a standard deviation less than 2 μGal (1 Gal = 1 cm/s), where we have excluded the results from a single prototype instrument from the analysis. This result, represents the best agreement ever obtained in a comparison of absolute gravimeters. In addition, for the first time, we were able to quantify the effect of the operators on the instrument agreement. The result indicates that the contribution to the errors in the observations due to the operator are less than 1 μGal, i.e. within the observational errors. We also demonstrate that there are no systematic differences between observations taken with FG5’s incorporating the bulk interferometer and those using the fiber optic version of the interferometer.

Using GPS and gravity to infer ice mass changes in Greenland

Climate research indicates that global warming is occurring and will probably continue for the next several decades. One consequence of a global warming scenario is a global sea-level rise due to thermal expansion of the nearsurface ocean water and melting of the Antarctic and Greenland ice sheets and continental glaciers. The lack of appropriate data has made it difficult to determine the relationship and feedback mechanisms between climate, sea level, and ice mass changes. It is even unclear, for example, whether changes in the Greenland and Antarctic ice sheets over the last century have caused sea level to rise or fall.

Comment on “Nature of the recent vertical ground movements inferred from high‐precision leveling data in an intraplate setting: NE Ardenne, Belgium” by A. Demoulin and A. Collignon

[01] Using yearly leveling surveys performed from 1993 to 1998 in the Ardenne, as well as historic leveling results from 1948 and 1974, Demoulin and Collignon [2000] (hereinafter referred to as D&C) observed that the total vertical ground movement (less than 1 cm) over a 20-to 30year period is barely higher than the yearly displacements measured to a few millimeters per year. They eliminated various sources of errors from their measurements and subsequently interpreted the resulting short-term oscillating crustal displacements as true tectonic motions within the upper crust accompanying the long-term deformation of uplifting areas in intraplate settings. Their justification is that the leveling discontinuities coincide with faults they assume to be active. In their conclusions, D&C suggested that such motions and their variations could be used to monitor potential seismogenic faults and even to foretell earthquakes. [2] Unfortunately, it is our opinion that the data analysis of D&C is inadequate and does not support the subsequent interpretation. Specifically, we interpret the apparent oscillations in the leveling data as the expected expression of the noisy character of leveling differences with amplitudes ranges of 0.5 to 1 cm. If this premise is accepted then there is also no evidence to suggest that the boundary faults in the study area are active. We thus feel that the conclusions of D&C are premature until their observations can be corroborated by improved observations or by other independent measurements. In this note, we present a careful statistical analysis to support our position.