Geodetic measurements in Greenland and their implications

We describe results from an ongoing experiment in Greenland, in which we ar e using absolute gravity and continuous Global Positioning System (GPS) mea surements 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 wes tern ice margin, and Kulusuk, located along the eastern ice margin at about the same latitude as Kellyville. The GPS measurements suggest average crus tal 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 u plift rate at Kellyville is consistent with independent archeological and h istorical evidence that the southwestern edge of the continent has been sub siding over the last 3000 years, but it is inconsistent with estimates of t he Earth's 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 disc repancy 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 co nsistent 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 rem oved from altimeter measurements of Greenland ice sheet elevations, then th e altimeter solutions could underestimate the true snow/ice thickness chang e by 5-10 mm/yr along portions of the western margin of the ice sheet.