Terminus Changes of Tidewater Outlet Glaciers in Greenland: Environmental Controls and Links to Glacial Earthquakes
As of 2002, Degree of Master of Science (MS) Quaternary and Climate Studies published under the auspices of the Climate Change Institute.
The contribution of continental ice sheets to sea level rise has doubled in the last decade and the rapid acceleration of Greenland's outlet glaciers has been one of the dominant factors in this contribution. The largest changes in speed have been observed at the tidewater margins of these glaciers, implying a link between terminus position and glacier dynamics. By understanding the environmental controls on terminus position, we will be better able to predict future ice dynamic changes and ice sheet contributions to sea level. Here we examine decade-long records of terminus position for five major marine-terminating glaciers (Daugaard Jensen, Kangerdlugssuaq and Helheim glaciers in East Greenland, and Jakobshavn and Rink glaciers in West Greenland) to quantify seasonal and interannual variability in terminus changes. The time series are constructed from ~daily resolution MODIS satellite images. In order to understand the role of environmental conditions in governing the observed variability, we compile companion datasets of air temperature, sea surface temperature, and wind stress magnitude for regions adjacent to each outlet glacier. Results show that Greenland outlet glaciers undergo a seasonal pattern of summertime retreat and wintertime advance. The onset of retreat is highly variable, beginning as early as the start of March (before the start of the melt season) and as late as early August. Helheim and Kangerdlugssuaq glaciers show a substantial increase in seasonal variability following the onset of large changes in ice dynamics. Jakobshavn Isbrae has a more complex seasonal pattern, although it too has undergone a large change in ice dynamics. Rink and Daugaard Jensen glaciers exhibit relatively little interannual variability, which is consistent with their stable ice dynamics. There is a weak latitudinal pattern in the timing of the onset of retreat, but there is no obvious relationship between the length or intensity of the melt season and the duration or magnitude of retreat. These results indicate a complex interaction between environmental conditions and terminus change, and also point to a potentially important role played by local geometry (ice thickness, fjord shape and bathymetry) in controlling terminus position. Numerous long-duration (>30 s) seismic events equivalent to M~5 earthquakes are co-located with fjords in Greenland containing fast-flowing outlet glaciers. We use our terminus position time series to examine the hypothesis that these so-called glacial earthquakes are the result of mega-scale calving events. Results show that glacial earthquakes always occur during periods of terminus retreat, but not every retreat event generates a glacial earthquake. Daugaard Jensen and Rink glaciers were the least teleseismically-active glaciers in our sample, and were also the glaciers exhibiting the smallest amount of seasonal and interannual terminus change. Jakobshavn Isbrae experienced the largest terminus retreat in our sample, but generated relatively few glacial earthquakes, and none at all until the glacier retreated to a certain point in its fjord. We find that the generation of glacial earthquakes is relatively independent of the time of year or size of the calving event, but closely linked to a glacier's terminus position in the fjord where calved icebergs are more likely to rotate and come into contact with the fjord-bottom, coupling seismic energy to the solid earth.