Climate vs. dynamic forcing of Greenland's largest glacier: A study of Jakobshavn Isbræ's response to the Little Ice Age
Stewart, Heather Anne Marie
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The Greenland Ice Sheet’s (GIS) magnitude of change has been of the utmost importance in understanding cryospheric contributions to the earth’s climate change. Of particular interest is the long-term record of Jakobshavn Isbræ, one of the largest outlet glaciers draining an estimated 6.5% of the GIS. Its recent rapid thinning, associated with nearly doubled velocities, indicates that Greenland’s outlet glaciers are likely to make faster contributions to sea-level rise than previously believed. To evaluate whether ongoing observed changes are climatically significant, changes must be determined over longer time frames. Although the 35 km retreat of its calving front since the LIA (1850) is well documented, it cannot be used to accurately reconstruct the glacier’s history; in particular, since much of its recent retreat, the terminus was likely floating and thus susceptible to small and short-lived climate perturbations. Here, we combine a chronology of the LIA readvance and subsequent retreat determined from ice sheet threshold lake sediments, along with a 3D reconstruction of ice marginal retreat, measured from stereo imagery to investigate the evolution of the floating ice tongue and land-based margins in the Jakobshavn drainage basin. For this study, we constrain Jakobshavn Isbræ’s longer-term context with proglacial threshold lake sediments. Four AMS radiocarbon dates from macrofossils immediately below the LIA sediments from three lake basins to the north of the fjord reveal that Jakobshavn Isbrae reached its LIA maximum extent between 530±10 and 370±60 cal yr BP (1400-1640 AD). Two AMS radiocarbon dates from a lake south of the fjord state that Jakobshavn reached its LIA maximum between 2250±70 and 2420±60 cal yr BP. Furthermore, the continuous nature of the LIA-sediment units in all sites indicates that Jakobshavn remained at or near its LIA maximum position between 1400-1640 AD and into the 20th century. Using stereoscopic pairs of aerial images taken in 1985, and SPOT satellite images acquired in 2007, vegetation trimlines marking the LIA ice extent and 1985 and 2007 ice sheet margins were mapped in 3D by using a soft-copy workstation. Maximum retreat and thinning rates were measured at Jakobshavn Isbræ, where the calving front retreated at an average rate of 0.178 km yr -1 between the LIA and 1985. Retreat rates increased to 0.545 km yr -1 between 1985 and 2007. Land based margins in the Jakobshavn area record average retreat rates from the LIA to 1985 at .007 km yr -1 and increasing from 1985 to 2007 at 0.030 km yr -1 . However, an outlet glacier just 30 km south of Jakobshavn Isbræ, Alanngorliup Sermia, is at or just above its LIA margin, and has only retreated at a rate of 0.017 km yr -1 since 1985. Thinning rates had a similar trend of increasing at the calving front at a rate of -2.15 m yr -1 from the LIA to 1985 and increased to -4.48 m yr -1 between 1985 and 2007. Land based margins in the Jakobshavn Isbræ area averaged -0.67 m yr -1 from the LIA to 1985 and increased to -1.34 m yr -1 thinning rate between 1985 and 2007. Alanngorliup Sermia has no thinning rate recorded between the LIA and 1985 because of its position at the LIA trimline and has thinned at a rate of -0.31 m yr -1 between 1985 and 2007. These results suggest the greatest retreat and thinning occurred between 1985 and 2007. They also suggest different termini environments respond differently to the same climatic changes. Varied patterns of retreat and elevation indicate dynamic controls of the Jakobshavn study area. These data suggest that although climate may be the greater driving force of the Jakobshavn margin, ice dynamics play a key role in the marginal evolution since the LIA. The net loss of ice from the GIS plays an important role in global sea-level rise, and therefore more detailed investigations of the causes for marked changes of margins are needed to assess ongoing future changes.