Dating Greenland Ice Margin Changes
Shortcut URL: https://serc.carleton.edu/69143
Location
Continent: North America
Country: Greenland
State/Province:
City/Town:
UTM coordinates and datum: none
Setting
Climate Setting: Polar
Tectonic setting: Craton
Type: Chronology
Figure 1. Map of Greenland Details
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Description
The Greenland Ice Sheet (Fig. 1), one of two ice sheets on the planet, and capable of raising global sea level by ~7 meters if completely melted, is changing rapidly. Concerns about global climate change amplifying in the Arctic are leading to intense scrutiny of the huge ice mass. But, an important question remains: How quickly can the Greenland Ice Sheet shrink? Answering this question is difficult, because we have only had a brief period of time over which to observe the response of the Greenland Ice Sheet to ongoing climate change.
One way to gain understanding on how the Greenland Ice Sheet responds to climate change is to study how it has responded to known climate changes in the past. For example, during the Holocene Epoch (the last 11,700 years), Greenland has experienced both warm spells and short-lived cooling events. By using various dating approaches, geologists can determine how the ice sheet responded to past climate events, and thus address questions regarding the responsiveness of the Greenland Ice Sheet to present and future climate change.
Past ice sheet change is recorded in the geologic record around the perimeter of the Greenland Ice Sheet. Assessing the history of the ice sheet then becomes a two-part problem. First, one needs to know where the ice margin was in the past. Second, one needs to know when it was there. Moraines and glacier-derived sediments deposited in adjacent oceans and lakes archive the location of the former ice margin through time. To address the timing part of the problem, geologists use a variety of techniques.
1. Cosmogenic nuclide exposure dating of boulders on moraines provides a direct age constraint for moraine deposition (Fig. 2). For example, Levy et al. (2012) used 10Be dating to determine the age of the Orkendalen Moraine on western Greenland at 6.8±0.3 ka. The Orkendalen Moraine lies just beyond the present ice margin, not far from the town of Kangerlussuaq. Prior to deposition of the Orkendalen Moraine, the ice sheet had steadily retreated from positions farther west, during the relatively warm early Holocene period. The date of the Orkendalen Moraine provides the last time that the Greenland Ice Sheet was more extensive than its current position; after the deposition of the Orkendalen Moraine, this sector of the Greenland Ice Sheet retreated to a location inland of its present position.
2. Cosmogenic nuclide exposure dating of bedrock and perched erratics can be used to provide the timing of deglaciation (Fig. 3). Corbett et al. (2011a) focused on the problem of when the western Greenland Ice Sheet retreated during the early Holocene warm period. However, instead of dating boulders on a moraine, they dated ice-sculpted bedrock samples and erratic boulders perched atop bedrock along a transect away from the present ice sheet margin (Fig. 2). In their study area, near Ilulissat, Corbett et al. (2011a) determined that the ice margin retreated rapidly between 8 and 7 ka.
3. Radiocarbon dating in lake sediments can be used to constrain the timing of past glacier advances. Kaplan et al. (2002) focused on the sediment stratigraphy preserved in a lake just beyond a young moraine (termed the historical moraine) on Greenland that delimits the recent advance of the Greenland Ice Sheet during the Little Ice Age cool period. The ice sheet margin advanced to near the target lake, then stopped and deposited a moraine just short of the lake, and then in the last century, retreated. During the time of the glacier advance, the glacier meltwater stream flowed into the lake, depositing rock flour (fine-grained glacier-eroded sediment) in the lake bottom (Fig. 4). However, prior to and following the advance, the lake bottom accumulated organic-rich sediments datable with radiocarbon dating. Thus, Kaplan et al. (2002) collected sediment cores from the lake and used radiocarbon dating on the organic sediments just below the contact with the overlying glacier-derived sediments to determine that the glacier advance occurred after ~1621-1740 AD (the range stems from uncertainties in radiocarbon dating).
4. Varve dating in lake sediments also can be used to constrain the timing of glacier advance. This approach takes advantage of the situation described above in which the ice sheet margin advances close to a lake and deposits rock flour only during its maximum phase. In some cases, the rock flour has annual banding, or varves. Thus, if the varves can be recovered in sediment cores, and carefully examined back in the lab, one can develop a varve chronology – or the number of varves represented in the rock flour layer. In cases where the varves extend to the surface, all one needs to do is count backward from the year of core recovery to determine the age of the oldest varve layer. Briner et al. (2011) used this technique to determine the timing of ice margin advance near Jakobshavn Isbrae (near Ilulissat), in western Greenland, during the Little Ice Age. Although radiocarbon dating constrained the advance of the glacier to its historical moraine to have taken place after ~1600 AD, the varve dating constrained the advance directly to 1820 AD.
Geologists use these techniques in combination to generate reconstructions of past changes of the Greenland Ice Sheet margin during the Holocene. These reconstructions can be compared with independent records of climate change to determine the response of ice margin change to discrete periods of past warm or cold episodes. As one final example, Young et al. (2011b) combined several of these techniques to document the Greenland ice margin response to a 160-year-long cold snap that occurred 8200 years ago, within which temperature dropped ~3°C in just a few decades. The ice sheet advanced in response to this cold snap revealing that even large ice masses like Greenland can adjust to climate change on human timescales.
One way to gain understanding on how the Greenland Ice Sheet responds to climate change is to study how it has responded to known climate changes in the past. For example, during the Holocene Epoch (the last 11,700 years), Greenland has experienced both warm spells and short-lived cooling events. By using various dating approaches, geologists can determine how the ice sheet responded to past climate events, and thus address questions regarding the responsiveness of the Greenland Ice Sheet to present and future climate change.
Past ice sheet change is recorded in the geologic record around the perimeter of the Greenland Ice Sheet. Assessing the history of the ice sheet then becomes a two-part problem. First, one needs to know where the ice margin was in the past. Second, one needs to know when it was there. Moraines and glacier-derived sediments deposited in adjacent oceans and lakes archive the location of the former ice margin through time. To address the timing part of the problem, geologists use a variety of techniques.
1. Cosmogenic nuclide exposure dating of boulders on moraines provides a direct age constraint for moraine deposition (Fig. 2). For example, Levy et al. (2012) used 10Be dating to determine the age of the Orkendalen Moraine on western Greenland at 6.8±0.3 ka. The Orkendalen Moraine lies just beyond the present ice margin, not far from the town of Kangerlussuaq. Prior to deposition of the Orkendalen Moraine, the ice sheet had steadily retreated from positions farther west, during the relatively warm early Holocene period. The date of the Orkendalen Moraine provides the last time that the Greenland Ice Sheet was more extensive than its current position; after the deposition of the Orkendalen Moraine, this sector of the Greenland Ice Sheet retreated to a location inland of its present position.
2. Cosmogenic nuclide exposure dating of bedrock and perched erratics can be used to provide the timing of deglaciation (Fig. 3). Corbett et al. (2011a) focused on the problem of when the western Greenland Ice Sheet retreated during the early Holocene warm period. However, instead of dating boulders on a moraine, they dated ice-sculpted bedrock samples and erratic boulders perched atop bedrock along a transect away from the present ice sheet margin (Fig. 2). In their study area, near Ilulissat, Corbett et al. (2011a) determined that the ice margin retreated rapidly between 8 and 7 ka.
3. Radiocarbon dating in lake sediments can be used to constrain the timing of past glacier advances. Kaplan et al. (2002) focused on the sediment stratigraphy preserved in a lake just beyond a young moraine (termed the historical moraine) on Greenland that delimits the recent advance of the Greenland Ice Sheet during the Little Ice Age cool period. The ice sheet margin advanced to near the target lake, then stopped and deposited a moraine just short of the lake, and then in the last century, retreated. During the time of the glacier advance, the glacier meltwater stream flowed into the lake, depositing rock flour (fine-grained glacier-eroded sediment) in the lake bottom (Fig. 4). However, prior to and following the advance, the lake bottom accumulated organic-rich sediments datable with radiocarbon dating. Thus, Kaplan et al. (2002) collected sediment cores from the lake and used radiocarbon dating on the organic sediments just below the contact with the overlying glacier-derived sediments to determine that the glacier advance occurred after ~1621-1740 AD (the range stems from uncertainties in radiocarbon dating).
4. Varve dating in lake sediments also can be used to constrain the timing of glacier advance. This approach takes advantage of the situation described above in which the ice sheet margin advances close to a lake and deposits rock flour only during its maximum phase. In some cases, the rock flour has annual banding, or varves. Thus, if the varves can be recovered in sediment cores, and carefully examined back in the lab, one can develop a varve chronology – or the number of varves represented in the rock flour layer. In cases where the varves extend to the surface, all one needs to do is count backward from the year of core recovery to determine the age of the oldest varve layer. Briner et al. (2011) used this technique to determine the timing of ice margin advance near Jakobshavn Isbrae (near Ilulissat), in western Greenland, during the Little Ice Age. Although radiocarbon dating constrained the advance of the glacier to its historical moraine to have taken place after ~1600 AD, the varve dating constrained the advance directly to 1820 AD.
Geologists use these techniques in combination to generate reconstructions of past changes of the Greenland Ice Sheet margin during the Holocene. These reconstructions can be compared with independent records of climate change to determine the response of ice margin change to discrete periods of past warm or cold episodes. As one final example, Young et al. (2011b) combined several of these techniques to document the Greenland ice margin response to a 160-year-long cold snap that occurred 8200 years ago, within which temperature dropped ~3°C in just a few decades. The ice sheet advanced in response to this cold snap revealing that even large ice masses like Greenland can adjust to climate change on human timescales.
Associated References
- Briner, J.P., Young, N.E., Thomas, E.K., Stewart, H.A.M., Losee, S., and Truex, S. (2011). Varve and radiocarbon dating support the rapid advance of Jakobshavn Isbrae during the Little Ice Age. Quaternary Science Reviews, v. 30, p. 2476-2486.
- Corbett, L.B., Young, N.E., Bierman, P.R., Briner, J.P., Neumann, T.A., Graly, J.A., and Rood, D.H. (2011). 10Be concentrations in bedrock and boulder samples resulting from early Holocene ice retreat near Jakobshavn Isfjord, western Greenland. Quaternary Science Reviews, v. 30, p. 1739-1749.
- Kaplan, M.R., Wolfe, A.P., and Miller, G.H. (2002). Holocene environmental variability in southern Greenland inferred from lake sediments. Quaternary Research, v. 58, p. 149-159.
- Levy, L.B., Kelly, M.A., Howley, J.A., and Virginia, R.A. (2012). Age of the Orkendalen moraines, Kangerslussuaq, Greenland: constraints on the extent of the southwestern margin of the Greenland Ice Sheet during the Holocene. Quaternary Science Reviews, v. 52, p. 1-5.
- Young, N.E., Briner, J.P., Axford, Y., Csatho, B., Babonis, G.S., Rood, D.H., and Finkel, R.C. (2011). Response of a marine-terminating Greenland outlet glacier to abrupt cooling 8200 and 9300 years ago. Geophysical Research Letters, v. 38, L24701.