Using cosmogenic isotopes to resolve the history of fjord glaciation
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Continent: North America
Country: Canada
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UTM coordinates and datum: none
Setting
Climate Setting: Polar
Tectonic setting: Craton
Type: Process
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Description
Introduction
In high-latitude regions, the edges of continents are dissected with large, spectacular fjords. Although we know that fjords are deeply carved glacial valleys that have their bottoms below sea level, understanding the history and style of glaciation of fjords during late Pleistocene glaciation has been more difficult. Two contrasting models of fjord glaciation have been debated for decades.
Field evidence from fjord landscapes commonly reveals fresh, glacially-eroded bedrock within fjord valleys, and highly weathered bedrock on the uplands adjacent to fjords. One model of former glaciation interprets the field evidence as indicating that fjords contained ice sheet outlet glaciers that were confined within the fjord valleys, much like some outlet glaciers draining the Greenland Ice Sheet (Figure 1). According to the "outlet glacier model," outlet glaciers eroded the valley bottoms and sides (making striated and polished bedrock), and left bedrock on the adjacent uplands ice-free and thus allowed to weather (Ives, 1978). The other model suggests that a thick ice sheet smothered the entire landscape, in which case ice streams like Jakobshavn Isbræ occupied fjords (Figure 2). According to the "Jakobshavn model," the base of the ice streams would have slid across the valley floor, flowed very fast, and caused intense glacial erosion (Sugden, 1978). Sectors of the ice sheet that covered the upland areas adjacent to fjords would have been frozen to their beds, which would have essentially protected the bedrock from glacial erosion. So, how do we decide which model to follow?
Cosmogenic isotopes
Cosmogenic isotopes are produced on Earth as a result of bombardment by cosmic radiation. Some cosmogenic isotopes are those that are produced in the Earth's surface, where they accumulate in the uppermost two to three meters of surface material. The accumulation rate of cosmogenic isotopes in earth surfaces is known, and thus, scientists use the amount of cosmogenic isotopes in a surface sample to determine when it first became exposed (Cockburn and Summerfield, 2004). Geomorphic events that either create new earth surfaces (e.g., a lava flow, or the top surface of a landslide deposit) or erode through the upper two to three meters of the earth's surface (e.g., river incision, glacial erosion), can be dated by measuring the concentration of cosmogenic isotopes in the affected areas. In this way, glacial geologists can determine when past glaciations took place by measuring the concentration of cosmogenic isotopes in bedrock, or glacial boulders, that were eroded by glaciers during past glaciations (Balco, 2012).
Cosmogenic isotopes can be used in glacial landscapes in other ways too. In an area that was glaciated, the concentration of cosmogenic isotopes can reveal where there was glacial erosion and where there was not – or, where there was more glacial erosion and where there was less. For example, where there was intense glacial erosion, the concentration of cosmogenic isotopes will be relatively low. And, where there was little to no glacial erosion, the concentration would be relatively high because the glacier did not remove previously accumulated isotopes during its occupation of the terrain. In some cases, where the timing of former glaciation can be constrained by other dating techniques, the concentration of cosmogenic isotopes can even be used to constrain the specific amount of glacier erosion (Briner and Swanson, 1998). Used in these ways, cosmogenic isotopes have improved geologists' ability to determine when former glaciations took place, and also to constrain the overall patterns of glacial erosion in formerly glaciated areas.
Findings from Baffin Island fjords
The recent application of cosmogenic isotope analysis in rock samples collected from fjord landscapes on Baffin Island (Figure 3) has added new knowledge (Briner et al., 2006). Bedrock surfaces within fjord valleys provide cosmogenic isotope ages of around 12,000 to 10,000 years ago, indicating the time when the vast Laurentide Ice Sheet retreated out of Baffin Island fjords. Bedrock surfaces on the upland areas adjacent to the fjords contained much higher concentrations of cosmogenic isotopes, equivalent to many 10s to 100s of thousands of years of exposure. These measurements so far are consistent with both models of former ice cover. However, in addition to measuring cosmogenic isotopes in the upland bedrock, cosmogenic isotopes have also been measured in glacial boulders perched on top of bedrock in the uplands (Figure 4). Previously, it was argued that the glacial (erratic) boulders were deposited on the uplands prior to the last Ice Age, and some even argued that the boulders were not glacial in origin. However, the cosmogenic isotope concentrations of boulders from the uplands yielded the same age as those ages from the fjord valleys (Figure 5). This indicates that the Laurentide Ice Sheet occupied both the fjords and the adjacent uplands during the late Pleistocene, and that both regions were deglaciated around the same time.
Ultimately, these data favor the "Jakobshavn model" of fjord glaciation for Baffin Island fjords. The results demonstrate that highly weathered bedrock with a high concentration of cosmogenic isotopes is not necessarily evidence for the lack of glaciation. Rather, ice sheets can be fickle eroders, gouging deep valleys in some locations, and protecting landscapes from significant erosion in others. In fact, it may be that once ice sheets produce deep fjords, they funnel most of their ice through these locations, which become foci for intense ice sheet erosion. In turn, those areas adjacent to the fjords are spared of fast-flowing, overriding ice, and become locations where underlying bedrock is preserved from glacial erosion. Almost certainly, the "outlet glacier model" and the "Jakobshavn model" of fjord glaciation represent part of a continuum of how fjords can be occupied by ice sheets.
In high-latitude regions, the edges of continents are dissected with large, spectacular fjords. Although we know that fjords are deeply carved glacial valleys that have their bottoms below sea level, understanding the history and style of glaciation of fjords during late Pleistocene glaciation has been more difficult. Two contrasting models of fjord glaciation have been debated for decades.
Field evidence from fjord landscapes commonly reveals fresh, glacially-eroded bedrock within fjord valleys, and highly weathered bedrock on the uplands adjacent to fjords. One model of former glaciation interprets the field evidence as indicating that fjords contained ice sheet outlet glaciers that were confined within the fjord valleys, much like some outlet glaciers draining the Greenland Ice Sheet (Figure 1). According to the "outlet glacier model," outlet glaciers eroded the valley bottoms and sides (making striated and polished bedrock), and left bedrock on the adjacent uplands ice-free and thus allowed to weather (Ives, 1978). The other model suggests that a thick ice sheet smothered the entire landscape, in which case ice streams like Jakobshavn Isbræ occupied fjords (Figure 2). According to the "Jakobshavn model," the base of the ice streams would have slid across the valley floor, flowed very fast, and caused intense glacial erosion (Sugden, 1978). Sectors of the ice sheet that covered the upland areas adjacent to fjords would have been frozen to their beds, which would have essentially protected the bedrock from glacial erosion. So, how do we decide which model to follow?
Cosmogenic isotopes
Cosmogenic isotopes are produced on Earth as a result of bombardment by cosmic radiation. Some cosmogenic isotopes are those that are produced in the Earth's surface, where they accumulate in the uppermost two to three meters of surface material. The accumulation rate of cosmogenic isotopes in earth surfaces is known, and thus, scientists use the amount of cosmogenic isotopes in a surface sample to determine when it first became exposed (Cockburn and Summerfield, 2004). Geomorphic events that either create new earth surfaces (e.g., a lava flow, or the top surface of a landslide deposit) or erode through the upper two to three meters of the earth's surface (e.g., river incision, glacial erosion), can be dated by measuring the concentration of cosmogenic isotopes in the affected areas. In this way, glacial geologists can determine when past glaciations took place by measuring the concentration of cosmogenic isotopes in bedrock, or glacial boulders, that were eroded by glaciers during past glaciations (Balco, 2012).
Cosmogenic isotopes can be used in glacial landscapes in other ways too. In an area that was glaciated, the concentration of cosmogenic isotopes can reveal where there was glacial erosion and where there was not – or, where there was more glacial erosion and where there was less. For example, where there was intense glacial erosion, the concentration of cosmogenic isotopes will be relatively low. And, where there was little to no glacial erosion, the concentration would be relatively high because the glacier did not remove previously accumulated isotopes during its occupation of the terrain. In some cases, where the timing of former glaciation can be constrained by other dating techniques, the concentration of cosmogenic isotopes can even be used to constrain the specific amount of glacier erosion (Briner and Swanson, 1998). Used in these ways, cosmogenic isotopes have improved geologists' ability to determine when former glaciations took place, and also to constrain the overall patterns of glacial erosion in formerly glaciated areas.
Findings from Baffin Island fjords
The recent application of cosmogenic isotope analysis in rock samples collected from fjord landscapes on Baffin Island (Figure 3) has added new knowledge (Briner et al., 2006). Bedrock surfaces within fjord valleys provide cosmogenic isotope ages of around 12,000 to 10,000 years ago, indicating the time when the vast Laurentide Ice Sheet retreated out of Baffin Island fjords. Bedrock surfaces on the upland areas adjacent to the fjords contained much higher concentrations of cosmogenic isotopes, equivalent to many 10s to 100s of thousands of years of exposure. These measurements so far are consistent with both models of former ice cover. However, in addition to measuring cosmogenic isotopes in the upland bedrock, cosmogenic isotopes have also been measured in glacial boulders perched on top of bedrock in the uplands (Figure 4). Previously, it was argued that the glacial (erratic) boulders were deposited on the uplands prior to the last Ice Age, and some even argued that the boulders were not glacial in origin. However, the cosmogenic isotope concentrations of boulders from the uplands yielded the same age as those ages from the fjord valleys (Figure 5). This indicates that the Laurentide Ice Sheet occupied both the fjords and the adjacent uplands during the late Pleistocene, and that both regions were deglaciated around the same time.
Ultimately, these data favor the "Jakobshavn model" of fjord glaciation for Baffin Island fjords. The results demonstrate that highly weathered bedrock with a high concentration of cosmogenic isotopes is not necessarily evidence for the lack of glaciation. Rather, ice sheets can be fickle eroders, gouging deep valleys in some locations, and protecting landscapes from significant erosion in others. In fact, it may be that once ice sheets produce deep fjords, they funnel most of their ice through these locations, which become foci for intense ice sheet erosion. In turn, those areas adjacent to the fjords are spared of fast-flowing, overriding ice, and become locations where underlying bedrock is preserved from glacial erosion. Almost certainly, the "outlet glacier model" and the "Jakobshavn model" of fjord glaciation represent part of a continuum of how fjords can be occupied by ice sheets.
Associated References
- Balco, G. (2011). Contributions and unrealized potential contributions of cosmogenic-nuclide exposure dating to glacier chronology, 1990-2010. Quaternary Science Reviews 30, 3-27.
- Briner, J.P., and Swanson, T.W. (1998). Using inherited cosmogenic 36Cl to constrain glacial erosion rates of the Cordilleran Ice Sheet. Geology 26, 3-6.
- Briner, J.P., Miller, G.H., Davis, P.T., and Finkel, R.C. (2006). Cosmogenic radionuclides from fiord landscapes support differential erosion by overriding ice sheets. Geological Society of America Bulletin 118, 406-420.
- Cockburn, H.A.P., and Summerfield, M.A. (2004). Geomorphological applications of cosmogenic isotope analysis. Progress in Physical Geography 28, 1-42.
- Ives, J.D., (1978). The maximum extent of the Laurentide ice sheet along the east coast of North America during the last glaciation. Arctic 31, 24–53.
- Sugden, D.E., (1978). Glacial erosion by the Laurentide ice sheet. Journal of Glaciology 83, 367–391.