COSMOGENIC EXPOSURE DATING OF LATE PLEISTOCENE TERMINAL MORAINES OF THE SOUTHWESTERN LAURENTIDE ICE SHEET

2020 ◽  
Author(s):  
Keith A. Brugger ◽  
◽  
Benjamin J. Laabs
Keyword(s):  
Late Pleistocene ◽  
Ice Sheet ◽  
Laurentide Ice Sheet ◽  
Exposure Dating

scholarly journals Surface profiles of the laurentide ice sheet in its marginal areas

1974 ◽  
Vol 13 (67) ◽  
pp. 37-43 ◽  
Author(s):  
W. H. Mathews
Keyword(s):  
Late Pleistocene ◽  
Water Pressure ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Laurentide Ice Sheet ◽  
Active Movement ◽  
Shear Stresses ◽  
Marginal Areas ◽  
Image Position ◽  
Basal Shear

Surface slopes of ice lobes can be estimated from the gradients of their margins as shown by ice limits, by contemporaneous recessional moraines, or by lateral melt-water channels, with allowance being made for the dip of an ice lobe laterally, as well as forward, toward its extremities. Profiles can be fitted approximately to a parabola with the equation in which h is the height above and x the distance up-stream from the terminus, in the same units, and A is a coefficient which varies from glacier to glacier. The coefficient A has a value of 4.7 m1 for both the Antarctic ice sheet inland from Mirny and the west central Greenland ice sheet. Several examples of late Pleistocene ice lobes within mountainous terrain of North America and New Zealand have values of A ranging from 2.9 ml to about 4.1 m1. For several ice lobes in the south-western part of the late Pleistocene Laurentide ice sheet, however, values are from about 0.3 to 1.0 m1, corresponding to basal shear stress of from about 0.07 to 0.22 bar. A major problem exists in accounting for the active movement of ice here under such low surface gradients and basal shear stresses. Evidence of basal slip, aided by high subglacial water pressure, should be looked for in the field. Alternatively, other possibilities for the explanation of such low surface gradients should be sought.

Surface exposure dating of the Pierre Sublobe of the James Lobe, Laurentide Ice Sheet

2020 ◽  
Vol 97 ◽  
pp. 88-98
Author(s):  
Stephanie L. Heath ◽  
Thomas V. Lowell ◽  
Brenda L. Hall
Keyword(s):  
Late Quaternary ◽  
Ice Sheet ◽  
Age Dating ◽  
Laurentide Ice Sheet ◽  
Des Moines Lobe ◽  
Exposure Dating ◽  
Des Moines ◽  
Contrast Exposure ◽  
Ice Sheet Dynamics ◽  
Exposure Ages

AbstractThe Laurentide Ice Sheet of the last glacial period terminated in several lobes along its southern margin. The timing of maximum extent may have varied among the terminal lobes owing to internal ice sheet dynamics and spatially variable external controls. Some terminal ice lobes, such as the westernmost James Lobe, remain poorly dated. To determine the timing of maximum ice extent in this key location, we have mapped glacial deposits left by the Pierre Sublobe in South Dakota and applied 10Be surface exposure age dating on boulders on moraine ridges associated with three distinct late Quaternary glacial drifts. The oldest and most extensive “Tazewell” drift produced variable 10Be surface exposure ages spanning 20–7 ka; the large range is likely attributable to moraine degradation and subsequent boulder exhumation. The oldest ages of about 20 ka are probably limiting minimum ages for the Tazewell moraine surfaces. By contrast, exposure ages of the youngest “Mankato” drift of the easternmost Pierre Sublobe tightly cluster at about 16 ka. This age for the Pierre Sublobe is consistent with the nearby Des Moines Lobe, suggesting both acted together.

scholarly journals Beryllium-10 dating of the Foothills Erratics Train in Alberta, Canada, indicates detachment of the Laurentide Ice Sheet from the Rocky Mountains at ~15 ka

2019 ◽  
Vol 92 (2) ◽  
pp. 469-482 ◽  
Author(s):  
Martin Margold ◽  
John C. Gosse ◽  
Alan J. Hidy ◽  
Robin J. Woywitka ◽  
Joseph M. Young ◽  
...  
Keyword(s):  
Rocky Mountains ◽  
Ice Sheet ◽  
Younger Dryas ◽  
Rocky Mountain ◽  
Laurentide Ice Sheet ◽  
Exposure Dating ◽  
Mountain Front ◽  
Ice Retreat ◽  
Cordilleran Ice Sheet ◽  
Latitudinal Trend

AbstractThe Foothills Erratics Train consists of large quartzite blocks of Rocky Mountains origin deposited on the eastern slopes of the Rocky Mountain Foothills in Alberta between ~53.5°N and 49°N. The blocks were deposited in their present locations when the western margin of the Laurentide Ice Sheet (LIS) detached from the local ice masses of the Rocky Mountains, which initiated the opening of the southern end of the ice-free corridor between the Cordilleran Ice Sheet and the LIS. We use 10Be exposure dating to constrain the beginning of this decoupling. Based on a group of 12 samples well-clustered in time, we date the detachment of the western LIS margin from the Rocky Mountain front to ~14.9 ± 0.9 ka. This is ~1000 years later than previously assumed, but a lack of a latitudinal trend in the ages over a distance of ~500 km is consistent with the rapid opening of a long wedge of unglaciated terrain portrayed in existing ice-retreat reconstructions. A later separation of the western LIS margin from the mountain front implies higher ice margin–retreat rates in order to meet the Younger Dryas ice margin position near the boundary of the Canadian Shield ~2000 years later.

Revised chronology of northwest Laurentide ice-sheet deglaciation from beryllium-10 exposure-dated erratics on the western Canadian Shield

2020 ◽  
Author(s):  
Alberto Reyes ◽  
Anders Carlson ◽  
Jesse Reimink
Keyword(s):  
Climate Change ◽  
Sea Level Rise ◽  
Sea Level ◽  
Ice Sheet ◽  
Canadian Shield ◽  
Laurentide Ice Sheet ◽  
Exposure Dating ◽  
Poor Region ◽  
A Site ◽  
Exposure Ages

<p>The timing of northwest Laurentide ice-sheet deglaciation is important for understanding how ice-sheet retreat, and associated meltwater discharge, may have been involved in abrupt climate change and rapid sea-level rise at the end of the last glaciation. However, the deglacial chronology across the western Canadian Shield is poorly understood, with only a handful of minimum-limiting <sup>14</sup>C dates and sparse cosmogenic nuclide exposure dates constraining the timing and pattern of northwest Laurentide ice-sheet retreat across >1000 km of ice-sheet retreat to the marine limit west of Hudson Bay. We present cosmogenic <sup>10</sup>Be surface exposure dating of glacial erratics at two sites, within a ~160,000 km<sup>2</sup> region with no reliable temporal constraints on ice-margin retreat, to directly date the timing of northwest Laurentide ice-sheet deglaciation. Six erratics perched directly on bedrock at a site on the western edge of the Slave Craton have exposure ages between 12.8±0.6 and 12.2±0.6 thousand years ago (ka; ±1sigma). Five erratics on bedrock, sampled at a site 115 km up-ice to the east, yielded exposure ages between 10.8±0.5 and 11.6±0.5 ka. When corrected for decreased atmospheric depth due to isostatic uplift since deglaciation, the error-weighted mean ages for the two sites indicate that the Laurentide ice sheet retreated through this region of the western Canadian Shield between 13.3±0.2 and 11.8±0.2 ka, or at least 1 kyr earlier than inferred from the canonical compilation of minimum-limiting <sup>14</sup>C dates for deglaciation and paleo-glaciological models. We tentatively infer a preliminary ice-margin retreat rate of ~0.1 m kyr<sup>-1 </sup>over this interval spanning much of the Younger Dryas which, compared to earlier estimates, implies a substantially lower volume of meltwater flux from the retreating northwest Laurentide ice sheet at this time.  Additional exposure ages on glacial erratics across this data-poor region are needed for validation of existing deglacial ice-sheet models, which can in turn contribute to comprehensive testing of hypotheses related to northwest Laurentide ice-sheet retreat rate, abrupt deglacial sea-level rise, and potential forcing of associated climate change events.</p>

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