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

Abstract

The 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.

Late Upper Paleolithic occupation at Cooper’s Ferry, Idaho, USA, ~16,000 years ago

“…(95.4% confidence level) within this timeframe of initial population expansion. The migration route these peoples employed to initially enter North America is hypothesized to have occurred either via an interior migration from eastern Beringia southward through a deglaciated ice-free corridor (IFC) that opened between continental ice sheets during the late Pleistocene (16,35) or by a combination of boat transport and walking south along the margin of glaciated and unglaciated Pacific shorelines (35)(36)(37)(38)(39)(40). Models favoring migration through an IFC argue for its opening by~14,800 cal yr B.P., providing time for humans to migrate from eastern Beringia and move throughout the Americas shortly before the appearance of the CPT (36).…”

Using10Be dating to determine when the Cordilleran Ice Sheet stopped flowing over the Canadian Rocky Mountains

“…3b), (4) eastwardly transported glacial erratics along the Rocky Mountain Foothills (Bednarski and Smith, 2007), and (5) the presence of volcanic-rich Cordilleran till east of the Rocky Mountains (Mathews, 1978;Bednarski and Smith, 2007;Hartman et al, 2018). Margold et al (2018Margold et al ( , 2019 incorporated ice flow over the Hart Ranges into a broader, regional picture of the ice sheet configuration, showing the ice drainage pattern at, and shortly after, the lLGM. According to their reconstructions, which integrated data on the glacial geomorphology (Margold et al, 2015a, b) with information from glacial isostasy (Peltier, 2004;Peltier et al, 2015, Lambeck et al, 2017 and numerical ice sheet modelling (Tarasov and Peltier, 2004;Tarasov et al, 2012), an ice saddle connecting the CIS and Laurentide Ice Sheet (LIS) was located east of the Rocky Mountains at the lLGM.…”

“…The approximate position of the ice divide at the lLGM is shown with thick blue lines; ice flow direction is indicated by thin blue lines. The locations of Mount Morfee and Mount Spieker are given by the red stars; remaining stars show the locations of selected 10 Be cosmogenic nuclide ages relating to the deglaciation of the CIS from Margold et al (2014Margold et al ( , 2019, Darvill et al (2018), and Lesnek et al (2018Lesnek et al ( , 2020. All ages have been recalculated using the online calculator of Balco et al (2008;version 3.0) and corrected for glacioisostatic rebound.…”

“…Ice flow over the Rocky Mountains has thus far remained unconstrained in time, with the lLGM being defined by sub-till radiocarbon ages along the marine-terminating margin (e.g., Clague, et al, 1980;Blaise et al, 1990;Ward et al, 2003;Al-Suwaidi et al, 2006) and by the timing of the early stages of ice retreat along the ice sheet perimeter (Stroeven et al, 2010(Stroeven et al, , 2014Darvill et al, 2018;Lesnek et al, 2018Lesnek et al, , 2020Margold et al, 2019). Here we use in situ produced cosmogenic 10 Be dating on glacial erratic boulders located on two summit peaks on opposite sides of the Hart Ranges of the Rocky Mountains to determine when Cordilleran ice stopped flowing over the mountain range.…”

Whole‐genome resequencing reveals persistence of forest‐associated mammals in Late Pleistocene refugia along North America’s North Pacific Coast

“…Coastal refugia, and specifically those along the NPC, may have also played an integral role in human colonization of the Americas by providing a viable maritime dispersal corridor. New evidence shows the Pacific Northwest was inhabited by humans as early as >15-16 thousand years ago (ka; Davis et al, 2019;Devièse et al, 2018) and the Alexander Archipelago at least >10 ka, and likely >13 ka (Carlson & Baichtal, 2015;Dixon et al, 2014;Lesnek et al, 2018;Mackie et al, 2018;McLaren et al, 2018), potentially predating the opening of an ice-free dispersal corridor through central Alberta, Canada (≤14.8 ka; Margold et al, 2019). An incomplete fossil record and cosmogenic 10 Be exposure dating and 14 C age estimates from raised marine sediments, however, have created uncertainty over the existence of hypothesized coastal refugia and their associated paleoecological communities (Lesnek et al, 2018(Lesnek et al, , 2020.…”