Cosmogenic radionuclides from fiord landscapes support differential erosion by overriding ice sheets

Abstract

The interpretation of differentially weathered mountainous areas along the fringes of Pleistocene ice sheets is fundamental for determining ice-sheet behavior and thickness during the last glaciation. Two existing interpretations are either that highly weathered uplands remained as nunataks while freshly eroded troughs held outlet glaciers during the last glaciation or that uplands and lowlands were equally covered by ice, but that it was differentially erosive as a function of its spatially variable basal thermal regime. Cosmogenic radionuclide measurements from 33 bedrock samples and 27 upland erratics from differentially weathered fi ord landscapes on northeastern Baffi n Island shed light on Laurentide Ice Sheet (LIS) dynamics and thickness. Tors on weathered upland surfaces have minimum 10 Be ages between ca. 50 and ca. 170 ka (n = 12), whereas the majority of erratics perched in the uplands range from ca. 10 to ca. 13 ka (n = 14), indicating that the whole landscape was glaciated during the Last Glacial Maximum (LGM). Glacially sculpted bedrock (n = 7) near sea level refl ects the age of deglaciation, and increasing amounts of isotopic inheritance in highelevation sculpted bedrock (n = 3), higher elevation intermediately weathered bedrock (n = 11), and highest elevation intensely weathered bedrock refl ect the weakening of erosive power as the ice sheet transitioned from fi ords to interfi ord plateaus. Cold-based glaciation of uplands was contemporaneous with warm-based glaciation in fi ords, suggesting the presence of ice streams. 26 Al and 10 Be concentrations measured in 10 bedrock samples indicate that tors have experienced a complex history of alternating periods of exposure, burial, and limited glacial erosion. The paired isotope data reveal a minimum duration of burial of 80-500 k.y., presumably by cold-based ice, and indicate that the tors have an age of at least 150-580 ka. These data, together with other reconstructions of ice streams in the eastern Canadian Arctic, suggest that ice streams were a large infl uence on northeastern LIS dynamics throughout the late Quaternary. Thus, the northeastern LIS was sensitively tied with North Atlantic thermohaline circulation and abrupt climate change.

Landscape evolution of Antarctica

“…In such situations ice can be essentially protective and leave little sign of erosion. There is debate as to how protective the ice may be (Cuffey et al, 2000), but recent work on cosmogenic isotope analysis demonstrating the age of exposure and the time buried beneath ice has shown that the hypothesis holds in many areas of the Northern Hemisphere (Briner et al, 2006;Stroeven et al 2002).…”

Deglaciation of the Laurentide Ice Sheet from the Last Glacial Maximum

“…The most dramatic changes to the LLGM extent of the LIS in the last decade has seen the ice margin extended to cover the >70,000 km 2 Banks Island in the Western Canadian Arctic (England et al, 2009;Lakeman et al, 2012Lakeman et al, , 2013 and the recognition that it likely extended to (or close to) the continental shelf edge in Baffin Bay (Briner et al, 2006) and in Atlantic Canada (Shaw et al, 2006) (see Fig 4). Banks Island had long been regarded as an ice-free refugium (Prest, 1969;Vincent, 1982;Dyke and Prest, 1987) and was portrayed as such in the most recent Dyke et al (2003) synthesis (see also Dyke, 2004).…”

Constraining multi-stage exposure-burial scenarios for boulders preserved beneath cold-based glacial ice in Thule, northwest Greenland

“…Cold-based ice also existed in mid-latitude regions, especially at high elevations (Bierman 45 et al, 2015) and along thin ice sheet margins (Colgan et al, 2002). Since cosmic rays attenuate 46 as they pass through Earth materials at a rate controlled by density, burial by ~10 m of ice causes 47 production of nuclides by spallation to become negligible (Lal, 1988 1999; Briner et al, 2003;Briner et al, 2006;Corbett et al, 2013;Håkansson et al, 2008;Kaplan 56 et al, 2001;Marquette et al, 2004;Stroeven et al, 2002;Sugden et al, 2005).…”