Felsenmeer persistence under non-erosive ice in the Torngat and Kaumajet mountains, Quebec and Labrador, as determined by soil weathering and cosmogenic nuclide exposure dating

Abstract

Soil analyses and terrestrial cosmogenic nuclide exposure dating are combined and a conceptual model proposed to explain altitudinal weathering contrasts in high-latitude highlands. We show that summits in the Torngat and Kaumajet mountains were covered by ice during the Last Glacial Maximum, and that their felsenmeer cover probably survived multiple glaciation events. For similar lithologies, soils on felsenmeer covered summits are signigicantly more weathered than those below the felsenmeer limit, displaying higher concentrations of crystalline iron, amorphous aluminium, and silicium extracted with oxalate. Secondary minerals such as gibbsite and kaolinite occur in felsenmeer soils, whereas those formed in till lacked these secondary minerals. 10 Be and 26 Al exposure ages for nine of ten samples, from high-elevation tors and autochthonous felsenmeer blocks, range from 73 ± 6 to 157 ± 15 ka. By contrast, ages of 11.4 ± 1.0 and 11.7 ± 1.0 ka are measured for bedrock in the much lower Saglek zone, indicating extensive (>3 m) glacial erosion of this zone during Late Wisconsinan glaciation. 26 Al/ 10 Be ratios demonstrate that exposure of the high-elevation surfaces was interrupted during at least one cosmic ray shielding event by either ice or till cover. In either case, Late Wisconsinan glaciers could not have extensively eroded these surfaces. Five erratics dated above the Saglek zone, including one in the felsenmeer zone, have exposure ages ranging from 11.6 ± 1.0 to 13.6 ± 0.7 ka. This indicates that valley and high-elevation ice persisted through the Younger Dryas Chron and provides further evidence that the highlands were not nunataks during the Late Wisconsinan period.Résumé : Des analyses de sols et la datation d'affleurements ayant des nucléides cosmogéniques terrestres sont combinées et un modèle conceptuel est proposé pour expliquer les contrastes d'altération selon l'altitude dans les hautes terres à des latitudes élevées. Nous démontrons que les sommets dans des monts Torngat et Kaumajet étaient couverts par de la glace au cours du dernier maximum glaciaire et que leur couverture de felsenmeer a probablement subi de multiples événements de glaciation. Pour les lithologies semblables, les sols formés dans les zones de felsenmeer sont significativement plus altérés que ceux formés aux plus basses altitudes. Les tills ont des concentrations plus élevées de fer cristallin, d'aluminium amorphe et de silice extraite par oxalate. Les minéraux secondaires tels que la gibbsite et la kaolinite se retrouvent dans les sols des felsenmeers, alors que ceux formés dans le till ne présentent pas ces minéraux secondaires. Les âges d'exposition obtenus par 10 Be et 26 Al, pour neuf des dix échantillons, provenant de roches isolées de haute élévation et de blocs de felsenmeer autochtones, varient de 73 ± 6 à 157 ± 15 ka. Cependant, des âges de 11,4 ± 1,0 ka et de 11,7 ± 1,0 ka sont mesurés pour le socle dans la zone beaucoup plus basse de Saglek, indiquant une forte (>3 m) érosion glaciaire dans cette zone durant la gla...

Landscape evolution of a bedrock peneplain on the southern Tibetan Plateau revealed by in situ-produced cosmogenic 10Be and 21Ne

“…This inference, however, does not preclude the accumulation of cold-based ice in the peneplain region. For instance, flat summit surfaces in Labrador, Canada, were buried under non-erosive ice during past glaciations as revealed by erosion island plots based on cosmogenic 10 Be and 26 Al (Marquette et al, 2004). In our case, the combination of cosmogenic 10 Be and 21 Ne concentrations does not provide any evidence for significant periods of burial, because six out of seven samples have a nuclide inventory that is consistent with a simple exposure history (Fig.…”

The altitude of the last ice sheet in Caithness and east Sutherland, northern Scotland

“…The observed altitudinal contrast between glacially eroded terrain, and summits that support tors, blockfields and shattered bedrock, can be explained in two ways (Ballantyne 1997(Ballantyne , 2007Ballantyne et al 1998Ballantyne et al , 2006Ballantyne et al , 2007Fabel et al 2002;Marquette et al 2004;Sugden et al 2005). First (hypothesis 1), the boundary between the two may represent the upper limit of the last ice sheet at the LGM, implying that glacially unmodified summits and plateaux remained above the level of the last ice sheet as nunataks, thus constraining maximum ice-sheet thickness.…”

“…Of the remaining species detected, both kaolinite and gibbsite are significantly better represented in samples from above the inferred upper limit of glacial erosion, at p < 0.02 and p < 0.1 respectively ( 2 test). Gibbsite has been shown to be significantly better represented in soil samples from summits that escaped LGM glacial erosion elsewhere in Scotland (Ballantyne 1994;Ballantyne et al 1998) as well as in Wales (McCarroll & Ballantyne 2000), Ireland (Rae et al 2004;Ballantyne et al 2006Ballantyne et al , 2007 and Québec-Labrador (Marquette et al 2004). Studies of gibbsite in montane soils and lowland grusses in Scotland show that it represents a product of pre-Devensian weathering and develops preferentially in acid lithologies in the early stages of saprolite formation (Wilson 1985;.…”

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).…”