Very low inheritance in cosmogenic surface exposure ages of glacial deposits: A field experiment from two Norwegian glacier forelands

Abstract

Terrestrial cosmogenic nuclide dating has been widely used to estimate the surface exposure age of bedrock and boulder surfaces associated with deglaciation and Holocene glacier variations, but the effect of inherited age has been rarely directly addressed. In this study, small clasts, embedded in flute surfaces on two cirque glacier forelands in Jotunheimen, southern Norway and deposited within the last ~60 years, were used to test whether such clasts have the modern surface exposure age expected in the absence of inheritance. Two different approaches were taken involving dating of (1) a single clast of cobble size from the proglacial area of Austanbotnbreen, and (2) 75 clasts mostly of pebble size from the proglacial area of Storbreen crushed and treated as a single sample. 10 Be surface exposure ages were 99 ± 98 and 368 ± 90 years, respectively, with 95% confidence (±2σ). It is concluded that (1) these small glaciers have eroded and deposited rock fragments with a cosmogenic zero or near-zero concentration, (2) the likelihood of inherited cosmogenic nuclide concentrations in similar rock fragments deposited by larger warm-based glaciers and ice sheets should be small, and (3) combining a large number of small rock particles into one sample rather than using single large clasts of boulder size may provide a viable alternative to the commonly perceived need for five or more independent estimates of exposure age per site.

<sup>10</sup>Be-based exploration of the timing of deglaciation in two selected areas of southern Norway

“…The knowledge of the vertical dimension of the LGM ice sheet could provide crucial information on palaeoenvironmental factors like sea-level changes, atmospheric and oceanic circulation, (de-)glaciation patterns, icesheet erosion rates, landscape evolution, and englacial thermal boundaries (Winguth et al, 2005;Rinterknecht et al, 2006;Goehring et al, 2008). The interpretation of bedrock with different degree of weathering in mountain areas affected by Quaternary glaciation can, therefore, be important for determining ice-sheet behaviour and thickness during the last glaciation periods (Brook et al, 1996;Briner et al, 2006;McCarroll, 2016). There are several concepts to explain the limit between differently weathered bedrock (trimline) separating highly weathered uplands comprising blockfields and tors from relatively unweathered lower exposures of freshly eroded glacial features (Rea et al, 1996;Briner et al, 2006).…”

Investigating Holocene mountain glaciations: a plea for the supremacy of glacial geomorphology when reconstructing glacier chronologies

“…Field experiments and selected tests have persuasively shown that the zero-age assumption (i.e. fulfillment of the 'no-inheritance' requirement) with the cosmogenic nuclide dating of rock fragments recently deposited at modern temperate glaciers is a valid one (Putnam et al 2012;ScHimmelPfenniG et al 2014;mattHeWS et al 2017). ScHaefer et al (2009) come to the same conclusion, yet surprisingly miss to draw related consequences for the procedure to obtain their final age estimates for boulder clusters.…”