Abstract
Calculating cosmogenic-nuclide surface-exposure ages is critically dependent on a knowledge of the altitude of the sample site. Changes in altitude have occurred through time as a result of glacial isostatic adjustment (GIA), potentially altering local nuclide production rates and, therefore, surface-exposure ages. Here we assess the impact of GIA on surface-exposure dating by calculating global time-dependent production rates since the Last Glacial Maximum using surface elevations that were corrected and uncorrected for GIA. We find that the magnitude of the GIA effect is spatially and temporally variable. Nuclide production could be reduced by up to 50% in the interior of large ice masses (in North America, Scandinavia and West Antarctica) at times of maximum glacial isostatic depression. Although smaller, the effect is still significant at ice sheet margins, where nuclide production is reduced by >5% and potentially >10%, making exposure ages older in those areas. Away from the ice sheet margins, land surfaces can be isostatically elevated, which can increase nuclide production by >5% and, therefore, make exposure ages younger. Areas that were more recently exposed or that are distal to large ice masses will generally be less affected. Importantly, we find that the effect at the primary 10Be production calibration sites is <1%. Applying a GIA correction to surface-exposure data may help resolve mismatches between some chronologies, but not necessarily in all regions, implying that additional factors may need to be considered. Past atmospheric changes can amplify or reduce the impact of GIA on nuclide production, and the combined effects should be fully accounted for in the future. These time-dependent influences on surface-exposure dating have potentially important implications for interpreting chronologies and for using the data to constrain ice sheet models.
Highlights
Cosmogenic-nuclide surface-exposure dating is a widely used approach for constraining the timing of past geomorphic events and, in particular, for reconstructing ice margin history during the Quaternary
Our calculations were done for grid cells inside as well as outside of former ice sheet margins in order to highlight the effects at potential ice-free areas in the ice sheet interior
Production at the peripheral bulge of large ice masses increases by up to 8% when land surface elevations are corrected for glacial isostatic adjustment (GIA) (Fig. 2)
Summary
Cosmogenic-nuclide surface-exposure dating is a widely used approach for constraining the timing of past geomorphic events and, in particular, for reconstructing ice margin history during the Quaternary. The technique is heavily dependent on a knowledge of the nuclide production rate and how this rate has varied through time (Gosse and Phillips, 2001; Lifton et al, 2014). The altitude of a rock surface exerts a fundamental control on nuclide production and, if not appropriately estimated, can lead to an inaccurate surface-exposure age. Over glacial-interglacial cycles, the expansion and recession of ice sheets caused the altitude of the land surface to change through a process called glacial isostatic adjustment (GIA) The aim of this paper is to provide a first global assessment of the impact of GIA on nuclide production and the implications for surfaceexposure dating.
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