Schmidt-hammer R-values from glacially-scoured bedrock surfaces across glacier-foreland boundaries: Insights into Holocene weathering rates with implications for exposure-age dating

Schmidt -hammer R -values were measured on glacially -scoured bedrock outcrops located inside and outside of 11 'Little Ice Age ' glacier -foreland boundaries in the Jotunheimen and Jostedalsbreen regions of southern Norway. Analysing paired samples differing in exposure age by -10,000 years constitutes a field experiment on chemical weathering rates within and between regions. Mean R -values ( +/- 95 % confidence intervals) from inside sites were 65.9 +/- 0.6 and 66.9 +/- 0.6 for rock surfaces composed of pyroxene-granulite gneiss in Jotunheimen at altitudes of 990 - 1360 m above sea level and granite and granitic gneiss in the Jostedalsbreen region at 270 - 620 m a.s.l. The corresponding values from outside sites of 39.9 +/- 0.9 and 39.0 +/- 0.9 were significantly lower, indicating a higher degree of chemical weathering. In contrast, regional differences in mean R -values were insignificant. A similar pattern is reflected in indices of rock weathering (39.0 % for Jotunheimen and 41.6 % for Jostedalsbreen), and weathering rate (2.8 R -value units and 3.0 units per 1000 years, respectively). These results imply an estimated minimum age resolution of Schmidt -hammer exposure -age dating of -265 - 315 years and a maximum age range of -20,000 years. They suggest potential application of the Schmidt hammer to both studies of weathering rates and exposure -age dating at the regional scale, despite lithological variation associated with different rock types and climatic variation associated with altitudinal differences of up to 1000 m between the sites.