Minor inheritance inhibits the calibration of the 10Be production rate from the AD 1717 Val Ferret rock avalanche, European Alps

To calibrate the in situ Be-10 production rate, we collected surface samples from nine large granitic boulders within the deposits of a rock avalanche that occurred in AD 1717 in the upper Ferret Valley, Mont Blanc Massif, Italy. The Be-10 concentrations were extremely low and successfully measured within 10% analytical uncertainty or less. The concentrations vary from 4829 +/- 448 to 5917 +/- 476 at g(-1). Using the historical age exposure time, we calculated the local and sea level-high latitude (i.e. >= 60 degrees) cosmogenic Be-10 spallogenic production rates. Depending on the scaling schemes, these vary between 4.60 +/- 0.38 and 5.26 +/- 0.43 at g(-1) a(-1). Although they correlate well with global values, our production rates are clearly higher than those from more recent calibration sites. We conclude that our Be-10 production rate is a mean and an upper bound for production rates in the Massif region over the past 300 years. This rate is probably influenced by inheritance and will yield inaccurate (e.g. too young) exposure ages when applied to surface-exposure studies in the area. Other independently dated rock-avalanche deposits in the region that are approximately 10(3) years old could be considered as possible calibration sites.