Regional Be production rate calibration for the past 12 ka deduced from the radiocarbon-dated Grøtlandsura and Russenes rock avalanches at 69° N, Norway |
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| Authors: | Cassandra R Fenton Reginald L Hermanns Lars H Blikra Peter W Kubik Charlotte Bryant Samuel Niedermann Anette Meixner Mirjam M Goethals |
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| Institution: | aHelmholtz-Zentrum Potsdam, Deutsches GeoForschungsZentrum, Telegrafenberg, D-14473 Potsdam, Germany;bInternational Centre for Geohazards, Norges Geologiske Undersøkelse, Leiv Eirikssons vei 39, N-7491 Trondheim, Norway;cÅknes/Tafjord Early-Warning Centre, Ødegårdavegen 176, N-6200 Stranda, Norway;dLaboratory of Ion Beam Physics ETH Zurich, CH-8093 Zurich, Switzerland;eNERC Radiocarbon Laboratory at Scottish Universities Environmental Research Centre, Scottish Enterprise Technology Park, East Kilbride G75 0QF, United Kingdom;fDepartment of Geosciences, University of Bremen, 28334 Bremen, Germany;gGeologisch-Paläontologisches Institut, Westfälische Wilhelms-Universität Münster, Corrensstr. 24, D-48149 Münster, Germany |
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| Abstract: | Two rock avalanches in Troms County – the Grøtlandsura and Russenes – were selected as CRONUS-EU natural cosmogenic 10Be production-rate calibration sites because they (a) preserve large boulders that have been continuously exposed to cosmic irradiation since their emplacement; (b) contain boulders with abundant quartz phenocrysts and veins with low concentrations of naturally-occurring 9Be (typically < 1.5 ppb); and (c) have reliable minimum radiocarbon ages of 11,424 ± 108 cal yr BP and 10,942 ± 77 cal yr BP (1σ), respectively. Quartz samples (n = 6) from these two sites contained between 4.28 × 104 and 5.06 × 104 at 10Be/g using the 1.387 Myr 10Be half-life. Determination of these concentrations accounts for topographic and self-shielding, and effects on nuclide production due to isostatic rebound are shown to be negligible. Persistent, constant snow and moss cover cannot be proven, but if taken into consideration they may have reduced 10Be concentrations by 10%. Using the 10Be half-life of 1.387 Myr and the Stone scaling scheme, and accounting for snow- and moss-cover, we calculate an error-weighted mean total 10Be production rate of 4.12 ± 0.19 at/g/yr (1σ). A corresponding error-weighted mean spallogenic 10Be production rate is 3.96 ± 0.16 at/g/yr (1σ), respectively. These are in agreement within uncertainty with other 10Be production rates in the literature, but are significantly, statistically lower than the global average 10Be production rate. This research indicates, like other recent studies, that the production of cosmogenic 10Be in quartz is lower than previously established by other production-rate calibration projects. Similarly, our findings indicate that regional cosmogenic production rates should be used for determining exposure ages of landforms in order to increase the accuracy of those ages. As such, using the total 10Be production rate from our study, we determine an error-weighted mean surface-exposure age of a third rock avalanche in Troms County (the Hølen avalanche) to be 7.5 ± 0.3 kyr (1σ). This age suggests that the rock avalanche occurred shortly after the 8.2 kyr cooling event, just as the radiocarbon ages of the Grøtlandsura and Russenes avalanches confirm field evidence that those rock-slope failures occurred shortly after deglaciation. |
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| Keywords: | Cosmogenic Spallogenic Beryllium-10 Quartz Norway Landslides Production rate calibration |
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