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We assess if variations in the in situ cosmogenic 26Al/10Be production ratio expected from nuclear physics are consistent with empirical data, knowledge critical for two-isotope studies. We do this using 313 samples from glacially transported boulders or scoured bedrock with presumed simple exposure histories in the Informal Cosmogenic-nuclide Exposure-age Database (ICE-D) from latitudes between 53°S to 70°N and altitudes up to 5000 m above sea level. Although there were small systematic differences in Al/Be ratios measured in different laboratories, these were not significant and are in part explained by differences in elevation distribution of samples analyzed by each laboratory. We observe a negative correlation between the 26Al/10Be production ratio and elevation (p = 0.0005), consistent with predictions based on the measured energy dependence of nuclear reaction cross-sections and the spatial variability in cosmic-ray energy spectra. We detect an increase in the production ratio with increasing latitude, but this correlation is significant only in a single variate model, and we attribute at least some of the correlation to sample elevation bias because lower latitude samples are typically from higher elevations (and vice versa). Using 6.75 as the 26Al/10Be production ratio globally will bias two-isotope results at higher elevations and perhaps higher latitudes. Data reported here support using production rate scaling that incorporates such ratio changes, such as the LSDn scheme, to minimize such biases.
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Technical note: Altitude scaling of 36 Cl production from Fe
Abstract. Cosmogenic nuclide production rates depend on the excitation functions of the underlying nuclear reactions and the intensity and energy spectrum of the cosmic-ray flux. The cosmic-ray energy spectrum shifts towards lower average energies with decreasing altitude (increasing atmospheric depth), so production rate scaling will differ for production reactions that have different energy sensitivities. Here, we assess the possibility of the unique scaling of 36Cl production from Fe by modeling changes in the 36ClFe/36ClK and 36ClFe/10Beqtz production ratios with altitude. We evaluate model predictions against measured 36Cl concentrations in magnetite and K-feldspar and 10Be concentrations in quartz from granitic rocks exposed across an elevation transect (ca. 1700–4300 ma.s.l.) in western North America. The data are broadly consistent with model predictions. The null hypothesis that 36ClFe/10Beqtz and 36ClFe/36ClK production ratios are invariant with altitude can be rejected at the 90 % confidence level. Thus, reaction-specific scaling factors will likely yield more accurate results than non-reaction-specific scaling factors when scaling 36Cl production in Fe-rich rocks and minerals.
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- Award ID(s):
- 2300559
- PAR ID:
- 10575406
- Publisher / Repository:
- European Geophysical Union
- Date Published:
- Journal Name:
- Geochronology
- Volume:
- 6
- Issue:
- 4
- ISSN:
- 2628-3719
- Page Range / eLocation ID:
- 541 to 552
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/gchron-6-541-2024
