A Multi-Radionuclide Approach for in-situ produced Terrestrial Cosmogenic Nuclides ¹⁰Be, ²⁶Al, ³⁶Cl and ⁴¹Ca

In-situ produced cosmogenic nuclides have proved to be valuable tools for quantifying Earth's surface processes. Here, the work-horses are ¹⁰Be and ²⁶Al in quartz-rich minerals, and ³⁶Cl in Ca- or K-rich minerals. Several attempts to find new matrix-product-pairs have been yet performed, especially with respect to broaden the time-scale to both more ancient [1] and more recent (historic) times. Thus, we have investigated other nuclides than ³⁶Cl as possible dating tools by cross-calibration in accompanied calcite- and quartz-rich samples from Antarctica (DV3 & Joh) and Southern France (Ciot).
AMS measurements of ¹⁰Be and ²⁶Al have been performed at the French 5 MV-AMS facility ASTER, ³⁶Cl at CAMS, LLNL, USA, and ⁴¹Ca at the Maier-Leibnitz-Laboratory. As we could only perform a single run for ⁴¹Ca measurements, all results can be regarded as preliminary.
Ratios between different nuclides from the same matrix (CaCO₃) and ratios of ¹⁰Be or ²⁶Al from CaCO₃ and SiO₂ can be compared with pure physical model calculations [2] giving us experimental terrestrial production rates for ¹⁰Be, ²⁶Al and ⁴¹Ca from Ca and CaCO₃.
As shown earlier [3], cosmogenic ¹⁰Be is highly contaminated with atmospheric ¹⁰Be and cannot be removed quantitatively from calcite samples, even by an improved chemical cleaning procedure [4]. Only working on clay-free calcite provides correct ¹⁰Be data, giving a 2.7 times higher production rate of ¹⁰Be from CaCO₃ than from SiO₂. Though, the production rate of ²⁶Al is only ~4.6% (CaCO₃ relative to SiO₂), ²⁶Al can be easily determined in calcite, as the low intrinsic ²⁶Al concentration yields to nearly as high ²⁶Al/²⁷Al as within corresponding quartz.
The measurement of ⁴¹Ca, mainly produced via thermal-neutron-capture, is hindered by very low ⁴¹Ca/Ca: <2.9x10^-15. Of course, ⁴¹Ca counting statistics are poor: Measured ⁴¹Ca/Ca values are based on total counts of 1-5. Nevertheless, the reproducibility (Joh & Joh-WC) is excellent. All our data are lower than the already published one from rock samples, i.e. lower than the six surface samples (3-63 x 10^-15) of Henning et al. [5] and Kutschera et al. [6] and lower than the surface and strongly shielded sample at 11 m depth (3.4-7.6 x 10^-15) of Middleton et al. [7].
The low ⁴¹Ca/Ca ratios make it very unlikely that ⁴¹Ca could be generally used for in-situ dating of calcareous environments, especially as there is little hope that background level for CaF₂-targets will improve, thus asking for very sophisticated and time-consuming CaH₂-target preparation and handling [9].

Acknowledgments
We appreciate the help of D. Lal (UCSD), M. Arnold and G. Aumaître (CEREGE), J. Lachner and I. Dillmann (TU Munich), S. Nardon (ENI S.p.A., Milan) and J. Borgomano (U Marseille). This work was partially funded within the framework of CRONUS-EU (Marie-Curie Action 6^th FP; #511927).
References
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[2] J. Masarik et al., NIMB 259 (2007) 642.
[3] S. Merchel et al., Quat. Geochronol. 3 (2008) 299.
[4] S. Merchel et al., NIMB, in review.
[5] W. Henning et al., Science 236 (1987) 725.
[6] W. Kutschera et al., Radiocarbon 31 (1989) 311.
[7] R. Middleton et al., Radiocarbon 30 (1989) 305.
[8] G. Korschinek and W. Kutschera, NIM144 (1977) 343.