The constancy of galactic cosmic rays as recorded by cosmogenic nuclides in iron meteorites

Smith, Thomas; Cook, David L.; Merchel, Silke; Pavetich, Stefan; Rugel, Georg; Scharf, Andreas; Leya, Ingo (2019). The constancy of galactic cosmic rays as recorded by cosmogenic nuclides in iron meteorites. Meteoritics & planetary science, 54(12), pp. 2951-2976. Meteoritical Society at the University of Arkansas, Dept. of Chemistry and Biochemistry 10.1111/maps.13417

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We measured the He, Ne, and Ar isotopic concentrations and the ¹⁰Be, ²⁶Al, ³⁶Cl, and ⁴¹Ca concentrations in 56 iron meteorites of groups IIIAB, IIAB, IVA, IC, IIA, IIB, and one ungrouped. From ⁴¹Ca and ³⁶Cl data, we calculated terrestrial ages indistinguishable from zero for six samples, indicating recent falls, up to 562 ± 86 ka. Three of the studied meteorites are falls. The data for the other 47 irons confirm that terrestrial ages for iron meteorites can be as long as a few hundred thousand years even in relatively humid conditions. The ³⁶Cl-³⁶Ar cosmic ray exposure (CRE) ages range from 4.3 ± 0.4 Ma
to 652 ± 99 Ma. By including literature data, we established a consistent and reliable CRE
age database for 67 iron meteorites. The high quality of the CRE ages enables us to study
structures in the CRE age histogram more reliably. At first sight, the CRE age histogram
shows peaks at about 400 and 630 Ma. After correction for pairing, the updated CRE age
histogram comprises 41 individual samples and shows no indications of temporal
periodicity, especially not if one considers each iron meteorite group separately. Our study
contradicts the hypothesis of periodic GCR intensity variations (Shaviv 2002, 2003),
confirming other studies indicating that there are no periodic structures in the CRE age
histogram (e.g., Rahmstorf et al. 2004; Jahnke 2005). The data contradict the hypothesis
that periodic GCR intensity variations might have triggered periodic Earth climate changes.
The ³⁶Cl-³⁶Ar CRE ages are on average 40% lower than the ⁴¹K-K CRE ages (e.g.,
Voshage 1967). This offset can either be due to an offset in the 41K-K dating system or due to a significantly lower GCR intensity in the time interval 195–656 Ma compared to the
recent past. A 40% lower GCR intensity, however, would have increased the Earth
temperature by up to 2 °C, which seems unrealistic and leaves an ill-defined ⁴²K-K CRE
age system the most likely explanation. Finally, we present new ²⁶Al/²¹Ne and ¹⁰Be/²¹Ne
production rate ratios of 0.32 ± 0.01 and 0.44 ± 0.03, respectively.

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