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

We measured the He, Ne, and Ar isotopic concentrations and the Be-10, Al-26, Cl-36, and Ca-41 concentrations in 56 iron meteorites of groups IIIAB, IIAB, IVA, IC, IIA, IIB, and one ungrouped. From Ca-41 and Cl-36 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 Cl-36-Ar-36 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 Cl-36-Ar-36 CRE ages are on average 40% lower than the K-41-K CRE ages (e.g., Voshage 1967). This offset can either be due to an offset in the K-41-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 degrees C, which seems unrealistic and leaves an ill-defined K-41-K CRE age system the most likely explanation. Finally, we present new Al-26/Ne-21 and Be-10/Ne-21 production rate ratios of 0.32 +/- 0.01 and 0.44 +/- 0.03, respectively.