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 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 of zero for six samples, indicating recent falls, up to 562+86 ka. Three of the studied meteorites are falls. The data therefore 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 improved quality of the CRE ages enables us to study structures in the CRE age histogramm more reliably. At first sight, the CRE age histogram shows peaks at about 400 Ma and 630 Ma. After correction for pairing, the updated CRE age histogram consists of 41 individual samples and shows no indications for periodic structures, especially not if one considers each group separately. Our study therefore clearly contradicts the popular hypothesis of periodic GCR intensity variations (Shaviv 2002,2003) but confirms other studies arguing that there are no periodic structures in the CRE age histogram (e.g., Rahmstorf et al. 2004; Jahnke 2005). Consequently, 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 significant lower GCR intensity in the time interval 195 Ma to 656 Ma compared to the recent past. A 40% lower GCR intensity, however, could increase the Earth temperature by up to 2°C, which seems unrealistic and leaves an ill-defined ⁴¹K-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 0.33+0.02, respectively, which we consider as reliable and robust.