Cosmic Ray Diffusion from the Galactic Spiral Arms, Iron Meteorites, and a Possible Climatic Connection

We construct a Galactic cosmic ray (CR) diffusion model. The CR flux reaching the Solar System should periodically increase with each crossing of a Galactic spiral arm. We confirm this prediction in the CR exposure age record of iron meteorites. We also find that although the geological evidence for the occurrence of ice-age epochs in the past eon is not unequivocal, it appears to have a nontrivial correlation with the spiral arm crossings and the CR flux variability—agreeing in period and phase.

Figure 1

The past eon: (a) Past Galactic spiral arm crossings assuming $\Delta \Omega =10.9(\mathrm{k}\mathrm{m}{\mathrm{s}}^{-1})/\mathrm{k}\mathrm{p}\mathrm{c}$. (b) The CRF reaching the Solar System using the CR diffusion model. Note that the CRF lags behind spiral arm crossings. This is portrayed by the hatched regions, which qualitatively show the predicted occurrences of IAEs if the CRF required to trigger them is the average flux. Arrows mark the middle of the spiral crossing and the expected midglaciation point. (c) Qualitative descriptions of the geologically recorded IAEs—its top half, as summarized by Crowell [9], who concluded that the current evidence is insufficient to support a claim of periodicity, while the bottom half, as summarized by Frakes et al. [10], who claim a periodicity exists. By fine-tuning the observed pattern speed of the arms to best fit the IAEs, an intriguing correlation appears between the IAEs and their prediction. Note that the correlation need not be absolute since additional factors may affect the climate. Other factors that should be considered and noted in the graph are the following: (1) The mid-Mesozoic glaciations are significantly less extensive as others. (2) It is unclear to what extent the period around 700 Myr BP was warmer than the IAEs before or after. (3) The first IAE of the neo-proterozoic (if indeed distinct) is very uncertain. (4) Since Norma’s crossing is an extrapolation from smaller Galactic radii, its location is uncertain. If the arm’s structure at smaller radii is indeed different [11], its preferred location will lag by about 20 Myr. (d) A 1-2-1 averaged histogram of the ${}^{41}\mathrm{K}/{}^{40}\mathrm{K}$ exposure ages of Fe meteorites, which are predicted to cluster around the CRF minima. The cluster-IAE correlation further suggests an extraterrestrial trigger for the glaciations.

Figure 2

A folded 1-2-1 averaged histogram (over the IAE periodicity) of the Fe meteorites’ exposure ages. A statistically significant signal is obtained for a period of $143\pm 10\mathrm{M}\mathrm{y}\mathrm{r}$. The dashed line is the CR diffusion model prediction. We find that the predicted phase of the cluster peaks agrees with the actual exposure age clustering. The clustering peaks 100 Myr after the spiral crossing (dotted lines), implying that it is unlikely to be a real age clustering peak, which should be related directly to the spiral crossing (e.g., by Oort cloud comets that are perturbed during arm crossing, that subsequently break asteroids more frequently, and that somehow affect the terrestrial climate).