Distribution of Cosmic Rays in the Galaxy -- a Link to Biodiversity Cycles

The spectral analysis of fluctuations of biodiversity (Rohde \& Muller, 2005) and the subsequent re-analysis of the diversity record, species origination and extinction rates, gene duplication, etc (Melott \& Liebermann, 2007) indicate the presence of a 62$\pm$3My cyclicity, for the last 500My. Medvedev \& Melott (2006) proposed that the cyclicity may be related to the periodicity of the Solar motion with respect to the Galactic plane, which exhibits a 63My oscillation, and the inhomogeneous distribution of Cosmic Rays (CR) throughout the Milky Way, which may affect the biosphere by changing mutation rate, climate, food chain, etc. Here we present a model of CR propagation in the Galactic magnetic fields, in the presence of both the mean field gradient and the strong MHD turbulence in the interstellar medium. We explore the ``magnetic shielding effect'' as a function of CR energy and composition and estimate the resultant flux of mutagenic secondary muons at the Earth surface.