Physical Driver for the 62-Myr Cycle in Fossil Diversity

The observation that fossil diversity is strongly modulated by two cycles, 62 Myr and 140 Myr, is one of the strangest discoveries of the last few years in geology and paleontology. (1) The cycles show strongest among those genera that are short-lived, i.e. endured for 45 Myr or less. In an attempt to identify patterns, we have subdivided the data into 170 groups consisting of various phyla, orders and classes, to examine the strength of the cycles; these results will be presented. As examples, the 62 Myr cycle is absent in the segmented worms (annelida), strong in the porifera, strong in the echinoderms, moderate in the bryosoa, moderate in the older anthropoda, very weak in the foraminifera, insignificant in the bivalves, very strong in the spiriferids, strong in trilobites, and very weak in fishes. No clear pattern has emerged from analysis of these taxonomic units. Several models will be discussed that could account for the cycles. One is the possibility of a helium-3 instability in the sun. Helium-3 is produced in the inner regions of the sun, but burned and destroyed in the deepest part of the core; only a layer outside the core remains. A regular overturn of the layer mixing it into the deeper core would result in a cycle of solar activity with a few percent modulation and a period that could be between 50 to 200 Myr.(2) This idea once received a great deal of attention when it was thought to be an explanation for the low flux of neutrinos from the sun. A second model to explain the cycle is the passage of the sun through variations in the mass density of the Milky Way Galaxy. Higher density puts an increased torque on comets in the Oort clould; that diminishes the angular momentum of half of them, and increases the eccentricity of the orbits. Doubling of the density (e.g. caused by passage through a giant molecular cloud or an arm of the galaxy) would result in a doubling of the number of comets hitting the earth. The passage through the arms of the galaxy has too long a period for the 62 Myr cycle, but could account for the 140 Myr cycle. We know that cometary impacts have major consequences for the diversity of life. We will discuss the possibility of such structure in the Milky Way. We will also present a description of other proposed explanations for the cycles, most of which can be ruled out. References: 1. Cycles in fossil diversity, Robert A. Rohde and Richard A. Muller, Nature vol. 454, pp. 208-210 (March 10, 2005). 2. The Solar Spoon, F. W. W. Dilke and D. O. Gough, Nature vol. 240, pp. 262-264; 293-294 (Dec. 1, 1972)