GALACTIC DARK-MATTER AND TERRESTRIAL PERIODICITIES

Analysis of recent geological databases reveals the presence of a simi lar to 26.5 Myr periodicity in the terrestrial record of the last simi lar to 200 Myr. The same periodicity is found to be present, at a high confidence level, in the record of impact cratering on Earth. It thus appears that global disturbances are modulated or caused largely by e xogenous forcing. The likely source is a periodic comet influx caused by Oort cloud disturbance as the Sun oscillates vertically through the Galactic disk. The Earth may thus be regarded as a probe of the disk environment; and to account for the periodicity, the Galactic disk is required to have a substantial dark matter component (similar to 0.15 M. pc(-3)). The purpose of this paper is to review the relevant observ ational framework, and to describe the basic astro- and geophysical pr ocesses which now seem to underpin evolution on Earth. Attention is dr awn in particular to recent oceanic sediment evidence which seems to c onfirm a continuous sequence of events from periodic Galactic forcing through the influx of giant comets (diameter greater than or similar t o 100 km) into stable sub-Jovian orbits at the rate of similar to 10 M yr(-1) and the resulting climatic disturbances due to their disintegra tion in near-Earth space. This rate, producing episodic multiple threa ts to the Earth by bodies in the sub-kilometre range, is numerically c omparable to the long-term cratering flux to Earth owing to 'one-off' greater than or similar to 1 km asteroids and comets. The resulting cu rrent hazard to Earth, amounting to a more substantial input and explo sion megatonnage in toto, is both more complex and more profound.