CLIMATIC OSCILLATIONS IN THE BIOSPHERE

Abstract

The history of the biosphere is a peculiar mixture of patterned and random processes, which are not easily untangled. Three sets of patterned processes are here discussed: A major climatic cycle with a period (?) of around 300 m.y.; an intermediate one in the 30 m.y. range; and a family of cycles in the 10,000 to 100,000 year domain. The long cycle is here interpreted as one in atmospheric carbon dioxide pressure, leading to an alternation of “greenhouse” and “icehouse” states. The origin is attributed to changes in the patterns and vigor of mantle convection, and two related phenomena: volcanism and fluctuations in sea level. The causes of the 30 m.y. rhythm remain enigmatic. The 10,000-100,000 year rhythms seem to be related to the Earth's orbital perturbations, as suggested long ago. These environmental fluctuations stress organic communities in various ways, and have been instrumental in steering their evolution.