Renewed Debate about Icehouse-Greenhouse Oscillations
Vrije Universiteit/Earth & Life Sciences, Amsterdam, Netherlands
The concepts of icehouse world and greenhouse world are widely used to characterize variations of the ocean-atmosphere system and predict characteristics of the associated sediment record. However, it is important to realize that the earlier unanimity on timing and causes of the icehouse-greenhouse oscillations has given way to a rather controversial debate. There now are two competing models that alternately invoke endogenic and celestial drivers. The classical icehouse-greenhouse cycle of the early 80's postulates that in the past 600 Ma the Earth has oscillated between a glacial mode and one of warm, equable climate; correlative changes in mineralogy of marine carbonates and evaporites as well as abundance of igneous intrusions point to an endogenic cause, such as variations in the rate of mantle convection. However, the 300-Ma icehouse-greenhouse cycle leaves important data unexplained, e.g. the Ordovician glaciation and the growing evidence for Late Jurassic and Early Cretaceous glaciations. The history of glaciations as well as sea-surface temperatures derived from oxygen isotopes indicate oscillations with wave lengths of 130–150 Ma. Such oscillations have been observed in the reconstructed cosmic-ray flux of the past 1000 Ma; they may modulate climate via low-level cloud cover. Observational evidence for both 300 Ma and 130-150 Ma oscillations in the Phanerzoic and late Proterozoic is strong and the sediment record probably reflects the combined effect of endogenic and celestially driven oscillations of the ocean-atmosphere system.