ONE-WAY AND QUASI-PERIODIC CLIMATE CHANGES: GEOLOGIC EVIDENCE

Abstract

During the last three billion years, the Earth has been experiencing a slow global cooling, expressed in the progressively increasing frequency and scale of glacial events. Based on these indications, geologic history can be divided into three climatic stages: (a) non-glacial (Early and Middle Archean), (b) with episodic glaciations (Late Archean to Middle Riphean), and (c) with frequent periodic glaciations (Late Riphean-present). The cooling has apparently been caused by changes in the density and composition of the atmosphere, by the waning of heat flow from the Earth's interior, and by an increase in the planet's albedo in response to continental growth. The irreversible climate trend was modulated and obscured by numerous superposed temperature oscillations of varying periodicity and amplitude. A broad spectrum of such oscillations, from ultralong-term (a few hundreds of million years) to decadal-scale, have been established. The Phanerozoic exhibits a hierarchy of coordinated periods of climatic oscillations of 10-12 ranks. Climatic oscillations of the first two or three ranks are to be detected in Proterozoic glacial sequences. In light of the above, the Earth's climate history appears to be naturally determined, the slow irreversible cooling of the Earth's surface being superposed by a rather regular pattern of periodic oscillations. The inference suggests itself that the Earth's highly complex and open climate system has been rather close to a stationary state, and that among the multitude of nonlinear processes bearing on climate there were a small number of those most influential, "governing" processes with periodic attractors. Glaciation maxima preceded principal tectonic phases, and they are clearly correlative to suprasubduction volcanic maxima, whereas the periods of degradation or cessation of glaciation coincide with tectonic, granitization, metamorphic, and, occasionally, mantle-plume volcanic maxima. This suggests that onsets of glaciation were triggered by explosive volcanism and by the ensuing volcanic winters. Deglaciations must have been due to increases in atmospheric transparency, to carbon oxidation and partial decarbonatization in sedimentary sequences of orogens, and to pulses of mantle plume volcanism. A similar explanation applies to long-term climatic oscillations that lasted 30 m.y. on average. Short-term climatic oscillations are explicable by variations in the astronomical parameters of our planet, and ultrashort-term ones, apparently, by solar activity variations.