PALAEOCLIMATES 131 PALAEOCLIMATES B W Sellwood, University of Reading, Reading, UK P J Valdes, University of Bristol, Bristol, UK ß 2005, Elsevier Ltd All Rights Reserved Introduction An evaluation of the climatic regime under which sedimentary successions accumulated has often been integral to their overall interpretation It is an essentially uniformitarian approach Palaeoclimatology, the understanding of past climatic regimes (palaeoclimates), has only relatively recently become a subject in its own right and is currently undergoing a major phase of expansion One reason for this is the light thrown by palaeoclimatic studies on the natural variability that exists in the Earth’s climate in the context of future, anthropogenically influenced, climate changes Another is the realization that many of the Earth’s sedimentary resources (e.g coal, hydrocarbons, and other extractable minerals) formed under particular types of climatic regime Through time, the climate of the Earth has undergone many changes – the most recent ice age ended a mere 500 human generations ago Astronomical data suggest that the Sun is of a type that steadily increases its heat output as it ages, increasing by about 1% per 100 Ma (see Solar System: The Sun) This presents a problem (the ‘faint young sun paradox’) Simply stated, in its early days the Sun would have emitted about 45% less than its current heat output, so Earth should have been frozen, but there is ample evidence from Earth’s most ancient rocks (e.g in Greenland, South Africa, and Australia) to suggest that aqueous processes were operating This paradox can be overcome if it is accepted that Earth’s atmosphere was much denser and much richer in greenhouse gases (such as carbon dioxide and methane) during its early history The rock record makes it clear that the Earth was periodically affected by major freezes during its early history, and controversy exists over the possibility that the Earth periodically froze from the poles to the equator (the ‘snowball Earth hypothesis’) during Proterozoic and older times (see Atmosphere Evolution) Despite the possibility that the Sun was significantly cooler 500 Ma ago, abundant and widespread geological evidence suggests that through much of the subsequent time the Earth has been significantly warmer than at present Since the Cambrian there appear to have been only three episodes that were cooler than now One was in the Ordovician, when glaciation was centred on the present Sahara (see Palaeozoic: Ordovician), and one was in the Late Carboniferous (see Palaeozoic: Carboniferous) and Early Permian, with glaciation centred on southern Gondwana (see Gondwanaland and Gondwana) The latest ‘ice ages’ took place during the Quaternary (see Tertiary To Present: Pleistocene and The Ice Age), from about 1.67 Ma, but a growing body of evidence suggests that this latest phase had its origins at around the Eocene–Oligocene boundary (36 Ma), when ice began to accumulate over eastern Antarctica For around 250 Ma, since the Late Permian, there appears to have been a period of extreme warmth, often referred to as the ‘greenhouse world’ The Earth’s past climate regimes can now be simulated using general circulation models (GCMs) similar to those used in long-term weather forecasting Orbital Forcing of Climate In 1875, James Croll was the first to describe the cyclic way in which the Earth’s orbit behaves The eccentricity varies from near-circular to elliptical over a cycle of about 110 Ka and affects the total amount of solar radiation reaching the Earth Changes in the tilt of the Earth’s axis of rotation (obliquity; currently about 23.4 but varying between 21.8 and 24.4 ), which affect the latitudinal distribution of received solar radiation, take around 40 Ka In addition, a rotational wobble in the Earth’s axis of rotation (as if the poles describe a circle against the background of the stars) causes the precession of the equinoxes and takes around 22 Ka to complete a cycle In the 1930s and 1940s, Milutin Milankovitch, improving on Croll’s work, realized that each of these cycles interacts with the others to cause changes in the amount of insolation received by the Earth that are sufficient to promote climate changes (see Earth: Orbital Variation (Including Milankovitch Cycles)) These cycles are now named after Milankovitch and have been used to predict the time-frame of glacial cycles in the Quaternary Changes in palaeoclimate have been recognized in Neogene oceanic sediments, which provide a metronomic time signal, synchronized with the Milankovitch timeframe, and hence a means of short-term correlation that was unavailable until recently Geological Proxies of Palaeoclimate (Figures 1A and 1B) Sedimentary facies and palaeontological data generally provide qualitative evidence of palaeoclimates