Long-term palaeoclimate: the origin of the ice ages Professor Simon K. Haslett Centre for Excellence in Learning and Teaching Simon.email@example.com 16rd September 2010
Introduction What stimulated the global cooling that led to the development of the continental ice sheets that characterised the Quaternary and other ice ages through geological time? The search for the answer to these major climate questions has generated some fascinating research that has become the focus of much media attention. However, a number of different theories have been proposed to account for the origin of the ice ages, and there is intense argument between the supporters of the different theories – a very controversial topic. This presentation hopes to discuss what caused the ice ages, and describes the radical theories relating to climatic changes.
Differing theories The main proponents of the argument are:
Maureen Raymo and her colleagues at the Massachusetts Institute for Technology, who suggest that the uplift of the Tibetan Plateau and other mountains during the Late Cenozoic stimulated global cooling; and
Robert Berner and his colleagues who believe that long-term trends in the carbon cycle coupled with the Greenhouse Effect are responsible.
These theories invoke a relationship between atmospheric CO2, tectonic activity, and the carbonate-silicate cycle.
Geochemical or steady-state model 1 The ‘geochemical’ or ‘steady-state’ model was proposed by Berner in 1990, and suggests that tectonic activity releases CO2 through a process called decarbonation, which increases atmospheric CO2. This in turn enhances the Greenhouse Effect which results in global warming, and enhances continental weathering. Rocks weather more rapidly in warm/moist conditions. Weathered products are transported to the sea where they promote carbonate (CaCO3) formation (i.e. shells) which sink to the sea-floor. C B A (a) Foraminifera secrete calcareous ‘tests’ and inhabit both the sea-bed and the water column. (b) Cut blocks of fenwood peat. Peat consists of partially decomposed organic material which acts as a sink for carbon because of the anaerobic conditions in which it is found. Note the coin for scale. (c) Volcanic activity releases millions of tonnes of CO2 into the atmosphere every year (Terceira island, Azores).
Geochemical or steady-state model 2 In conjunction with the weathering processes themselves (which consume atmospheric CO2), detrital rain in the water column takes carbon out of circulation until the sediments are decarbonated. This process maintains a steady-state relationship between weathering, decarbonation, and atmospheric CO2 levels, and so essentially weathering is controlled by decarbonation. The variation in CO2 throughout the Phanerozoic is apparently related to the relationship of tectonic activity, rise of vascular plants, and the burial of organic matter. The Quaternary ice ages are attributed by Berner (1990) to a general decrease in decarbonation over the last 100 Ma and an increase in the burial of organic matter.
Uplift or non-steady-state model 1 Raymoet al.’s (1988) model (called the uplift or non-steady-state model) suggests that CO2 levels are not controlled by decarbonation, and promotes the idea that the process of uplift alone can stimulate weathering, stripping CO2 out of the atmosphere, increasing CaCO3 sedimentation in the oceans, so causing global cooling and the Quaternary ice ages. Thus, this model operates in a non-steady-state because the carbon cycle is being influenced by factors from outside the system (i.e. uplift). Formation of scree slopes indicates physical and chemical weathering and erosional processes acting on a rock face (Andalusia, Spain).
Uplift or non-steady-state model 2 During the Late Cenozoic a number of uplift events have occurred (e.g. Tibetan Plateau, Himalayas, Andes, Alps etc) which may have provided the stimuli for global cooling. Indeed there is evidence for increased weathering at this time (e.g. strontium content of deep-sea sediments). Tectonically driven uplift of mountain ranges , e.g. the Alps, has been suggested to have initiated glaciation.
Consolidation of the theories Sundquist (1991) constructs a complex ocean-atmosphere-sediment model to evaluate these conflicting theories. He concludes that both of these theories operate in nature, but on different time scales, with the steady-state model operating over longer periods of geological time, whilst the non-steady-state may interfere over shorter periods and specific to uplift events. A useful figure derived from his model is that a lagged response time of 300-400 ka exists between either uplift or decarbonation and increased weathering capable of depleting atmospheric CO2.
Practical – Milankovitch cycles 1 Examine the SPECMAP graph below of an oxygen isotope record taken from a deep-sea sediment core (the top of the core is the modern sea-floor surface). Oxygen isotopes vary depending on changes in global ice volume. Using your knowledge of oxygen isotopes and Milankovitch cycles, answer the questions that follow:
Practical – Milankovitch cycles 2 Make a copy of the preceding graph. Identify perturbations in the oxygen isotope record due to eccentricity, obliquity and precession cycles (annotate examples on your graph where appropriate). Construct a general chronology (in 1000’s of years) for the core and draw a timescale up the side of your graph. Reconstruct palaeoclimate change represented by the oxygen isotope record (annotate the graph to show palaeoclimate extremes). E = Eccentricity of orbit T = Obliquity of the Ecliptic (tilt) P = Precession of the Equinoxes Source: FAQ 6.1. Fig 1. IPCC, 2007. Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Avervt, K.B., Tignor, M. and Miller, H.L. (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 996 pp.
Summary The origin of the ice ages remains a very controversial topic. There are two opposing theories that have implications for the origin of Quaternary ice ages. Both theories are in agreement that the Quaternary ice ages are a function of CO2 in the atmosphere and the Greenhouse Effect. Robert Berner believes that the ice ages are essentially a consequence of carbon storage mechanisms corresponding with a decrease in decarbonation. Maureen Raymoet al., however, argues that as the Himalayas grew, heavy monsoon rains combined with CO2 in the air eroded the newly exposed rock, removing so much CO2 out of the atmosphere that global temperatures dropped. In 1991, Eric Sundquist concluded that both theories have their place in nature, but operate over different timescales.
References Berner, R.A. 1990. Atmospheric carbon dioxide levels over Phanerozoic time. Science, 249: 1382-1386. Broecker, W.S. and Denton, G.H. 1990. What drives glacial cycles? Scientific American, 262(1): 48-56. Harris, S.A. 2002. Global heat budget, plate tectonics and climate change. GeografiskaAnnaler, A84: 1-9. Hays, J.D., Imbrie, J. and Shackleton, N.J. 1976. Variations in the earth’s orbit: pacemaker of the ice ages. Science, 194: 1121-1132. Molnar, P. and England, P. 1990. Late Cenozoic uplift of mountain-ranges and global climate change – chicken or egg? Nature, 346: 29-34. Paterson, D. 1993. Did Tibet cool the world? New Scientist, 2nd July issue, 29-33. Raymo, M.E. and Ruddiman, W.F. 1992. Tectonic forcing of late Cenozoic climate. Nature, 359: 117-122. Raymo, M.E., Ruddiman, W.F. and Froelich, P.N. 1988. Influence of late Cenozoic mountain building on ocean geochemical cycles. Geology, 16: 649-653. Ruddiman, W.F. and Kutzbach, J.E. 1991. Plateau uplift and climatic change. Scientific American, 264(3): 66-. Sundquist, E.T. 1991. Steady- and non-steady-state carbonate-silicate controls on atmospheric CO2. Quaternary Science Reviews, 10: 283-296.
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