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TEACHING EARTH SCIENCES ● Volume 31 ● Number 4, 2006 Extreme Earth: Climate Change Through Deep Time DR HOWARD FALCON-LANG Climate change is nothing new. Over the billions of years of deep time, the Earth’s climate has fluctuated between periods of extreme warmth and cold. We can think of these extremes as experiments that nature has undertaken on our behalf. These experiments teach us how the Earth’s System works, and give us a better idea of the implications of our current meddling with planetary dynamics. In this article, I review some of the ways that geologists investigate ancient climate and give examples of what Earth was like during some climatic extremes. Figure 1 The author with a Carboniferous fossil tree at Joggins, Nova Scotia, Canada. These fossils are remains of the earliest tropical rainforests. The study of ancient climates has a long history. One of the first palaeoclimatic investigations was made by Lun Sun in China way back in the twelfth century AD. He discovered a fossil forest of what he thought was petrified bamboo. As bamboo did not grow in that area, he argued that the climate must have been different when the rocks were formed. Palaeoclimatic research in a modern sense really began in the nineteenth century with the work of Charles Lyell. He sought to explain the mounting geological evidence for past climate change by means of observable natural causes rather than catastrophic events. Palaeoclimate toolbox So how can we study what the Earth’s climate was like millions of years ago? There are four main approaches to palaeoclimatology. One of the most useful for field geologists is sedimentary evidence. For example, tillites, the deposits of ancient glacial moraines, imply permanently freezing conditions; evaporites, such as gypsum and halite, suggest sufficient aridity to precipitate out salts; whereas laterites, iron-rich soil profiles, indicate very humid climates under which all other soil constituents were weathered away. In fact, ancient soils, or palaeosols, are one of the best sedimentary indicators of past climates as the global distribution of soil types is strongly controlled by temperature and rainfall, as well as bedrock. Another popular approach to palaeoclimate research involves fossils. Here, inferences are based on the known climate tolerance of the nearest living relative of the fossil in question. For example, crocodiles are today limited to environments with a mean annual temperature of 14°C. So when fossil crocodiles turned up in the Paleogene of northern Canada, geologists painted a picture of a subtropical Arctic Eden. Similarly, the widespread discovery of palm trees in the Tertiary London Clay points to England having had a much warmer climate in the distant past. Isotopes represent a third method for exploring ancient climates. One particularly useful isotope is oxygen, which comes in two main forms, light oxygen-16 and heavy oxygen-18. Because light oxygen fits into the ice lattice more easily that heavy oxygen, their ratio in seawater is influenced by the size of the polar ice caps. In short, the larger the ice caps, the greater the proportion of heavy oxygen in seawater. Study of the oxygen isotopic ratio of marine foraminifera therefore provides one of the most detailed records of the various ice ages that have affected the Quaternary. Computer models represent a final palaeoclimatic technique. The Met Office has developed accurate simulations of the Earth’s climate based on general physical principles. These General Circulation Models (GCMs) can also be applied to the geological past by changing the various input parameters such as continental position and atmospheric composition. For example, modelling of the Triassic period, when all landmasses were joined together and atmospheric carbon dioxide levels were higher, has suggested very hot arid conditions at the heart of Pangaea. Such model www.esta-uk.org 10
TEACHING EARTH SCIENCES ● Volume 31 ● Number 4, 2006 results can then be tested against the other kinds of palaeoclimatic indicators. Carboniferous Icehouse Earth From a palaeoclimatic perspective, the latter part of the Carboniferous period, 315-290 million years ago, may be one of the most interesting phases of Earth history. Like today, Carboniferous climate was cold with large polar ice caps. Geologists refer to this kind of climate as an Icehouse world. When Alfred Wegener proposed his theory of continental drift in the early twentieth century he used evidence for these ice caps to support his argument. He noticed that Carboniferous tillites occurred widely across Antarctica, South America, Africa, India and Australia, and proposed that they had once comprised a single landmass (Gondwanaland) positioned over the South Pole. The remaining continents of Europe and North America comprised Euramerica, which lay over the equator. Every geologist knows that British Carboniferous rocks are rich in coal (Carboniferous literally means coal-bearing), the compacted remains of primitive tropical rainforests (Figure 1). One interesting feature of these successions is that they contain distinct cycles of coal interbedded with marine bands. These ‘cyclothems’ record repeated sea-level fluctuations of about 70 metres, and average cycles are estimated to have been about 100,000 years in duration. They were probably formed as the Gondwanan ice cap waxed and waned in size through successive ice ages. The cause of cyclic ice build up and retreat was probably linked to wobbles in the Earth orbit (Milankovitch cycles), which changed the amount of solar energy that reached the surface. The Quaternary ice ages were driven by the same phenomenon. What was the effect of these ice ages on the Carboniferous coal forests? Lying on the equator, they were far away from the nearest ice sheets, nevertheless there is evidence that climate exerted a major influence. During ice ages, the Earth’s atmosphere becomes drier as cold air cannot carry so much moisture, and this is notably the case in the tropics. Tracking fossil plants through Carboniferous cyclothems shows that coal forests flourished during the warm, wet interglacial phases, but during subsequent ice ages were replaced by drought-adapted vegetation. One big debate raging at the moment is what happened to the Amazon rainforest during the height of the last ice age, 18,000 years ago. Some say it contracted, while others argue that it remained intact. Carboniferous studies contribute to this debate showing that the earliest rainforests to evolve did indeed respond to ice age fluctuations. Cretaceous Greenhouse Earth Another interesting period for palaeoclimatologists is the Cretaceous, some 144-65 million years ago. At this time, our planet appears to have experienced the other climatic extreme, a Greenhouse phase. Computer models predict that global mean annual temperature may have been 10°C greater compared to today. So what was Greenhouse Earth like? For one thing, the polar ice caps appear to have almost entirely melted, as there are no known tillites and only limited evidence of icerafted debris near the Cretaceous poles. Consequently, sea level was raised by as much as 200 metres. The familiar Chalk seas of Europe, and similar marine deposits in North America, are some of the most tangible evidence for such a global sea-level rise. Today, the formation of polar sea ice is the main driver of ocean circulation. As seawater freezes, salt is expelled from the ice lattice, and cold, dense brines form that sink to the ocean floors creating currents. In a more or less ice-free Earth, as envisaged for the Cretaceous, one might expect that ocean circulation would be much reduced. This has, in fact, proved to be the case, as Cretaceous successions contain common black shales. These organic-rich deposits accumulated in stagnant seas where there was too little oxygen to break down organic matter. These relics of Cretaceous oceanic stagnation are of enormous economic importance, as black shales are source rocks for many oil fields. If an ice-free Earth with stagnant oceans sounds pretty hard to believe, one final aspect of the Cretaceous world was even more bizarre: the existence of temperate rainforests over both poles, similar to those of present day Chile (Figure 2). Some of the best fossil forests have been discovered on Antarctica, which, like today, lay over the South Pole in Cretaceous times. Growing at more than 75° of latitude, an intriguing feature of these ecosystems is the fact that they would have had to endure months of continuous darkness during the winter. However, studies of tree-rings in fossil woods show that the trees actually thrived in these environments. Annual rings are typically more than 2 mm in width, and Antarctic forests were as productive as any English woodland. Figure 2 Did Antarctica once look like this? Monkeypuzzle forests in Chile are the nearest living relatives to Cretaceous polar forests. 11 www.esta-uk.org
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