Study Guide for Atmosphere, Earth and LifeAtmosphere, Earth and Life This is the third book that you will study as part of the Course. Several of the Course video programmes are relevant to the material in the book, especially ‘Volcanoes and the Atmosphere’, ‘Biosphere 2’ and ‘Daisyworld’. The book is assessed in TMA 03. (Note that there are self-assessment questions in the Multiple-choice questions and answers booklet. You should complete these as you study the text and they will also be useful for revision of the Course.) Chapter 1 Oxygen and the Earth This introductory chapter introduces the chief reservoirs of oxygen in the Earth, and shows that there are two reservoirs of free molecular oxygen available for metabolism. A number of quantitative estimates are made of the amounts of oxygen in various reservoirs. These calculations involve some quite large numbers, so you will need to be confident about manipulation of powers of ten, but they are otherwise straightforward. Chapter 2 Oxygen and life Oxygen is produced by photosynthetic processes when biomass is formed from atmospheric carbon dioxide and water by using sunlight. However, photosynthesis is balanced by respiration, so oxygen can only accumulate in the atmosphere if the system is unbalanced by removal of organic carbon, by burying it in sediments. This chapter examines the processes of respiration and photosynthesis, and quantifies the residence time of oxygen in the atmosphere. It also describes the various ways of describing gases quantitatively. Many basic physics concepts are involved, with which you may already be familiar. However, some revision may be helpful. Chapter 3 The fossil record This fairly straightforward chapter describes the various kinds of fossil evidence from animals, plants and algae that provide clues to atmospheric oxygen levels at successive times during the Earth’s history. An important section deals with the use of carbon isotope evidence for photosynthesis in the very early Earth. Chapter 4 The rock record For most of the Earth’s history, there is no macroscopic fossil evidence for the existence of life at all. There are many essentially inorganic pointers to atmospheric oxygen levels in the early Earth which are related to the oxidation state of iron, notably red beds, banded iron formations and paleosols. These consistently indicate a significant increase in atmospheric oxygen levels about 2 billion years ago. Some of the chemistry involved in paleosols is quite complex – you could skip this if you find it hard. Chapter 5 Evolution of the Earth’s atmosphere during the Cryptozoic What was the atmosphere like before the rock record commenced? This chapter tackles the formation of the Earth’s primary and secondary atmosphere in the Hadean Era, and also examines briefly the formation of oxygen by photolytic dissociation – an entirely inorganic process. Given the evidence that life evolved very early in the Earth’s history, it is appropriate to enquire what happened to the oxygen formed then – why did it not accumulate to levels comparable to today? Evidence that geological processes – and their rates – were different then is discussed. Chapter 6 Modelling the evolution of atmospheric oxygen levels during the Phanerozoic Two American scientists have used data on rates of sedimentation and carbon burial, etc. to model atmospheric oxygen levels during the Phanerozoic. Importantly, they concluded that there was a ‘spike’ in oxygen levels about 300 million years ago. This may have had important implications for the evolution of animal life. Chapter 7 A closer look at the regulation of atmospheric oxygen One of the most intriguing questions in Earth System Science is how or why oxygen has remained at about the same level since about 300 million years ago. In absolute terms, the mass of oxygen in the atmosphere is rather small. So what regulates it? Several mechanisms have been proposed – fire is an intuitively obvious one. By looking more closely, however, the arguments turn out to be non-intuitive, and rest on some subtle aspects of the chemistry of phosphorus; one of life’s key nutrients. This is a challenging but important chapter. Chapter 8 Ozone and life Ozone and oxygen are inextricably linked. This chapter looks briefly at the biological importance of ozone, which may have played a crucial role in the evolution of life by providing a screen for damaging ultraviolet radiation. The chapter examines some of the underlying physical considerations, and touches briefly on evidence for ozone levels over geological history. Chapter 9 Carbon dioxide and the Earth This chapter introduces the second main theme of the book. Although it has to be regarded as only a trace gas in the atmosphere, with a concentration of only 360 p.p.m., carbon dioxide is enormously important to the Earth System for two reasons: in the biological realm of photosynthesis and respiration, it is the counterpart to oxygen, while in the physical realm it plays an important part in regulating the Earth’s surface temperature via the greenhouse effect. Chapter 10 An empirical model of atmospheric carbon dioxide levels during the Cryptozoic Constraining variations in atmospheric carbon dioxide through the Earth’s history is difficult, especially for the earliest times. This chapter examines some of the indirect approaches that have been used, for example surface temperature estimates derived from evidence of glaciations have been used to infer the varying contribution of carbon dioxide to the atmospheric greenhouse as the solar flux has increased. No assumptions are made about the processes or rates at which carbon dioxide moves around the Earth System to reach these inferences. Chapter 11 A process-based model of atmospheric carbon dioxide levels during the Phanerozoic Much more data are available for rates and magnitudes of geological processes during the Phanerozoic. This chapter, therefore, follows a similar approach to that used in Chapter 6 to estimate atmospheric carbon dioxide levels over time from first principles, using data on rates of volcanism, sedimentation, weathering, etc. The model used – known as GEOCARB – has important ramifications, because it can also relate to atmospheric temperature levels. Thus it is possible to investigate, for example, the extent to which increased weathering rates associated with mountain uplift could lead to a drawdown in carbon dioxide and thus a decrease in temperature. Chapter 12 An overview – the Earth’s atmosphere in a lifeless world Because the evolution of the Earth’s atmosphere has been so intimately associated with the history of life, it is difficult to disentangle the two. This final brief chapter attempts to highlight the inter-relationship by examining what would happen to the atmosphere if life were abruptly extinguished. This thought-experiment is useful for highlighting the important sources and sinks of oxygen and carbon dioxide. Required background and the most difficult sections Discovering Science (S103) or its predecessor (S102), and at least one other second level science course or Earth science related course would be helpful in providing the background and skills to study the material in this book. In addition to those topics already raised earlier in the Course, it will be useful to have some understanding of: •basic physical concepts including pressure, density, temperature, relative atomic and molecular masses, especially in the context of gases; •basic chemical and biological processes, especially oxidation and reduction, photosynthesis and respiration; •simple mathematical manipulations, especially those involving powers of ten; •the nature of isotopes and conventions for manipulating carbon isotope ratios (covered in Origins of Earth and Life); •concepts of energy and wavelength especially in their application to the electromagnetic spectrum; •the nature of the geological record, and especially the terminology used to describe various periods of time. Inevitably, some parts of the text are more challenging than others. You may find the sections on carbon isotopes in Sections 3.3, 5.5 and 7.5 difficult. Section 4.2.4 on the chemistry of paleosols may be tricky if you have little background in chemistry. In Chapter 7, some of the arguments about the phosphorus cycle and its role in oxygen regulation are rather subtle and elusive, but they are important, so please persevere! Some of the physics involved in Chapter 8 on ozone may also be new to you. Chapters 10 and 11 necessarily involve a good deal of the chemistry involved in the carbon cycle, as carbon moves from the atmosphere to soils and water. This will require some application, but it is important, and is covered elsewhere in the Course, especially in The Dynamic Earth. Finally, recall that while you are required to understand them, you will not be required to memorize formal definitions of terms and concepts used. These will be included in the Glossary. Also, note that while you will see a number of chemical and mathematical equations in the text, these are all straightforward. The mathematical manipulations involve only addition, subtraction, multiplication, division, use of powers of ten, square roots and occasional logarithms. Video programmes There are three video programmes (Video Bands 4–6) linked to this book and Video Bands 4 and 5 will also have relevance to other areas of the Course. Thus, there are few direct requirements to view a programme at any particular point. You may want to view these again later with relevant texts. Video Band 4: ‘Volcanoes and the Atmosphere’ Throughout geological history, volcanoes have been pumping large quantities of gases into the atmosphere. Until the advent of humans, they were effectively the only sources of some gases such as SO2. This programme examines the consequences of pumping large volumes of SO2 into the atmosphere during two kinds of eruption: large explosive eruptions, like that of Mount Pinatubo in 1991, and flood basalt eruptions, specifically those of the Columbia River basalt province in North America. The focus is on the climatic and environmental consequences of large eruptions, but the programme also provides insights into the role of volcanoes in atmospheric evolution. Video Band 5: ‘Biosphere 2’ This programme concerns the experiment carried out in the Arizona desert, aimed at constructing a microcosm of the Earth’s environment, partly so that the requirements of a permanently manned orbiting space colony could be investigated. ‘Biosphere 1’ of course, is the Earth itself, in which all of us are participants in an uncontrolled experiment on changing surface conditions. Biosphere 2 highlights the problems of regulating atmospheric composition, and provides some fascinating insights into the sources and sinks of gases. Video Band 6: ‘Daisyworld’ Modelling has played a key role in understanding many aspects of how the atmosphere has evolved. This programme is largely concerned with the efforts of one scientist, James Lovelock, to model an Earth System which is self-regulating. Lovelock discusses the background to his ‘Gaia’ hypothesis, and describes how he devised a specific response to critics who argued that Lovelock’s Gaian ideas were untenable in terms of biological natural selection. His simplest Daisyworld model involves only black and white daisies and a steadily warming Sun. He concluded from his model that self-regulation is an emergent property. If correct, this concept has some far-reaching implications. Video Band 3: ‘Did Tibet Cool the Earth?’ and Video Band 7: ‘The Cretaceous Greenhouse’, are also relevant to this book. You may wish to watch Video Band 3 again and to preview Video Band 7. Website Read the following sections on the S269 website: Atmosphere, Earth and Life – Topics Course Items and TMAs Other Resources Copyright © 2003 The Open University SUP 78370 6 Back to S269 |