The focus is on geobiochemistry, with only occasional references to disciplines such as palaeontology, and the approach is a mix of straight science and narrative accounts following recent research. The details of people and research programmes in the latter sometimes become a bit dense and there are a few places where Canfield — a major player himself — briefly ventures into travelogue or memoir, with the text dropping a little abruptly into present tense, sometimes just for one or two paragraphs. ("To better understand this, I join a cruise onboard the Vidal Gomez, a rickety old American oceanographic ship recommissioned into the Chilean Navy. ...") The outline summary in the remainder of this review ignores all of that and restricts itself to the science.
Canfield begins with an overview of the Earth, with the presence of water, the thermal balance, and the latter's connection with the carbon cycle. He goes on to consider life before oxygen, looking at the workings of sulfur- and iron-based ecosystems driven by sources of chemical energy.
A key event was the evolution of oxygenic photosynthesis, where Canfield goes into some biochemical detail, looking at chlorophylls, reaction centres, the oxygen-evolving complex, and rubisco. A separate chapter covers the higher level ecology and physiology of cyanobacteria: microbial mats, adaptations for different light regimes, specialised cells for nitrogen fixation, and so forth.
What controls atmospheric oxygen? Over the long-term, this is intimately tied up with the carbon cycle: the burial of organic carbon and pyrite releases oxygen to the atmosphere and their uplift and then weathering and oxidation remove it.
How far back cyanobacteria and the biological production of oxygen can be dated hinges on analysis of carbon isotope ratios and attempts to find fossil evidence or other biomarkers. There is also geological evidence for early oxygen levels from sulfur isotope fractions and molybdenum concentrations.
Despite some "whiffs" of oxygen, there seems to have been considerable delay, perhaps waiting for "the mantle to quiet", between the first cyanobacteria and the Great Oxidation Event (GOE). "The geologic record demonstrates that around 2.3 billion years ago the oxygen content of Earth's atmosphere increased dramatically."
Following that, the focus is on Canfield's own idea (the "Canfield Ocean") that "the GOE likely increased the flux of sulfate to the oceans through the oxidative weathering of sulfides on land, thus enhancing rates of sulfate reduction to hydrogen sulfide in the ocean. ... dissolved iron was removed from the oceans by reaction with sulfide ... the deep oceans remained anoxic".
The Neoproterozoic saw the rise of animals, which has an uncertain relationship with oxygen levels. It is possible that "motile animals evolved into an environment that was already 'permissible' for some time before their appearance" or alternatively that "animals themselves engineered the conditions of environmental change".
The final chapter on the Phanerozoic (the last five hundred million years) explains how rates of organic carbon and pyrite sulfur burial have been measured and used to model oxygen concentrations. The notable feature of the latter is "the large positive oxygen excursion seen during the Carboniferous and Permian Periods", driven by coal deposits and ended by a shift to the deposition of sandy red beds containing iron oxides.
Canfield's explanations are clear, making good use of diagrams and halftones, and there are eight attractive colour plates. The more technical material, including all the chemical equations, has been relegated to sixteen pages of endnotes, but Oxygen remains moderately difficult by popular science standards, assuming some confidence with the basic ideas of chemistry. There's no simple, neat story, either, with Canfield's approach capturing the complexities of a field that is the subject of active, ongoing research. And the subject material has less broad an appeal than that of a book like Nick Lane's Oxygen: The Molecule That Made the World, which covers such topics as medicine, metabolism, aging, and sex.
Oxygen: A Four Billion Year History will be an entertaining and informative read, however, for anyone with a serious interest in the long-term history of the Earth: students contemplating working in the area and specialists in related disciplines as well as engaged general readers.
April 2014
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