Nordic Center for Earth Evolution director Canfield (Ecology/Univ. of Southern Denmark; co-editor: Fundamentals of Geobiology, 2012 etc.) delivers “the history of atmospheric oxygen on Earth.”
The author’s project is directly relevant to efforts to discover life elsewhere in the universe. He explains that geobiologists are currently attempting to correlate the evolution of life on Earth to transformations in the levels of atmospheric oxygen that began somewhere around 580 million years ago. He sets the stage by looking back to the earlier history of life, beginning with the first organic molecules. The evolution of oxygen-producing cyanobacteria, which released oxygen into the atmosphere through photosynthesis, was a major step along the way. Around 2.3 billion years ago, the rate of oxygen production exceeded the flux of hydrogen. Canfield, however, believes that this is only part of the story. The question remaining to be answered is, “when and how did oxygen become more than a whiff and a permanent feature of Earth's atmosphere?” The author claims that a crucial element in our current understanding of the abundance of oxygen in our atmosphere is the process through which it was concentrated in the oceans. He explains that scientists believe this process began with “the evolution of tiny animal plankton, so-called zooplankton, [which] completely changed the carbon cycle and the distribution of oxygen in the oceans.” They produced “fast-sinking fecal pellets” that slowly dissolved as they sank to the bottoms of the oceans. In the author's opinion, it was animal activity that created a major “redistribution of oxygen in the oceans…rather than an increase in the levels of atmospheric oxygen”—and that created a tipping point. Canfield’s text will be illuminating for scientific-minded readers but difficult for those not already somewhat familiar with the concepts.
A mixed-success attempt at a popular treatment of the complexities of a fascinating subject.