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University of California in San Diego came up with a new method in 2000 to use the relative numbers of sulfur isotopesfound in rocks of known age to tell us when particular kinds of life might have arisen.
    Farquhar and Thiemens analyzed the pattern of sulfur isotopes in sedimentary rocks from Archean to Paleozoic time, finding large variations in sulfur isotopes prior to about 2.4 billion years ago. But in rocks younger than these the fluctuations disappear, and the best interpretation is that this change was caused by a lack of ultraviolet radiation hitting molecules of SO 2 in Earth’s atmosphere. This could have happened only through the formation—the first formation at that—of the ozone layer that exists to this day. If there is no oxygen, there is no ozone screen, and we now have evidence that there was no ozone layer before about 2.4 billion years ago. After this, many other sedimentary indicators start to suggest the presence of atmospheric oxygen.
    So there was no oxygen before 2.4 billion years ago, at least not enough oxygen to create an ozone layer. But were there any cyanobacteria anywhere at all? Probably not. When it became clear that the major scientific drilling program in South Africa (funded by the Agouron Institute mentioned above) had missed the great oxygenation event, they allowed the team to drill two more holes through slightly younger sediments in South Africa, which certainly did cross this event. This is the time interval between ~2.4 and 2.2 GA, the earliest part of what is called the Paleoproterozoic. And they found something rather peculiar . As noted above, the minerals pyrite and uraninite, and the sulfur isotopes, are very strong indicators of the lack of oxygen. At the other end of this spectrum is the element manganese, which is usually an equally powerful indicator for the presence of free molecular oxygen. The new data show copious levels of sedimentary manganese oxide, but in the same rock that has the other indicators of the lack of oxygen!
    But it was more complex. Our junior colleague at Caltech, Woodward Fischer, working with graduate student Jena Johnson and Caltech alumnus Sam Webb (in charge of one of the microanalytical beam lines at the Stanford linear accelerator), decided to look further. 5 It turns out that the same sediments that have this slug of sedimentary manganese also have silt-sized grains of detrital pyrite and uraninite, and the isotopic sulfur signature that demands no freeoxygen (well, less than 1 ppm). This was completely unexpected, but it gets worse. Working with another Young Turk colleague at Caltech, Mike Lamb—an expert in the geophysics of mineral transport during sedimentation—they extended this no oxygen constraint to the entire depositional system. The silt at the edge of the delta where we sampled it had to have originally been eroded from a continent somewhere, then transported through a river system, through meandering streams, coastal estuaries, near-shore sedimentary environments, and out to the distal toe of the delta. None of these environments could have had even 1 ppm of free molecular oxygen 6 (and so were obviously not affected by glacial meltwater, which might have had a little bit). Oxygenic cyanobacteria have well-known nutrient requirements—principally iron and phosphorous 7 —that would have been provided in many places along this depositional pathway. They produce copious quantities of oxygen—bubbles—when they grow. If any of these “islands of oxygenic photosynthesis” had actually existed, then where were they? The worst place for them to grow would be far out at sea,away from these nutrient sources. That was the vision of Preston Cloud mentioned above, but it frankly does not make sense in this context. The survival of those sedimentary indicators of anoxia is totally incompatible with the presence of oxygen—and cyanobacteria—anywhere in the environments those grains traveled through.

    Overlap interval of