Oxygen in ancient oceans

Oxygen in the air and dissolved oxygen in the ocean are crucial for sustaining animal life on Earth. But that oxygen wasn’t always there. Researchers can gain some information about the history of ocean oxygenation from the compositions of sedimentary rocks, because dissolved O₂ affects the solubility and precipitation of certain elements. Now, an international team of researchers led by Tais W. Dahl (University of Southern Denmark) has studied the problem using a new technique based on molybdenum isotopes. In oxygenated water, Mo forms the soluble molybdate ion, MoO₄ ^2-, and precipitates in small quantities by a mechanism that favors lighter isotopes. But when hydrogen sulfide is present instead of O₂, all Mo isotopes precipitate quickly. By analyzing the Mo in 180 rock samples with ages spanning almost 2 billion years, the researchers concluded that O₂ levels increased twice: 550 million years ago and 400 million years ago, as shown by the blue and green bands in the figure. The first increase coincides with the emergence of the first motile animals, and the second coincides with the evolution of much larger animals in the phylum Chordata, including large predatory fish. The method does not provide a quantitative measure of O₂ levels over time. Still, based on the physiological requirements of fish, the researchers suggest that prior to 400 million years ago the ocean and atmosphere contained O₂ at just 15-50% of their present levels, which means that the earliest animals survived with much less oxygen than we breathe today. (T. W. Dahl et al., Proc. Natl. Acad. Sci. USA, in press, doi:10.1073/pnas.1011287107.)