Researchers examined microfossils and the rocks that contain them from the McArthur and Birrindudu basins in Northern Territory, Australia. The remains date to about 1.75 to 1.4 billion years ago, when the region was a shallow inland sea with lagoons, offshore mudflats and calm coastal waters. Atmospheric oxygen was only 1% or less and ocean oxygen was patchy.
The team prepared and sorted microfossils from drill core material and used sedimentology and geochemistry to describe ancient environments. Sediment type let them match fossil groups to four settings: lagoons, tidal areas, coastal regions and offshore waters. Mineral clues such as iron pyrite (FeS2) and concentrations of vanadium, molybdenum and uranium helped show how much oxygen was present.
The fossils are mainly in rock formed where the seafloor had oxygen. This pattern suggests early eukaryotes needed oxygen for part of their life cycle.
Difficult words
- microfossil — very small fossil of ancient lifemicrofossils
- basin — low area that holds water or sedimentbasins
- lagoon — shallow coastal water separated from the sealagoons
- sediment — small pieces of rock and soil in water
- mineral — natural solid material found inside rocks
- oxygen — gas in the air needed by many organisms
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Discussion questions
- Which of the four settings in the article (lagoons, tidal areas, coastal regions, offshore waters) would you like to visit? Why?
- Why do you think scientists use minerals to learn about ancient oxygen levels?
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