|Banded iron from Isua|
Banded iron formations are sedimentary rocks composed of alternating layers of silica and iron hydroxides. As they only form in marine environments, and as the constituent grains interact chemically with surrounding seawater, they preserve an excellent geochemical record of their deposition environment.
Analysis shows that the ratios of chromium and uranium isotopes in the banded iron are consistent with having been weathered from continental rocks by reactive oxygen species. These could only exist in an atmosphere containing free oxygen, if only temporarily. Oxygen is a chemically reactive element and is quickly stripped from the atmosphere. To have built up concentrations high enough to result in the deposition of banded iron formations requires a powerful source. This suggests that photosynthesis may have been in action 3.8 billion years ago, adding weight to the theory that life is older still.
'It is generally believed that the Early Earth was a completely anoxic, but our study shows that the surface of the Earth was exposed to a low oxygen atmosphere already this time,' said Professor Frei. 'This has far reaching implications for how we investigate the pace of evolution of life and its biodiversity on our planet.'