Tuesday, 23 February 2016

A Whiff Of Oxygen From The Early Archaean

From 2.4 to 2.2 billion years ago, across the Proterozoic Earth, billions of tonnes of iron oxides were laid down in thick deposits known as banded iron formations. They indicate something very special: the permanent appearence of oxygen in the atmosphere. The Great Oxidation Event, as it is known, was a revolutionary time in Earth's history. Recently, however, geological studies have shown that temporary pulses of oxygen did occur hundreds of millions of years before the permanent oxygenation of the atmosphere. These have been shown to extend back into the Archaean eon 3.2 billion years ago. The only process powerful enough to generate significant amounts of oxygen is photosynthesis and the only photosynthetic organisms in the Archaean were cyanobacteria. Such oxygen traces have therefore been taken as evidence of their existence at an equivalently ancient point in time.

Banded iron from Isua
Now a recent study has pushed the geochemical record of oxygen back by an incredible 600 million years into the earliest Archaean. A team of researchers, led by Professor Robert Frei from the University of Copenhagen, analysed 3.8 billion year old banded iron formations from Isua in Western Greenland.

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.'