Researchers Matthijs A. Smit (PCIGR, UBC EOAS) and Klaus Mezger (Universität Bern, Switzerland) have discovered a possible link between the long-term evolution of Earth’s crust and the emergence of Earth’s oxygen cycle 3.0 billion years ago. Their findings have been published in Nature Geoscience.
Cyanobacteria may have produced O2 as early as 3.7 billion years ago. Still, it took almost a billion years for that O2 to start accumulating in the shallow oceans and, subsequently, the atmosphere during a period commonly called the Great Oxidation Event (2.4-2.2 Gyr ago). The slow and stepwise rise of O2 in Earth’s atmosphere has always been somewhat of a mystery. Links to the changing composition of continents at the time have been proposed. However, with clear constraints on the average composition of the continents lacking, these links have been difficult to elucidate.
Smit and Mezger decided to approach this problem by letting nature do the sampling. They analyzed the Cr/U chemistry of land-sourced sediments, working from the idea that these rocks sample large areas of the exposed continents and thus may provide a good representation of their average composition.
Their research revealed a staggering change in continental composition between 3.0-2.4 billion years ago. Before this period, the continents were much more rich in Mg and poorer in Si than today, and contained Mg-rich minerals such as olivine. Present-day analogues show that hydration of such mafic crust causes local surface waters to have extremely high pH and high concentrations of O2 scavengers such as methane and dihydrogen. Although rare today, such reducing waters must have been common before 3.0 Gyr ago. Ultimately, it may have been the removal of Earth’s primitive proto-continents and the cleansing of the environment from O2 scavengers, which set System Earth on track towards having an oxygenated and habitable environment conducive to the evolution of higher life forms.
The findings are among the first reported from a new interdisciplinary research program on the formation and evolution of Earth’s earliest continents. In this program, lead-PI Smit and his students use the world-leading analytical facilities at PCIGR to analyze relics of Archean continental crust found in Canada, Greenland, India, South Africa and elsewhere.