Decades of careful geologic study by Russian geologists (and our own Professor Tony Prave) identified 2-billion-year-old rocks that contained evidence for minerals formed from the evaporation of ancient seawater. These rocks are in the Karelia region of Russia and recent drilling there obtained drill core that contained 600 meters of evaporites including both halite and anhydrite. This remarkable discovery has extended the geologic record of salt deposits on Earth by over 1 billion years. Furthermore, textures in the sulfates indicate that, amazingly, the deposits are primary and have not been dissolved and re-precipitated in all that time!
The presences of the various salts in the evaporite sequence revealed by XRD made it clear that this 2 billion year old seawater looked quite a bit like modern seawater. Prof Tony Prave, Dr Mark Claire and Dr Aubrey Zerkle joined with colleagues from the Karelian Science Centre, Tartu University in Estonia the Norwegian Geological Survey and Princeton University to study these evaporite minerals and found additional geochemical clues in sulfur, oxygen, and calcium isotopes that enabled us to constrain the sulfate concentrations to be a minimum of 10 mM, which is one third of the modern value. This is vastly greater than previous estimates by other workers and implies that the chemistry of the oceans and atmosphere had changed dramatically from its early reducing conditions. This salt can be seen as the “smoking gun” of the oxidative side of the Great Oxidation Event, and confirms that the event was as great as its name.