Chemicals in ancient ocean rocks hint that 3.2 billion years ago, the surface of a baby Earth was continent-free and covered by a global ocean.
In the new study, researchers Johnson and Wing turned to Panorama's unique landscape in the Australian outback. Its rocky scenery preserves a hydrothermal system dating to 3.2 billion years ago, "and records the entire ocean crust from the surface down to the heat engine that drove circulation," Johnson said. Preserved in that craggy seafloor were different versions, or isotopes, of oxygen; over time, the relationship between these isotopes can help scientists decode shifts in ancient ocean temperature and global climate.
However, the scientists uncovered something unexpected through their analysis of more than 100 sediment samples. They found that 3.2 billion years ago, oceans held more oxygen-18 than oxygen-16 (the latter is more common in the modern ocean). Their computer models showed that on a global scale, continental land masses leach oxygen-18 from the oceans. In the absence of continents, the oceans would carry more oxygen-18. And the ratio between these two oxygen isotopes hinted that at the time, there were no continents at all, the study found.
Other researchers have previously proposed the idea that Earth was once ocean covered, Johnson said. However, there's less agreement about how much of that crust was visible above sea level. This discovery "provides actual geochemical constraints on the presence of land above sea level," he explained.
The prospect of an ancient waterworld Earth also offers a new perspective on another intriguing question: where the planet's earliest forms of life appeared and how they evolved, the researchers wrote in the study. "There are two major camps for the origin of life: hydrothermal vents and ponds on land," Johnson said. "If our work is accurate, it means the number of environments on land for life to emerge and evolve was quite small or absent until sometime after 3.2 billion years ago."