Utrecht University: The oldest traces of land on the early Earth

The Earth’s oceans appeared between 4.4 and 4.2 billion years ago. At the time, they covered the entire surface of the planet. So when did the first land rise above sea level? Geologists from Utrecht, Bergen (Norway) and Münster (Germany) have recently dated the first significant landmasses to a very early period: 3.7 billion years ago. “That’s a billion years earlier than some estimates, and only 800 million years after the formation of the Earth.” Their research will be published today in the prestigious journal Nature Geoscience.

The geologists used a new method for determining the moment when the first landmasses emerged. Paul Mason, Professor of Petrology at Utrecht University and one of the researchers involved, explains: “We can’t look back in time to see the first land, but we can find evidence archived in ancient rocks. We look at minerals precipitated from seawater that have stored information about these first pieces of exposed land.”

Radioactive decay
The Earth’s earliest mountain chains were most likely made of granite, a relatively low density rock that easily rises above other rocks on the seafloor. Granite is rich in the element rubidium, which can undergo radioactive decay, to make a new isotope of the element strontium. “The strontium from this decay process can erode out of the granite as it interacts with rainwater and can be washed through rivers into the sea where it mixes with natural background strontium”, explains Desiree Roerdink, the lead author of the article, and former PhD student with Mason in Utrecht, who now works at the University of Bergen. “The ratio of strontium isotopes in seawater can then be used to argue for its origin from exposed granite rocks. In other words: land above sea level.”

Fingerprint
Since samples of ancient seawater cannot be found today, the researchers had to resort to an indirect method for its measurement. “We looked at the mineral barite, which forms on the seafloor. Barite collects the strontium from the seawater, and records the fingerprint of the isotopes that originally washed down from the first landmasses.”

Continental drift
Remarkably, the minerals that the research team studied were preserved in their original state for the past 3.5 billion years. They can be found in what is now South Africa, India and Australia. “That doesn’t mean that’s where the oldest land masses were located; only that those areas still hold traces of these ancient rocks”, Roerdink explains. “That’s because the Earth’s crust is constantly changing. Continents drift, rise and disappear, and a piece of rock you find today may have been originally created on the other side of the world.”

Islands
“We are talking here about the oldest traces of land that have yet been discovered”, Mason explains, but he goes on to caution “It doesn’t mean that there wasn’t land above sea level before that time. There might have been some small islands scattered around the world’s ocean before 3.7 billion years ago, but they were just too tiny to leave traces in the chemistry of seawater.”

Evolution
“The dating of the first land masses also says something crucial about the appearance of life on Earth”, Roerdink adds. “This is because the erosion of these rocks supplied nutrients to the oceans, many of which are essential for life, such as phosphorous. This shows us that the geological and biological evolution of the early Earth were closely related.”

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