Early Earth’s warm mantle may have led to the Archean ‘water world’

A vast global ocean may have covered early Earth at the start of the Archean period, 4 to 3.2 billion years ago, a side effect of having a warmer mantle than today, new research shows. .

The new findings challenge previous assumptions that the size of Earth’s global ocean has remained constant over time and offer clues as to how its size may have changed over geological time, the authors say. of the study.

Most of the Earth’s surface water is found in the oceans. But there is a second reservoir of water deep inside the Earth, in the form of hydrogen and oxygen attached to minerals in the mantle.

A new study in AGU Advances, which publishes high-impact, open-access research and commentary on earth and space science, estimates how much water the mantle could hold today and how much water it could have stored in the past.

The results suggest that since the early Earth was warmer than it is today, its mantle may have contained less water because the minerals in the mantle hold less water at higher temperatures. Assuming the mantle is currently over 0.3 to 0.8 times the mass of the ocean, a larger surface ocean could have existed at the start of the Archean. At that time, the mantle was around 1,900 to 3,000 degrees Kelvin (2,960 to 4,940 degrees Fahrenheit), compared to 1,600 to 2,600 degrees Kelvin (2,420 to 4,220 degrees Fahrenheit) today.

If early Earth had a larger ocean than it is today, it could have changed the makeup of the early atmosphere and reduced the amount of sunlight reflected back into space, the authors say. These factors are said to have affected the climate and habitat that supported early life on Earth.

“It’s sometimes easy to forget that the deep interior of a planet is actually important to what happens with the surface,” said Rebecca Fischer, a mineral physicist at Harvard University and co-author of the new study. “If the mantle can only hold a limited amount of water, it has to go somewhere else, so what happens thousands of miles below the surface can have pretty big implications.”

The sea level on Earth has remained fairly constant over the past 541 million years. The sea levels of the earliest times in Earth’s history, however, are more difficult to estimate, as little evidence has survived from the Archean Aeon. Over geologic time, water can move from the ocean surface inward through plate tectonics, but the size of this water flow is not well understood. Because of this lack of information, scientists had assumed that the overall size of the ocean remained constant over geological time.

In the new study, co-author Junjie Dong, a mineral physicist at Harvard University, developed a model to estimate the total amount of water the Earth’s mantle could potentially store based on its temperature. He incorporated existing data on how much water different mantle minerals can store and considered which of these 23 minerals would have occurred at different depths and times in Earth’s past. He and his co-authors then linked these storage estimates to the volume of the surface ocean when the Earth cooled.

Jun Korenaga, a geophysicist at Yale University who was not involved in the research, said it was the first time that scientists have linked mineral physics data on mantle water storage with the size of the ocean. “This link has never been mentioned in the past,” he said.

Dong and Fischer point out that their estimates of the mantle’s water storage capacity are highly uncertain. For example, scientists don’t fully understand how much water can be stored in bridgmanite, the main mineral in the mantle.

The new findings shed light on how the global ocean may have changed over time and may help scientists better understand water cycles on Earth and on other planets, which could be helpful in understanding where life can evolve.

“It is certainly useful to know something quantitative about the evolution of the global water balance,” said Suzan van der Lee, a seismologist at Northwestern University who was not involved in the study. “I think it’s important for basic seismologists like myself, who take pictures of the mantle’s current structure and estimate its water content, but it’s also important for people looking for exoplanets that contain water and which ask for the origin of our water. . “

Dong and Fischer now use the same approach to calculate how much water can be contained inside Mars.

“Today, March looks very cold and dry,” Dong said. “But a lot of geochemical and geomorphological evidence suggests that the beginning of Mars could have contained water on the surface – and even a small ocean – so there is a lot of interest in understanding the water cycle on Mars. ”