04/06/2021 at 09:15 CEST
A group of scientists has discovered that our planet was covered with a vast global ocean until 3,200 million years ago, due to the fact that the mantle was warmer than it is today. But from a certain moment, part of that water ended up in the Earth’s mantle, which today houses a kind of hidden ‘ocean’ in the form of hydrogen and oxygen attached to certain minerals. Scientists are beginning to understand the relationships between the seas on the planet’s surface and the water within.
A vast global ocean would have covered the primitive Earth, 4,000 to 3,200 million years ago, and this must have been a consequence of having a warmer mantle than the current one, according to new research, which provides relevant news about the formation of the seas.
The new findings challenge previously existing models, which assumed that the size of Earth’s global ocean has remained constant over time, and show that its size may have changed over geological ages, depending on the study authors.
Most of the surface water on Earth exists in the oceans. Nevertheless, there is a second reservoir of water deep inside the Earth, in the form of hydrogen and oxygen attached to the minerals of the mantle.
A new study in AGU Advances estimates how much water the mantle could potentially hold today and how much it could have stored in the past.
The findings suggest that since the early Earth was warmer than it is today, its mantle may have held less water, because the minerals in the mantle retain less water at higher temperatures.
“We found that the water storage capacity of the mantle decreases as the potential temperature of the mantle increases, and its estimated value depends on the water storage capacity of the bridgmanite mineral in the lower mantle,” the authors note.
Assuming that the mantle is currently more than 0.3-0.8 times the mass of the ocean, there could have been a larger surface ocean during the early Archaic, when that mantle was warmer.
At the time, the mantle was roughly 1,900-3,000 degrees Kelvin (2,960-4,940 degrees Fahrenheit), compared to 1,600-2,600 degrees Kelvin (2,420-4,220 degrees Fahrenheit) today.
This fact would have had important implications for the atmosphere. If the early Earth had a larger ocean than today, that could have altered the composition of the early atmosphere and reduced the amount of sunlight reflected back into space, according to the authors. These factors would have affected the climate and the habitat that supported the first life on Earth.
“Sometimes it’s easy to forget that the deep interior of a planet is really important to what happens to the surface,” according to Rebecca Fischer, a mineral physicist at Harvard University and co-author of the new study. “If the mantle can hold so much water, it has to go somewhere else; that is, what happens thousands of kilometers below the surface can have quite large implications.
Earth’s sea level has been fairly constant for the past 541 million years. However, sea levels from earlier in Earth’s history are more difficult to estimate, because little evidence has survived from the Archaic eon.
From the surface to the interior
Through geological time, water can move from the ocean surface to the interior through plate tectonics, but the volume of that water flow is not yet well known. Due to this lack of information, scientists had assumed that the size of the global 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 that the Earth’s mantle could potentially store based on its temperature.
It incorporated existing data on how much water the different mantle minerals can store and considered which of these 23 minerals would have been generated at different depths and times in Earth’s past.
He and his co-authors then linked those storage estimates to the volume of the ocean’s surface as the Earth cooled.
Jun Korenaga, a Yale University geophysicist who was not involved in the research, said this is the first time that scientists have linked mineral physics data on mantle water storage to the size of the ocean. “This connection has never been raised in the past,” he said.
Dong and Fischer note that their estimates of the mantle’s water storage capacity carry a lot of uncertainty. 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 other planets, which could be valuable in understanding where life may evolve.
Reference report: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2020AV000323