Archean Earth Was Covered by Global Ocean, New Study Suggests
The surface of Earth was likely covered by a global ocean 3.24 billion years ago (Archean Eon), according to a new study published in the journal Nature Geoscience.
“Our findings could help scientists to better understand how and where single-cell organisms first emerged on Earth,” said Dr. Boswell Wing, a researcher in the Department of Geological Sciences at the University of Colorado Boulder.
“The history of life on Earth tracks available niches. If you’ve got a waterworld, a world covered by ocean, then dry niches are just not going to be available.”
In the study, Dr. Wing and colleagues examined 3.24-billion-year-old hydrothermally altered oceanic crust from the Panorama district in the Pilbara Craton of Western Australia.
“There are no samples of really ancient ocean water lying around, but we do have rocks that interacted with that seawater and remembered that interaction,” said Dr. Benjamin Johnson, a scientist at Iowa State University.
“The process is like analyzing coffee grounds to gather information about the water that poured through it.”
To do that, the researchers analyzed data from more than 100 rock samples from across the dry terrain.
They were looking, in particular, for two different isotopes of oxygen trapped in stone: a slightly heavier atom called oxygen-18 and a lighter one called oxygen-16.
They discovered that the ratio of those two isotopes of oxygen may have been a bit off in seawater 3.24 billion years ago — with just a smidge more oxygen-18 atoms than you’d see today.
“Though these mass differences seem small, they are super sensitive,” Dr. Wing said.
Sensitive, it turns out, to the presence of continents.
“Today’s land masses are covered by clay-rich soils that disproportionately take up heavier oxygen isotopes from the water — like mineral vacuums for oxygen-18,” Dr. Wing said.
The study authors theorized that the most likely explanation for that excess oxygen-18 in the ancient oceans was that there simply weren’t any soil-rich continents around to suck the isotopes up. That doesn’t mean, however, that there weren’t any spots of dry land around.
“There’s nothing in what we’ve done that says you can’t have teeny, micro-continents sticking out of the oceans,” Dr. Wing said.
“We just don’t think that there were global-scale formation of continental soils like we have today.”
Which leaves a big question: when did plate tectonics push up the chunks of rock that would eventually become the continents we know and love?
The scientists aren’t sure. But they’re planning to scour other, younger rock formations at sites from Arizona to South Africa to see if they can spot when land masses first roared onto the scene.
“Trying to fill that gap is really important,” Dr. Johnson said.
B.W. Johnson & B.A. Wing. 2020. Limited Archaean continental emergence reflected in an early Archaean 18O-enriched ocean. Nat. Geosci 13, 243-248; doi: 10.1038/s41561-020-0538-9