The Earth of 3.2 billion years ago was a “water world” of submerged
continents, geologists say after analyzing oxygen isotope data from ancient
ocean crust that’s now exposed on land in Australia.
And that could
have major implications on the origin of life.
“An early Earth without emergent continents may have resembled a ‘water
world,’ providing an important environmental constraint on the origin and
evolution of life on Earth as well as its possible existence elsewhere,”
geologists Benjamin Johnson and Boswell Wing wrote in a paper just published
online by the journal Nature Geoscience.
Johnson is an
assistant professor of geological and atmospheric sciences at Iowa State
University and a recent postdoctoral research associate at the University of
Colorado Boulder. Wing is an associate professor of geological sciences at
Colorado. Grants from the National Science Foundation supported their study and
a Lewis and Clark Grant from the American Philosophical Society supported
Johnson’s fieldwork in Australia.
Johnson said his
work on the project started when he talked with Wing at conferences and learned
about the well-preserved, 3.2-billion-year-old ocean crust from the Archaean
eon (4 billion to 2.5 billion years ago) in a remote part of the state of
Western Australia. Previous studies meant there was already a big library of
geochemical data from the site.
Johnson joined
Wing’s research group and went to see ocean crust for himself — a 2018 trip
involving a flight to Perth and a 17-hour drive north to the coastal region
near Port Hedland.
After taking his
own rock samples and digging into the library of existing data, Johnson created
a cross-section grid of the oxygen isotope and temperature values found in the
rock.
(Isotopes are
atoms of a chemical element with the same number of protons within the nucleus,
but differing numbers of neutrons. In this case, differences in oxygen isotopes
preserved with the ancient rock provide clues about the interaction of rock and
water billions of years ago.)
Once he had two-dimensional
grids based on whole-rock data, Johnson created an inverse model to come up
with estimates of the oxygen isotopes within the ancient oceans. The result:
Ancient seawater was enriched with about 4 parts per thousand more of a heavy
isotope of oxygen (oxygen with eight protons and 10 neutrons, written as 18O)
than an ice-free ocean of today.
How to explain
that decrease in heavy isotopes over time?
Johnson and Wing
suggest two possible ways: Water cycling through the ancient ocean crust was
different than today’s seawater with a lot more high-temperature interactions
that could have enriched the ocean with the heavy isotopes of oxygen. Or, water
cycling from continental rock could have reduced the percentage of heavy
isotopes in ocean water.
“Our preferred
hypothesis — and in some ways the simplest — is that continental weathering
from land began sometime after 3.2 billion years ago and began to draw down the
amount of heavy isotopes in the ocean,” Johnson said.
The idea that
water cycling through ocean crust in a way distinct from how it happens today,
causing the difference in isotope composition “is not supported by the rocks,”
Johnson said. “The 3.2-billion-year-old section of ocean crust we studied looks
exactly like much, much younger ocean crust.”
Johnson said the
study demonstrates that geologists can build models and find new, quantitative
ways to solve a problem — even when that problem involves seawater from 3.2
billion years ago that they’ll never see or sample.
And, Johnson
said these models inform us about the environment where life originated and
evolved: “Without continents and land above sea level, the only place for the
very first ecosystems to evolve would have been in the ocean.”
Journal article: https://www.nature.com/articles/s41561-020-0538-9
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