Origins of Life on Earth Grow More Complex as Scientist Debunk Ancient Climate Theory

A group of scientists may have deepened the mystery behind a scientific conundrum known as the young, faint Sun paradox, by publishing a study debunking a theory as to why ancient Earth was warm enough to sustain life even though the Sun's energy was only three-quarters the strength it is today.

The Archean eon, between 3.8 billion and 2.4 billion years ago, saw the evolution of very basic forms of life, mainly bacteria and single-celled organisms -- but a weaker Sun thought to be present at the time would not have been warm enough to sustain life -- a conundrum that has puzzled researchers for decades, despite multiple theories as to why this happened.

"During the Archean the solar energy received at the surface of the Earth was about 20 to 25 percent lower than present," said study author Ray Burgess, from the University of Manchester's School of Earth, Atmospheric and Environmental Sciences. "If the greenhouse gas composition of the atmosphere was comparable to current levels then the Earth should have been permanently glaciated but geological evidence suggests there were no global glaciations before the end of the Archean and that liquid water was widespread."

Exponentially higher greenhouse gas levels -- regulators of the Earth's climate -- have been suggested as a possible explanation, but there is not scientific agreement around that theory.

"To counter the effect of the weaker Sun, carbon dioxide concentrations in the Earth's atmosphere would need to have been 1,000 times higher than present," said Bernard Marty, from the University of Lorraine. "However, ancient fossil soils -- the best indicators of ancient carbon dioxide levels in the atmosphere -- suggest only modest levels during the Archean. Other atmospheric greenhouse gases were also present, in particular ammonia and methane, but these gases are fragile and easily destroyed by ultraviolet solar radiation, so are unlikely to have had any effect."

Lead study author Marty and his team sought to test another theory -- that levels of nitrogen could have been higher in the ancient atmosphere, a condition that would have amplified the greenhouse effect of carbon dioxide while allowing for atmospheric conditions that would support an ice-free Earth.

To test this, the research team examined tiny pockets of air trapped in water bubbles entombed in pieces of quartz from a region in northern Australia known for its exceptionally old and well-preserved rocks.

"We measured the amount and isotopic abundances of nitrogen and argon in the ancient air," Marty explained. "Argon is a noble gas which, being chemically inert, is an ideal element to monitor atmospheric change. Using the nitrogen and argon measurements we were able to reconstruct the amount and isotope composition of the nitrogen dissolved in the water and, from that, the atmosphere that was once in equilibrium with the water."

Marty and his team learned that the partial pressure of nitrogen in the Archean atmosphere was similar, if not even lower, than it is at present, a find that rules out nitrogen as a main contender as a solution to the early climate puzzle.

"The amount of nitrogen in the atmosphere was too low to enhance the greenhouse effect of carbon dioxide sufficiently to warm the planet. However, our results did give a higher than expected pressure reading for carbon dioxide -- at odds with the estimates based on fossil soils -- which could be high enough to counteract the effects of the faint young Sun and will require further investigation," Burgess added.

The research is published in the journal Science.