Abundant carbon dioxide in the ancient Earth's atmosphere warmed the planet through the greenhouse effect, making up for the limited light cast by the young sun, scientists reported Monday in a study that examined how seawater chemistry billions of years ago differed from that of today.

The researchers modeled how reactions between dissolved carbon dioxide and rock in the ocean's crust could have influenced the types of oxygen that were present in seawater and eventually preserved in sediments. The team reported the findings in Proceedings of the National Academy of Sciences.

During the Archean Eon, which lasted from about 4 billion to 2.5 billion years ago, the sun was only about 70% to 80% as luminous as today, says Daniel Herwartz, an isotope geochemist at the Universität zu Köln in Germany and first author of the study. However, there's only limited evidence of ice prior to about 3 billion years ago, which indicates that the early Earth wasn't frigid. Additionally, the composition of ancient sediments found in places such as Pilbara, Western Australia implies that the oceans may have been as hot as 70 degrees Celsius (158 degrees Fahrenheit) early in our planet's history. 

This discrepancy is known as the faint young sun paradox. Climate modelers have struggled to explain why the early Earth wasn't freezing cold, Herwartz says.

To gauge the temperature of primordial seas, researchers analyze the ratios of different kinds, or isotopes, of oxygen in sediments such as carbonate rocks and cherts that precipitated from seawater. The relative difference between the amount of a heavier oxygen isotope known as ¹⁸O and a lighter one known as ¹⁶O lessens as temperatures rise.  

"As you go back in time, these isotopic ratios decrease further and further, which implies the ocean became hotter and hotter," Herwartz said. "And at some point these temperatures really get unreasonably high."

There are several explanations for these measurements. One is that ocean temperatures really were very toasty. Alternatively, it's possible that the isotopic composition of ancient seawater differed from what scientists observe in modern oceans, which would change how the sediments should be interpreted.

"We can now better explain why the oxygen isotopic composition of ocean water was probably a bit lower than we have thought before, and it can be decreased by increasing the carbon dioxide concentration in the atmosphere," Herwartz said.

Today, there are several processes that add or remove heavy and light oxygen isotopes from the ocean, the most important of which are continental weathering and hydrothermal alteration at the mid-ocean ridges. The former happens when minerals dissolve and form new minerals with different oxygen isotopic compositions, pulling ¹⁸O from the water. 

The opposite effect is achieved with hydrothermal alteration.  

"The minerals that reform have less of the heavy isotope, and so you basically push heavy ¹⁸O into the ocean water," Herwartz said.

A third mechanism occurs at high concentrations of carbon dioxide. In carbon dioxide weathering, dissolved gas reacts with the oceanic crust to produce carbonate and silicates, extracting heavy oxygen isotopes from the water in the process. 

Carbon dioxide weathering doesn't significantly affect the isotopic composition of the present-day ocean. However, Herwartz and his team propose, it could explain the low oxygen isotope ratios found in ancient sediments.

The researchers modeled how high carbon dioxide concentrations in the atmosphere could affect the ratios of ¹⁸O to ¹⁶O, and of another oxygen isotope known as ¹⁷O to ¹⁶O, in seawater. ¹⁷O behaves similarly to its slightly heavier cousin ¹⁸O, Herwartz said, and is found in low amounts relative to ¹⁶O in ancient sediments. 

He and his team found that increasing carbon dioxide concentrations, rather than increasing continental weathering or decreasing hydrothermal alteration, in their models best represented how the oceanic isotopic composition could change, leading to the low ratios of heavier to lighter oxygen isotopes observed in sediments from the Archean. 

The researchers estimate that carbon dioxide in the Archean atmosphere could have exerted an atmospheric pressure of 1 bar. This would be many times higher than present carbon dioxide levels on Earth, but much lower than the 90 bars estimated for Venus's dense, carbon dioxide-rich atmosphere. 

The researchers calculated that the ratio of ¹⁸O to ¹⁶O in seawater slowly changed between 3.2 billion and 2.6 billion years ago, rising from about 0.5% to 0.2% less than levels found in modern ocean water. This would have coincided with a major drop in atmospheric carbon dioxide during this period, as well as the start of plate tectonics around 3 billion years ago. 

Plate tectonics would have sharply reduced the carbon dioxide content of the atmosphere for a variety of reasons involving the cooling of the upper mantle and generation of land masses and topography. This provided a supply of weatherable rocks, making the carbon dioxide weathering process more effective, and a reservoir to store carbon. 

The findings suggest that the early oceans were warm — perhaps 40 degrees Celsius (104 degrees Fahrenheit) — but not hot, Herwartz says. The ratio of heavy-to-light oxygen isotopes in seawater was lower than it is today because there was so much carbon dioxide in the atmosphere. This abundant carbon dioxide would have warmed the planet through the greenhouse effect, counteracting the weak young sun.

One limitation of the research is that it dealt with computer models; in the future, Herwartz wants to find more direct evidence to quantify how the isotopic composition of seawater could change over time. 

Additionally, this study only used measurements taken from silicate minerals. The researchers next plan to analyze other sediments such as carbonates and phosphates to see if their oxygen isotopic ratios match that of the silicate minerals. This would verify that the oxygen isotopic ratios in the silicate minerals weren't influenced by some other, unrecognized process and give a fuller picture of the chemistry of the ancient ocean, Herwartz says.

The study, "A CO₂ greenhouse efficiently warmed the early Earth and decreased seawater ¹⁸O/¹⁶O before the onset of plate tectonics," published May 31 in Proceedings of the National Academy of Sciences, was authored by Daniel Herwartz, Universität zu Köln; Andreas Pack, Georg-August-Universität Göttingen; and Thorsten J. Nagel, Aarhus University.