Earth’s Ancient Carbon Cycle and the First Ice Age

Research published in Nature suggests that changes in atmospheric carbon dioxide levels due to continent formation could well be responsible for the presence of liquid water on early Earth.

During the first half of Earth's history, the Sun supplied less energy to our planet than at present. This fact might imply that early in Earth’s existence, much of the water on the planet was frozen. The geological record, however, shows otherwise, with the first ice age occurring only 2.4 billion years ago. Research published in the 6 June 2013 edition of Nature suggests that changes in atmospheric carbon dioxide levels due to continent formation could well be to blame [1].

Carbon dioxide can dissolve in rainwater, and in doing so makes the pH of the raindrops more acidic. This acid rain, when it falls on the ground, reacts with rocks such as basalts and granites, resulting in carbon becoming trapped there. Such a process results in gradual flux of carbon out of the atmosphere and into Earth’s crust.

Geologists have long posited that the atmosphere on early Earth was so rich in carbon dioxide, a so-called greenhouse gas, that it trapped far more of the Sun’s heat than it does today. This hypothesis would account for the presence of liquid water on the young planet. Direct evidence for this, however, has been lacking.

“This is something that has baffled scientists for years but our findings provide a possible explanation,” said study co-author Dr. Ray Burgess, from the University of Manchester’s School of Earth, Atmospheric and Environmental Sciences.

Earth was born with a high concentration of CO2 in the atmosphere. If all carbon now trapped in sedimentary rocks, such as carbonates, was released, atmospheric pressure would be 95 times what it is today. This situation would parallel that of Venus, where the surface temperature is 450°C due to the extreme abundance of greenhouse gases. Ancient sedimentary rocks here on Earth do not record such extreme partial pressures of CO2, nor such temperatures, and the first global glaciations appeared 2 billion years after Earth's formation. Something must have happened to drastically decrease the concentration of greenhouse gases.

“An increase in continental surfaces, and therefore in possibilities for CO2 sequestration by weathering and alteration, could be the key process " says Prof. Bernard Marty of CPRG-CNRS, the lead author of the study.

Pujol et al, using a sample of ancient water trapped in even more ancient rock from Western Australia, have changed this view. The 3.5 billion year-old water, trapped in extremely small bubbles in quartz, provided the team (a French-English collaboration based at CPRG-CNRS, Université de Lorraine, France, University of Manchester, UK, and IPGP, Paris, France) with a unique glimpse into the composition of the Archaean atmosphere. The most informative metric for them was the ratio of two different isotopes of Argon, 40Ar/36Ar. This ratio in the ancient water, when compared to the ratio seen in today’s atmosphere, was used to estimate the growth of Earth’s crust, and thus the emergence of the continents, over geological time. It was found that up to 80 % of the present-day volume of the continental crust was formed between 3.5 and 2.7 billion years ago. This drastic increase in crustal production led to the emergence of large continental surfaces, which might have acted as an efficient sink for atmospheric carbon dioxide

The data suggest that Earth’s growing continents represented an increasingly large sink for carbon. As acid rain fell, so it reacted more and more with newly born crustal rock. These reactions in turn gradually reduced the levels of carbon dioxide in the atmosphere, cooling Earth. After 2 billion years of this slow carbon flux, the temperature fell and water began to freeze, resulting in the first ice age.

"This study further demonstrates the need to better understand the carbon cycle during interactions between rocks and the environment, and between the deep Earth and the surface of the planet, and the DCO project is certainly an excellent frame to do so " says Marty.

 

Photo credit: A 3.5 billion year-old lava with a cavity containing hydrothermal quartz and carbonates (white minerals). The sample containing 3.5 billion year-old water trapped in tiny inclusions comes from such an assemblage; Bernard Marty

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