This was the climate on Earth 2.7 billion years ago
Oceans at 80°C and an atmosphere loaded with greenhouse gases, this is what the climate looked like in the Neoarchaean 2.7 billion years ago. However, a new study shows that the Arctic region still managed to maintain sub-zero temperatures.
Determining the climate of the past is important to understand the factors and mechanisms that control it and to better predict its future evolution. But climate is a complex concept, not globally uniform, and leaves only indirect traces in the fossil record.
If it is already difficult to accurately determine the climate that prevailed a few hundred thousand years ago, what about the climate of the primitive Earth? Thus, the entire first half of the history of our planet is drowning in the gray area.
However, a team of researchers provides data that allows us to go back in time and look at the climate in the Neoarchean, 2.67 billion years ago.
Oxygen isotope to track past climates
The continental crust began to form very early in Earth’s history and is believed to have formed on the surface of the oceans about 3.3 billion years ago, during the mid-Archaean. Now the soil appeared, says rock-atmosphere-hydrosphere interactions. The continental rise that characterized the Archaean period undoubtedly played a major role in the intensification of these geochemical exchanges and undoubtedly had a profound effect on the climate prevailing on the still young Earth and vice versa.
The hydrological cycle of the continental crust may therefore be a key element for a better understanding of Earth’s climate at the end of the Archaean. Based on this idea, researchers traced one of the isotopes of oxygen: 18O. This isotope actually has certain properties that make it an interesting indicator of the circulation of rainwater on Earth (meteorite water).
Oxygen 18 is a really heavy isotope. In fact, rainwater contains less than seawater, the humidity of the air is closely related to the phenomenon of evaporation. Therefore, there is more oxygen 16 (light) than oxygen 18 in the atmosphere. The ratio between these two isotopes in the sample is defined as δ.18He
Currently δ18The O of seawater is 0‰, and that of rainwater is negative and can reach extreme values (-60‰) under certain conditions. value of δ18Thus, it allows to estimate the humidity in the atmosphere, the humidity level itself is directly controlled by the evaporation temperature. Colder temperatures thus tend to further reduce the amount 18O in the vapor and therefore sink δ18In those extreme negative values.
Reconstructing climate from rock chemistry
However, as stated earlier, since their formation, the rocks of the continental crust have been interacting with rainwater. Thus, the circulation of meteorite water will slightly change the chemical composition of silicate minerals, which will then preferentially “capture” oxygen 16 isotopes.18O is less than 0, the value is related to the value of the water in question. Analysis of certain rocks and determination of their δ18It therefore allows us to reconstruct δ thanks to the inverse approach18O to have an idea of the meteoric waters and thus of the climatic conditions prevailing on the surface.
A frozen North Pole despite an extremely warm global climate
All that remained was to find the appropriate rock sample. It was in the Kola craton in Russia that researchers found an interesting geological witness. Dating back to the Neoarchean (2.67 billion years ago) period, these igneous rocks bear traces of meteoric water circulation and chemical changes. value of δ18From these rocks, it allowed us to make the first assumptions about the nature of precipitation and surface temperature in the region occupied by the Kola craton. According to paleomagnetic reconstructions, the craton was at latitudes near the North Pole 2.7 billion years ago.
The results were published in the journal Geology, indicating that this region is exposed to a cold climate with an average annual temperature below 0°C. According to the sedimentary record, the climate of the Archaean is generally considered to be very warm, with oceans at temperatures of 80 °C and associated with an atmosphere overloaded with CO.2 and methane.
The results presented by DOZakharova and colleagues therefore suggest that a dynamic atmospheric regime existed that allowed cold and frozen zones to be maintained (at least episodically) at the poles, despite the globally stifling environment at that time.
