10 April 2019

Climate fluctuations 2.5 billion years ago

Earth scientists at Utrecht University and University of Geneva have shown for the first time that the Earth’s climate was subject to regular changes 2.5 billion years ago, when conditions were fundamentally different from those today. The changes were due to periodic fluctuations in the shape of Earth’s orbit. They published their findings on 8 April in the leading journal, Nature Geosciences.

Milankovitch cycles

The shape of Earth’s orbit and the orientation of its axis undergo regular fluctuations over periods of many thousands to millions of years. The fluctuations, known as Milankovitch cycles (after the Serbian geophysicist Milutin Milankovitch), have a major effect on the distribution of sunlight over the planet’s surface, and consequently on climate.

Astronomically induced climate changes of this kind have been recorded in the Earth’s geological record of the past hundreds of millions of years, in the form of highly regular patterns of successive rock strata. Scientists have studied these in detail, enabling the changes to be identified precisely.

Photograph: Banded iron formation in a drill core

Fundamentally different

Hardly anything is known about climate changes still further back in time, however. Billions of years ago, conditions on Earth were fundamentally different from those at present: there was no free oxygen in the atmosphere yet, volcanic activity was more violent, and no vegetation or multicellular life had developed. Nevertheless, there must have been fluctuations in the Earth’s orbit and axis that may have affected the climate at the time, and therefore possibly the early biosphere and the chemical composition of the oceans.

Banded iron formations

Earth scientist Margriet Lantink and her fellow researchers examined whether Milankovitch cycles are recognisable in 2.5 billion-year-old banded iron formations. BIFs are iron-rich, characteristically layered rocks that were widely deposited on the ocean floor at that time and can now be found on the oldest extant parts of the Earth’s crust – in South Africa in this case.

Regular large-scale alternations (letters are 5m and numbers are 20m) in banded iron formation due to climatic fluctuations which form an identical pattern over a distance of 250 km.

Until now, scientists have explained the deposition of these iron formations and their regular layering as being due mainly to increased submarine volcanic activity, the hydrothermal source of the iron. In addition, the evolution of photosynthesis may have produced oxygen in the shallowest parts of the ocean, causing the iron dissolved in the water to be oxidised and fall to the bottom.

Linking regular alternations to cyclical changes

The researchers are the first scientists to have managed to link the regular alternations in the BIFs to cyclical changes in the Earth’s orbit around the sun, with periods of 405,000 years and 1.4 to 1.6 million years. They achieved this by combining the research results with very precise uranium-lead dating. The combined research shows that Milankovitch cycles 2.5 billion years ago were already having a major effect on the planet’s climate and on iron deposition in the oceans. Further research will be needed to show precisely how this happened.