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Record of Earth's history found in ancient rocks

Chemical signatures in ancient rocks hold the key to the evolution of oxygen and ozone in the Earth'

Chemical signatures in ancient rocks hold the key to the evolution of oxygen and ozone in the Earth's early atmosphere. Image: NASA

Geologists have discovered that sulphur isotopes, locked in rock formations, are a key to understanding the evolution of the Earth's atmosphere, and long term cycles of global warming and ozone depletion.

The study, published in Science, is an analysis of Precambrian sedimentary rocks up to 3.8 billion years old. The analysis reveals a profound change in the chemical reactions involving sulfur and oxygen that begins before 2.1 billion and extends to 2.5 billion years ago. This coincides with a period during which the oxygen levels in the atmosphere are known to have increased sharply.

"This is the first time that anyone has been able to see a record of oxygen from the ancient atmosphere," says Mark Thiemens from the University of California, San Diego (UCSD), "We now know it's possible to track the evolution on Earth of oxygen and ozone, which both coincide with the evolution of life and the build up of the conditions on the planet that led to a major shift in the atmosphere 2.2 billion years ago."

For half of the Earth's 5 billion year history there was little atmospheric oxygen. The first evidence of oxygen in the atmosphere is from 2.2 billion year old banded-iron formations, which imply that there was sufficient oxygen to oxidise the iron in the rocks in a process akin to rusting.

While scientists have long known of this major shift in the earth's atmosphere, up until now they had no way to track the record of oxygen and ozone in the atmosphere.

To track down oxygen the researchers looked for a recognisable signature that originated from chemical processes in the atmosphere - in this case, from the oxidation of sulfur-bearing gases.

From variations in the four most common isotopes of sulfur that were incorporated into sulfide and sulfate minerals in the rocks, the scientists were able to infer that the atmosphere 2.45 billion years ago had limited free oxygen and was the main arena for chemical reactions involving sulfur. That's contrasted to the present-day environment in which the atmosphere has significantly more free oxygen and in which chemical reactions involving sulfur are dominated by terrestrial processes.

"What we found is a geochemical indicator that originated in the atmosphere and it's clearly a global signature," says James Farquhar, a postdoctoral fellow at UCSD. "It appears in samples that are older than 2 billion years, but is most pronounced in samples older than 2.5 billion years."

For decades it's been assumed that the isotopic variations used by the UCSD researchers could only be found in meteorites and were a unique by-product of nucleosynthesis in stars. But in a recent paper, published in the July 13 issue of Nature, the scientists demonstrated that their presence in 20 million-year-old volcanic-ash deposits and 10 million-year-old gypsum deposits reflected chemical processes in the Earth's atmosphere.

The UCSD team's latest discovery pushes that window into the ancient atmosphere back to a critical period in the planet's history - when oxygen and ozone were accumulating in the atmosphere and the first terrestrial forms of life were expanding.

Besides improving knowledge about the ancient atmosphere, the UCSD finding has implications for improving the understanding of long-term atmospheric events in the future, such as global warming.

"One always hears the argument, 'Isn't global warming all part of a natural cycle?'" says Thiemens. "To answer that question, you really want to have a large-scale record. This will give it to us. We really need to understand the past in order to understand the present and the future."