New 'biosignature' to track remains of ancient life on Earth

Scientists have discovered a new 'biosignature' to track the remains of ancient life preserved in rocks, which are significantly altered over billions of years and could help identify life elsewhere in the solar system.

The finding can help scientists to track records of life on Earth in highly metamorphosed rocks over 3,700 million years old, with organic material often turning into the carbon-based mineral graphite, said researchers from University College London (UCL) in the UK.

In the first study, published in the journal Earth and Planetary Science Letters, the team analysed ten rock samples of banded iron formations (BIF) from Canada, India, China, Finland, US and Greenland spanning over 2,000 million years of history.

They noted that carbon preserved in graphite-like crystals --'graphitic carbon' -- located alongside minerals such as apatite, which our teeth and bones are made of, and carbonate, are the biosignatures of the oldest life forms on Earth.

Life on Earth is all carbon-based and over time, it decomposes into different substances, such as carbonate, apatite and oil. These become trapped in layers of sedimentary rock and eventually the oil becomes graphite during subsequent metamorphism in the crust," said Dominic Papineau from UCL.

"Our discovery is important as it is hotly debated whether the association of graphite with apatite is indicative of a biological origin of the carbon found in ancient rocks," Papineau said in a statement.

"We now have multiple strands of evidence that these mineral associations are biological in banded iron formations. This has huge implications for how we determine the origin of carbon in samples of extra-terrestrial rocks returned from elsewhere in the solar system," he said.

The team investigated the composition of BIF rocks as they are almost always of Precambrian age (4,600 million years old to 541 million years old) and record information about the oldest environments on Earth.

They analysed the composition of rocks ranging from 1,800 million years old to over 3,800 million years old using a range of methods involving photons, electrons, and ions to characterise the composition of graphite and other minerals of potential biogenic origin.