Ancient ‘biosignatures’ of Earth’s oldest life forms identified

It’s thought that microfossils from ancient, highly metamorphosed rocks are destroyed in the high temperatures and pressures associated with that process – but new research published today in the Journal of the Geological Society suggests some vital ‘biosignatures’, such as molecular, elemental and isotope signatures, may survive. The results help track the remains of ancient life preserved in some of Earth’s oldest rocks, and may even help identify life elsewhere in the Solar System.

Almost all evidence for the earliest traces of life on Earth comes from particles of graphitic carbon preserved in metamorphosed rocks. The potential source of that carbon is debated, with some suggesting it may be non biological or could have been introduced to the rock later on. The new study, alongside a study by the same group published in Earth and Planetary Science Letters, looks at graphitic carbon which occurs alongside carbonates and apatite – a mineral found in our teeth and bones. The researchers conclude that the minerals are most consistent with a biological origin from the remains of Earth’s oldest life forms.

‘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’ says Dr Dominic Papineau of UCL Earth Sciences, Centre for Planetary Science and the London Centre for Nanotechnology.

‘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.’

The first study, published in Earth and Planetary Science Letters, looks at ten rock samples of banded iron formations (BIF) from Canada, India, China, Finland, USA and Greenland, spanning over 2,000 years of Earth’s history.

More than a dozen different Banded Iron Formations from all over the world are now documented to contain graphite (red – or in white circles), apatite (turquoise) and carbonate (green). The thin sections (centre) are paper-thin slices of the rock mounted on one inch round glass.

‘Previously, it was assumed that finding apatite and graphite together in ancient rocks was a rare occurance’ says team member Dr Matthew Dodd (UCL Earth Science and the London Centre for Nanotechnology), ‘but this study shows that it is commonplace in BIF across a range of rock metamorphic grades.’

The researchers then analysed the same minerals from the ca. 1,850 million years old Michigamme silicate banded iron formation from Michigan. The formation, known to contain fossils and to have been metamorphosed at greater than 550 degrees Celcius, contained several biosignatures within graphitic carbon and associated apatite, carbonate and clays. The results of that study are published today in the Journal of the Geological Society.

Together, all of the observations of the composition are consistent with an origin from decayed biomass strongly altered by high temperatures.

Many micro-analytical techniques were used to visually and quantitatively characterize the composition of graphite, shown in shades of lightest gray in the upper left panel. The compositions revealed that the CHNOPS elements (carbon, hydrogen, nitrogen, oxygen, phosphorous, and sulphur) are preserved together even after being cooked at more than 550oC and after almost 2 billion years in the Earth’s crust.

‘Our new data provide additional lines of evidence that graphite associated with apatite in BIF is most likely biological in origin’ says Dr Papineau. ‘Moreover, by taking a range of observations from throughout the geological record, we resolve a long-standing controversy regarding the origin of isotopically light graphitic carbon with apatite in the oldest BIF. We’ve shown that biosignatures exist in highly metamorphosed iron formations from Greenland and northeastern Canada which are more than 3,850 million years old and date from the beginning of the sedimentary rock record.’

The work, partly funded by NASA, could help researchers identify biological signatures in rocks from extraterrestrial environments such as Mars.

  • Journal references:

Paper one: Journal of the Geological Society

‘Fossil biomass preserved as graphitic carbon in a late Paleoproterozoic banded iron formation metamorphosed at more than 550oC’ by Dominic Papineau, Bradley T. De Gregorio, James Sagar, Richard Thorogate, Jianhua Wang, Larry Nittler, David A. Kilcoyne, Hubertus Marbach, Martin Drost, Geoff Thornton

Paper two: Earth and Planetary Science Letters

‘Widespread occurrences of variably crystalline 13C-depleted graphitic carbon in banded iron formations’ by Matthew S. Dodd, Dominic Papineau, Zhen-Bing She, Chakravadhanula Manikyamba, Yu-Sheng Wan, Jonathan O’Neil, Juha A. Karhu, Hanika Rizo, Franco Pirajno

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