Researchers from The University of Western Australia have discovered that the carbon found in 3.5 billion-year-old black chert – rocks made up of fine-grained quartz crystals – came from organic, carbon-rich compounds deposited in hydrothermal vent systems.
The study, by Professor Birger Rasmussen and Dr Janet Muhling, from UWA's School of Earth Sciences, was published in Science Advances.
Previously the origin of the carbon found in chert from the North Pole area in Western Australia's Pilbara region, was thought to have come from fossilised microbes. This interpretation was later challenged and the 'fossils' were thought to be secondary artefacts, generating debate about the origin of some of the oldest 'fossils' and associated carbon.
Now new research, which used high-magnification microscopic techniques, has found much of the carbon in the black cherts is not from indigenous bacteria but was deposited from solutions containing organic compounds such as oil.
"This suggests that the carbon was deposited together with silica from hot fluids that moved through cracks in volcanic lavas below seafloor vent systems similar to modern black smokers or vent chimneys," Professor Rasmussen said.
"The deposition of organic compounds with silica produced a series of microscopic carbon-rich structures, including petrified oil droplets, which have previously been mistaken for fossils."
The study raised the question as to how organic compounds could form in an environment dominated by thick piles of volcanic lava.
"Some of the organic molecules could have formed by non-biological processes," he said. "For instance, when hot water circulates through igneous rocks such as volcanic lavas, water and carbon dioxide can react with mineral surfaces to form organic compounds."
Similar reactions have been proposed to explain the presence of organic matter in Martian meteorites and in igneous rocks on Earth.
"The results of the research offer a glimpse of an earlier lifeless planet when water-rock reactions at seafloor vents produced a cocktail of organic compounds that may have acted as a continuous feedstock for the assembly of the first living cells," Professor Rasmussen said.
"Carbon is the key element of life and its origin in ancient sedimentary rocks is central to questions about the emergence and early evolution of life."
