The isotopic composition of carbon in iron formations from the Saglek-Hebron complex in Nunatsiavut (northern Labrador) was considered evidence of the earliest traces of life on Earth. But a new study from the University of Ottawa, Carleton University and University College London suggests the opposite.

The study shows that the petrographic, geochemical and spectroscopic characteristics of the graphite (the crystalline form of carbon) found in the chemical sedimentary rocks of Saglek-Hebron are indeed “abiotic,” that is, they represent non-living physical or chemical aspects of an environment or that do not contain life.

This improves our understanding of how early biomass changed on Earth and highlights the interaction between non-biological processes and remains of ancient life. The study of graphitic materials is key to deciphering the carbon cycle on the early Earth.

This study is crucial to the search for early life on Earth and possibly on neighboring planets.

What the researchers did

Using micro-Raman spectroscopy, the researchers examined the isotopic signatures in these rocks. Their results show that graphite could have come from liquid substances containing carbon, hydrogen and oxygen, probably resulting from the decomposition of ancient organic materials.

“Our study focuses on chemical sedimentary rocks found in the Saglek-Hebron. These rocks are among the oldest on Earth, dating back 3.9 billion years. They were formed by oceanic precipitation. They contain banded ore formations that may have been formed by the activity of bacteria,” explains co-author Jonathan O’Neil, associate professor in the Department of Earth and Environmental Sciences at the University of Ottawa.

“They are ideal for studying ancient biological processes. Our study challenges the previous interpretation that the carbon isotope composition of these rocks indicates a biological origin, but their spectroscopic properties are more indicative of abiotic properties. This leads us to rethink the processes responsible for isotopic signatures and their connection to the action of microorganisms,” adds O’Neil.

Last year’s research focused on samples collected during a 2016 field campaign in Nunatsiavut. Petrological characterization was conducted in Ottawa and spectroscopic analysis of graphitic carbon was performed in London, UK.

Thin sections a SG-274 (banded ore formation; BIF), b SG-236 (BIF) and c SG-275 (marble) are shown at 2.5 cm width. The corresponding X-ray mineral maps of thin sections d SG-274 and e SG-236 show a dominance of quartz, magnetite and amphibole and f SG-275 where calcite, ankerite, olivine and antigorite dominate. g Selected area in SG-274 (red box in panel a) shows 576 calcite grains (green dots), 339 graphite grains (GraI; red dots) and 64 calcite-graphite associations (GraII; yellow dots). The petrographic occurrences show significantly more GraI and GraII near the central zone of coarse quartz and predominantly graphite-free calcite grains near the Fe-silicate and magnetite sections of the BIF. — Nature Communications

Shaped by liquids

“Graphitic carbon from chemical sedimentary rock samples was studied in three sedimentary rock samples that are almost 3.9 billion years old. Spectroscopic analysis of this graphitic carbon suggests that it was formed from metamorphic fluids (at temperatures above 500 °C) rather than by bacterial processes,” says O’Neil.

Research shows that graphite in rocks may have formed in the absence of organic life, possibly through a carbon extraction process. The degree of crystallization of graphite correlates with the metamorphism of the rock, suggesting that metamorphism influences the preservation and alteration of carbon-based materials.

The study, titled “Abiotic synthesis of graphitic carbons in the Eoarchean Saglek-Hebron metasedimentary rocks,” was published in Nature Communications. (Open Access)

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