Unfortunately, it looks like the northeastern Canadian territory of Labrador might not be the cradle of all life on Earth after all.
A dispute has erupted in the scientific world over whether the oldest evidence of life on this planet was really discovered several years ago in rocks around Saglek Bay, a fjord in northern Labrador’s majestic Torngat Mountains.
In September 2017 a team of nine scientists – led by Tsuyoshi Komiya of the University of Tokyo’s Department of Earth Sciences and Astronomy and Yuji Sano of the same university’s Atmosphere and Ocean Research Institute – announced in the journal Nature that they had discovered deposits of something that pointed to the existence of life almost 4 billion years ago. Specifically, what they found was graphite embedded in what had earlier been determined to be the oldest sedimentary rock on Earth, which had formed on top of the planet’s newly cooled crust about 3.95 billion years ago.
“The graphite grains occur as aggregates and elongated shapes (up to a few tens to hundreds micrometers long) mostly along grain boundaries of other minerals, parallel to the bedding planes,” reads their article. “Some graphite grains also occur as small globular inclusions of 1 to 2 micrometers in the quartz, garnet, biotites, plagioclase and amphibole grains.”
What is significant about this graphite, the authors explained, was that it had a very low ratio of carbon-13 in its composition. That, they concluded, meant life. To explain: Carbon-13 and carbon-12 start out in equal measures, but the autotrophic life (like algae) that consume simple substances prefer the light carbon isotopes. They eat the carbon-12 and when they die they sink to the bottom, leave the carbon-13 behind. When the sediment buries them and they decompose, what remains is mostly carbon-12.
Or, as Komiya and his colleagues wrote in their paper:
“We conclude that the graphite from clastic sedimentary rocks in the Saglek Block has a biogenic origin and the primary d13Corg and d13Ccarb values were estimated to be <-28.2 and >-2.6‰, respectively. As a result, the isotopic fractionation between graphite and carbonate (d13Ccarb-d13Corg) reached -25.6‰ more than those in turbidite-derived sedimentary rocks of the Isua supracrustal belt6. The large fractionation provides the oldest evidence for autotrophs, utilizing reductive acetyl-CoA pathway or the Calvin cycle, over 3.95 Ga.”
What this means, the scientists added, is a little easier to understand:
“The discovery of the biogenic graphite enables geochemical study of the biogenic materials themselves and will open the door to elucidating the early life not only in the Earth, but also in other planets.”
To be clear, they do not claim to have found the earliest life on Earth, since there are other indications it is several hundreds of millions of years older. In fact, 4.1-billion-year-old zircon geologically removed from its point of origin (which remains a mystery) and deposited in Australia was found to contain graphite with a low carbon-13 ratio – which, as mentioned above, is evidence of biological activity.
Also, based on an earlier 3.7-billion-year-old finding of carbon-12-rich graphite in Greenland by same mostly Japanese team working in Labrador, other scientists estimated that the complexity of life at the time meant it must have emerged more than 4 billion years ago, when the Earth was still largely molten.
“If these are really the figurative tombstones of our earliest ancestors, the implications are staggering,” Australian geologist Abigail Allwood wrote in 2016 about the Greenland discovery. “Earth's surface 3.7 billion years ago was a tumultuous place, bombarded by asteroids and still in its formative stages. If life could find a foothold here, and leave such an imprint that vestiges exist even though only a minuscule sliver of metamorphic rock is all that remains from that time, then life is not a fussy, reluctant and unlikely thing. Give life half an opportunity and it'll run with it.”
However, another group of scientists is throwing cold water on the claim that the Labrador graphite samples were found in 3.95-billion-year-old rock – therefore questioning the conclusion that life was that old, too.
Unfortunately, the article “On the true antiquity of Eoarchean chemofossils – assessing the claim for Earth’s oldest biogenic graphite in the Saglek Block of Labrador” that was published in the April 2019 issue of Precambrian Research is not freely available online, but the abstract provides an explanation:
“Unambiguous field relationships between ca. 3.9 Ga tonalitic gneiss and the graphite-bearing metasediments have not been demonstrated in the literature that the Saglek claim relies upon, and earlier U-Pb-Hf isotopic studies on zircon from metasediments at one of the localities used in the claim indicate a Mesoarchean to Neoarchean time of deposition,” wrote Martin Whitehouse and three other geologists from universities in Sweden, Poland, Germany and Australia. “We conclude that, irrespective of the validity of the carbon isotopic evidence, field relationships and geochronological evidence fail to demonstrate an age of >3.95 Ga for the potential traces of life.”
It would be a shame if they were right, although of course sentiment is no valid argument against scientific evidence. Labrador is already the location of one of the earliest signs of human civilization – a 7,000-year-old burial mound near L’Anse Amour – so it would be nice if it was also home to the oldest rocks and the oldest signs of life, as well. Stay tuned for further developments.
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