Isua supracrustal belt, southwest Greenland

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A belt of roughly 3.7–3.8 billion-year-old metasedimentary and metavolcanic rocks northeast of Nuuk, and one of the very few places on Earth where sedimentary rocks of that age survive at all. Isua is where the argument about the earliest life is conducted, because it offers two quite different kinds of claim. The first is geochemical: graphite particles in 3.7 Ga seafloor sediments are depleted in carbon-13 to a degree that on the modern Earth is characteristic of biological carbon fixation, reported by Rosing in 1999. The second is morphological: a 2016 paper reported 3.7 Ga conical structures interpreted as stromatolites, exposed by unusually recent snow-melt. A 2018 reassessment of the same outcrop argued the structures are deformation features produced by metamorphism rather than layered microbial build-ups. Isua has been metamorphosed to amphibolite facies, which is precisely the problem — heat and strain can both destroy biological signal and manufacture things that resemble it.