Reassessing evidence of life in 3,700-million-year-old rocks of Greenland
Abstract
The
Palaeoarchean supracrustal belts in Greenland contain Earth’s oldest
rocks and are a prime target in the search for the earliest evidence of
life on Earth. However, metamorphism has largely obliterated original
rock textures and compositions, posing a challenge to the preservation
of biological signatures.
A recent study of 3,700-million-year-old rocks
of the Isua supracrustal belt in Greenland described a rare zone in
which low deformation and a closed metamorphic system allowed
preservation of primary sedimentary features, including putative conical
and domical stromatolites1
(laminated accretionary structures formed by microbially mediated
sedimentation).
The morphology, layering, mineralogy, chemistry and
geological context of the structures were attributed to the formation of
microbial mats in a shallow marine environment by 3,700 million years
ago, at the start of Earth’s rock record. Here we report new research
that shows a non-biological, post-depositional origin for the
structures. Three-dimensional analysis of the morphology and orientation
of the structures within the context of host rock fabrics, combined
with texture-specific analyses of major and trace element chemistry,
show that the ‘stromatolites’ are more plausibly interpreted as part of
an assemblage of deformation structures formed in carbonate-altered
metasediments long after burial. The investigation of the structures of
the Isua supracrustal belt serves as a cautionary tale in the search for
signs of past life on Mars, highlighting the importance of
three-dimensional, integrated analysis of morphology, rock fabrics and
geochemistry at appropriate scales.
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