Earth’s Oldest Stromatolites and the Search for Life on Mars
Boulder, Colo., USA: The earliest morphological traces of life on Earth are
often highly controversial, both because non-biological processes can
produce relatively similar structures and because such fossils have often
been subjected to advanced alteration and metamorphism. Stromatolites,
layered organo-sedimentary structures reflecting complex interplays between
microbial communities and their environment, have long been considered key
macrofossils for life detection in ancient sedimentary rocks; however, the
biological origin of ancient stromatolites has frequently been criticized.
An article released Friday in the Geological Society of America journal Geology uses a range of advanced two- and three-dimensional
analytical techniques to establish the biological origins of Earth’s oldest
stromatolites from the 3.48-billion-year-old Dresser Formation, Pilbara,
Western Australia.
Although these stromatolites have undergone severe diagenesis and
weathering and preserve no organic materials, a team led by Dr. Keyron
Hickman-Lewis of the Natural History Museum, London, has used optical and
electron microscopy, elemental geochemistry, Raman spectroscopy, and
laboratory- and synchrotron-based tomography to identify numerous
characteristics indicative of a biological origin.
In addition to performing laboratory tomography of 3D stromatolitic
macrostructure, the team was able to achieve the first sub-micron pixel and
voxel sizes for imaging of Precambrian stromatolite microstructures via
phase contrast imaging using the SYRMEP beamline at the Elettra
Synchrotron, Trieste, Italy. This enabled the identification of non-uniform
layer morphologies, void spaces arising from the degassing of decaying
organic materials, and pillar-like vertical structures interpreted as
microbial palisade structure, a common indicator of phototrophic growth.
The Dresser Formation stromatolites have been mostly replaced by hematite
(iron oxide) due to recent weathering. While this renders organic
geochemical analyses impossible, this composition is highly relevant for
the search for life on Mars.
Sedimentary rocks at the surface of Mars have been subjected to similar
pervasive oxidation and also comprise mostly iron oxides in their upper
centimeters to meters. In this regard, the Dresser Formation stromatolites
may be uniquely relevant materials to inform us of a precise style of
biosignature preservation expected on Mars. As the Mars 2020 Perseverance rover continues its exploration of Jezero crater, we
should search for morphological expressions of life resembling those
identified in the Dresser Formation and prepare for advanced
multi-technique analyses when Martian samples are eventually returned to
Earth.
FEATURED ARTICLE
Advanced 2D-3D insights into Earth's oldest stromatolites (~3.5 Ga):
Prospects for the search for life on Mars
Keyron Hickman-Lewis and colleagues
Contact: keyron.hickman-lewis@nhm.ac.uk, The Natural History Museum,
Department of Earth Sciences, London, UK
URL:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50390.1/618747/Advanced-two-and-three-dimensional-insights-into
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