Earth’s oldest stromatolites and the search for life on Mars

The earliest morphological traces of life on Earth are often highly controversial, both because non-biological processes can produce relatively similar structures and because these fossils have often been subjected to advanced weathering and metamorphism.

Stromatolites, layered organo-sedimentary structures reflecting complex interactions between microbial communities and their environment, have long been considered key macrofossils for the detection of life in ancient sedimentary rocks; however, the biological origin of ancient stromatolites has often been criticized.

An article published Friday in the magazine Geology uses a range of advanced analytical techniques in two and three dimensions to establish the biological origins of Earth’s oldest stromatolites of the 3.48 billion year old Dresser Formation in Pilbara, Western Australia.

Although these stromatolites have undergone severe diagenesis and weathering and retain no organic matter, a team led by Dr Keyron Hickman-Lewis from the Natural History Museum in London used light and electron microscopy, elemental geochemistry, Raman spectroscopy and laboratory and synchrotron tomography to identify many features indicating a biological origin.

In addition to performing laboratory tomography of the 3D stromatolite macrostructure, the team was able to obtain the first submicron pixel and voxel sizes for imaging Precambrian stromatolite microstructures by phase contrast imaging using the SYRMEP beamline at the Elettra synchrotron, Trieste, Italy. This allowed the identification of non-uniform layer morphologies, void spaces resulting from outgassing of decaying organic materials, and vertical pillar-like 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. Although this makes organic geochemical analyzes impossible, this composition is highly relevant to the search for life on Mars.

Sedimentary rocks on the surface of Mars have been subjected to similar widespread oxidation and also include mostly iron oxides in their upper centimeters to meters. In this regard, stromatolites from the Dresser Formation may be particularly relevant materials to inform us of a precise style of preservation of the biosignature expected on Mars.

As the Mars 2020 Perseverance rover continues its exploration of Jezero Crater, we must search for morphological expressions of life resembling those identified in Dresser’s formation and prepare for advanced multi-technique analyzes when Martian samples are eventually returned to Earth.