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 such fossils have often undergone advanced alteration and metamorphism.

Stromatolites, layered organo-sedimentary structures that reflect complex interactions between microbial communities and their environment, have long been considered key macrofossils for detecting life in ancient sedimentary rocks. However, the biological origin of ancient stratoliths has often been criticized.

An article published Friday in the magazine Geology uses a range of advanced 2D and 3D analytical techniques to determine the biological provenance of Earth’s oldest stratoliths from the 3.48-billion-year-old Dresser Formation, Pilbara, Western Australia.

Although these stromatolites have undergone severe diagenesis and erosion and do not preserve organic materials, a team led by Dr. Keyron Hickman-Lewis of the Natural History Museum, London, used optical and electron microscopy, elemental geochemistry, Raman spectroscopy and laboratory-based synchrotron tomography to identify several features indicative of biological origin.

In addition to performing laboratory tomography of 3D stromatolite macrostructure, the team was able to achieve the first submicron pixel and voxel sizes for imaging Precambrian stromatolite microstructures via phase contrast imaging using the SYRMEP beamline at the Elettra Synchrotron, Trieste, Italy. This allowed the identification of nonuniform layer morphologies, voids resulting from degassing of decaying organic materials, and vertical pillar-like structures interpreted as microbial pile structure, a common indicator of phototrophic growth.

Dresser Formation stromatolites have been mostly replaced by hematite (iron oxide) due to recent weathering. While this makes organic geochemical analyzes impossible, this composition is very important to the search for life on Mars.

Sedimentary rocks on the Martian surface have undergone similar diffuse oxidation and also contain mainly iron oxides in their upper centimeters to meters. In this regard, Dresser Formation stromatolites may be uniquely relevant materials to inform us about a precise style of biosignature preservation expected on Mars.

As the Mars 2020 Perseverance rover continues its exploration of Jezero Crater, we should look for morphological expressions of life similar to those identified in the Dresser Formation and prepare for advanced multi-technological analyzes when the Martian samples are eventually returned to Earth.