The oldest stromatolites on Earth and the quest for life on Mars

The oldest morphological signs of life on Earth are frequently very contentious, both because non-biological processes can produce substantially comparable structures and because such fossils have frequently been exposed to extensive modification and metamorphism. Stromatolites, layered organo-sedimentary structures reflecting complex interplays between microbial communities and their environment, have long been regarded as key macrofossils for detecting life in ancient sedimentary rocks; however, the biological origin of ancient stromatolites has been frequently questioned.

Despite the fact that these stromatolites have been subjected to severe diagenesis and weathering and contain no organic materials, a team led by Dr. Keyron Hickman-Lewis of the Natural History Museum in London used optical and electron microscopy, elemental geochemistry, Raman spectroscopy, and laboratory- and synchrotron-based tomography to identify numerous characteristics suggestive of a biological origin.

In addition to achieving laboratory tomography of 3D stromatolitic macrostructures, the team was able to accomplish the first sub-micron pixel and voxel sizes for imaging of Precambrian stromatolite microstructures by phase contrast imaging at the Elettra Synchrotron in Trieste, Italy. This allowed the discovery of non-uniform layer morphologies, blank spaces caused by the degassing of decaying organic components, and pillar-like vertical structures interpreted as microbial palisade structure, a common sign of phototrophic development.

Due to recent weathering, the Dresser Formation stromatolites have been mostly replaced by hematite. While this makes organic geochemical investigations unfeasible, it is extremely important for the hunt for life on Mars.

Sedimentary rocks on Mars' surface have been subjected to comparable ubiquitous oxidation, and their upper centimetres to metres are largely iron oxides. In this light, the Dresser Formation stromatolites may be particularly useful in informing us of the precise manner of biosignature preservation that can be predicted on Mars. As the Mars 2020 Perseverance rover explores Jezero crater, we should look for morphological manifestations of life similar to those seen in the Dresser Formation and plan for advanced multi-technique analysis when Martian materials are returned to Earth.

(source : ANI)