HOW TO FABRICATE A MICROFOSSIL: NEW INSIGHTS ON THE FORMATION AND PRESERVATION OF FALSE MICROBIAL BIOSIGNATURES IN THE ROCK RECORD (Invited Presentation)

Our understanding or the early record of life on Earth relies on our ability to identify microbial biosignatures in the geological record. Among those, morphological signatures (i.e. microfossils), which often consist in microscopic spheres and filaments preserved in rocks, are particularly difficult to interpret.

Recently, we discovered that the reaction of sulfide with dissolved organic compounds can lead to the formation of micrometric spheres and filaments composed of organic carbon and elemental sulfur. This reaction occurs in modern sulfidic environments, and might have been widespread in the Proterozoic, when vast portions of the oceans were euxinic.

Here, we investigated the potential for these biomorphic carbon-sulfur microstructures to produce pseudofossils in the rock record. We performed experimental silicification of the microstructures to mimic early stages of fossilization in cherts, a rock type that hosts many Proterozoic putative microbial fossils. Silicification of the filamentous sulfur bacterium Thiothrix was performed in parallel as a positive control for microfossil formation. The products of these silicification experiments were characterized using an array of micro- to nano-scale analytical methods (SEM, TEM, micro-Raman, STXM) and bulk spectroscopic techniques (FTIR, S K-edge XANES).

Both carbon-sulfur microstructures and Thiothrix cells were morphologically preserved by the rapid precipitation (within a few days) of nano-colloidal silica at their surface. Within a few weeks, elemental sulfur minerals disappeared from the interiors of both microstructures and bacterial cells, while their organic walls became sulfurized. Sulfurization may favor the long-term preservation of both microfossils and pseudofossils in the rock record. After several months of silicification, the abiotic microstructures retained their biomorphic morphologies better than the bacterial cells. Microstructures could be discriminated from silicified Thiothrix based on their organic composition, as determined by FTIR spectroscopy and C K-edge and N K-edge STXM analyses. Future experiments at high temperature and pressure will determine how these morphological and chemical signatures evolve through diagenesis.