In order to understand where and how life emerged and evolved, it is critical to find fossil traces of life earliest stages. Among these traces, cellular microfossils give key insight into ecological and morphological evolution. However, when looking for microfossils in Archean rocks, micropaleontologists are confronted to several pitfalls. First, the hydrothermal environments which are believed to have hosted Early Life stages are also the place of abiotic processes creating artefacts resembling cellular remains. Second, Archean formations have undergone secondary hydrothermalism and metamorphism, which have strongly altered anycarbonaceous microstructures.
This aim of the PhD project is to better characterize the abiotic artefacts produced in alkaline hydrothermal environments, and to find criteria to distinguish these artefacts from actual cellular remains, even after metamorphism of the host rock.
In this perspective, silica-witherite biomorphs are first synthesized and characterized in conditions mimicking Archean alkaline hydrothermal systems. In a second step, biomorphs thus obtained are experimentally silicified. Indeed, siliceous cherts are ubiquitous in the oldest cratons, and they have been found to be exceptional preservation matrices. Thirdly, in order to simulate metamorphism of the host rock, the samples formed this way are experimentally altered at high-P and high-T in autoclaves. The products are subsequently analyzed through a series of in-situ techniques: optical and electronic microscopy, Raman and Infrared Spectroscopy, Mass Spectrometry (Nano-SIMS) and X-ray synchrotron spectroscopy (XANES). The results of these analyses will be used in order to make assessment of biogenicity in natural samples from the Pilbara Craton (Australia).
This PhD project is part of the ERC-funded project TRACES, directed by Dr Mark van Zuilen.