In situ imaging of organic sulfur in 700–800 My-old Neoproterozoic microfossils using X-ray spectromicroscopy at the S K-edge

The study of very ancient microfossils has recently raised contentious issues regarding interpretation of the biogenicity of the structures. In situ investigation of certain elements such as sulfur within potential microfossils is a powerful complement to other methods of investigation that can provide valuable information on biogenicity. We present here a first such study on Precambrian microfossils from the 700–800-My-old Neoproterozoic Draken Formation, Svalbard, using scanning X-ray microscopy (SXM) in the fluorescence mode and X-ray absorption near edge spectroscopy (XANES) at the sulfur K-edge. SXM allowed mapping of up to 300 ppm of probably endogenous sulfur within the kerogenous walls of Myxococcoides chlorelloidea microfossils. XANES showed that the sulfur is most likely contained in heterocyclic organic compounds, such as thiophene(s).