Microscopy evidence of bacterialmicrofossils in phosphorite crusts of the Peruvian shelf: Implications for phosphogenesis mechanisms

Phosphorites are sedimentary formations enriched in Ca-phosphate minerals. The precipitation of these minerals is thought to be partlymediated by the activity of microorganisms. The vast majority of studies on phosphorites have focused on a petrological and geochemical characterization of these rocks. However, detailed descriptions are needed at the sub-micrometer scale atwhich crucial information can be retrieved about traces of past ormodern microbial activities. Here, scanning electron microscopy (SEM) analyses of a recent phosphorite crust from the upwelling-style phosphogenesis area off Peru revealed that it contained a great number of rod-like and coccus-like shaped micrometer-sized (~1.1 μm and 0.5 μm, respectively) objects, referred to as biomorphs. Some of these biomorphs were filled with carbonate fluoroapatite (CFA, a calcium-phosphate phase common in phosphorites); some were empty; some were surrounded by one or two layers of pyrite. Transmission electron microscopy (TEM) and energy dispersive X-ray spectrometry (EDXS) analyses were performed on focused ion beam(FIB)milled ultrathin foils to characterize the texture of CFA and pyrite in these biomorphs at the fewnanometer scale. Non-pyritized phosphatic biomorphswere surrounded by a thin (5-15 nmthick) rimappearing as a void on TEM images. Bundles of CFA crystals sharing the same crystallographic orientations (aligned along their c-axis) were found in the interior of some biomorphs. Pyrite formed a thick (~35-115 nm) layer with closely packed crystals surrounding the pyritized biomorphs, whereas pyrite crystals at distance from the biomorphs were smaller and distributed more sparsely. Scanning transmission X-ray microscopy (STXM) analyses performed at the C K-edge provided maps of organic and inorganic carbon in the samples. Inorganic C, mainly present as carbonate groups in the CFA lattice, was homogeneously distributed, whereas organic C was concentrated in the rims of the phosphatic biomorphs. Finally, STXM analyses at the Fe L2,3-edges together with TEMEDXS analyses, revealed that some pyritized biomorphs experienced partial oxidation. The mineralogical features of these phosphatic biomorphs are very similar to those formed by bacteria having precipitated phosphate minerals intra- and extracellularly in laboratory experiments. Similarly, pyritized biomorphs resemble bacteria encrusted by pyrite. We therefore interpret phosphatic and pyritized biomorphs present in the Peruvian phosphorite cr