SEM IMAGING OF BIOSTRUCTURES IN UPPER CRETACEOUS SEDIMENTARY PYRITE: AN ASTROBIOLOGICAL APPROACH

The search for microbial fossils in sedimentary pyrite concerns in the better understanding of fossilization processes in sulfide minerals as well as in the astrobiological exploration, especially in planets in which this mineral could exist, like Mars. Recent works revealed that a wide diversity of microorganisms have been preserved in both Precambrian and Phanerozoic sedimentary pyrite from continental and/or marine environments. This work reports microscopic biostructures observed under Scanning Electron Microscope (SEM) in sedimentary pyrite lenses from the laminated limestone of the Cretaceous Agua Nueva Fm. (Cenomanian/Turonian) of the state of San Luis Potosí, Mexico. Electron Dispersive X-ray Spectrometry (EDS) analyzed the biostructures for elemental composition. The bioelements observed consist of 1) biomorphic structures and 2) non-biomorphic structures related to biological activity. Biomorphic structures consist of coccoid bacteria (0.2mm-5mm of diameter), bacilli (1mm-8mm of length), diatoms (2mm-14mm of length), spheres with smooth surfaces (5mm-9mm of diameter) and ornamented spheres (70mm of diameter). Non-biomorphic elements are represented by patches of smooth surfaces within the pyritic matrix (extracellular medium) interrupted by “teeth and socket” structures, and cylinder-like elements with hollow cores probably associated to microbial sheaths. EDS shows S and Fe in the biostructures indicating they consist of pyrite. The C is abundant in the biomorphic structures (76.39%), less common in the smooth surfaces (39.76%), and absent in the pyritic matrix. It suggests C is associated to the biostructures and probably has an organic origin. The presence of diverse biostructures in the analyzed pyrite represents the fossil record of living organisms that existed in the sediment and the water column during the deposition of the Agua Nueva Fm. or during the genesis of pyrite. On the other hand, recent astrobiological studies have suggested that sedimentary pyrite could be formed in aqueous scenarios in the early past of Mars. Given the high preservation potential of the sedimentary pyrite in Earth, the detection of this mineral in Martian sediments should represent a real opportunity to know the diversity of microorganisms in this planet, if they were present.