IDENTIFICATION OF BIOGENICITY BY ANALYTICAL GEOCHEMICAL TECHNIQUES IN EXCEPTIONALLY PRESERVED PLANT FOSSILS FROM THE MESOZOIC OF BRAZIL

Among the fossil plants preserved in the Triassic and Jurassic levels of the central region of Rio Grande do Sul (Brazil), it is common the presence of branches and leaves covered by millimetrically dark crusts, attributed to the deposition of iron oxides hydroxides. Internally, the crusts exhibit replicate anatomical features of leaves, such as epidermal cells and stomata, rarer in other preservation types. Two distinct compositional assemblages were identified, sharing a genesis under similar taphonomic constraints. The first represents an older Dicroidium flora (Ladinian-Carnian), meanwhile the second, of an end of Triassic or basal Jurassic age, is characterized by conifers and bennettites. However, both shares the same depositional context, represented by lacustrine laminated mudstones inserted in thick sandstones deposits of high sinuosity river channels. The analysis of the material shows a preferential covering of the more coriaceous or woody structures. The focus of this study was to evaluate the chemical nature of the associated crusts and inform about the first results obtained by the applying of distinct tools: scanning electron microscopy (SEM), dispersive energy spectrometry (EDS), Raman and Mössbauer spectrometry, X-ray diffractometry (DRx), and x-ray fluorescence (FRX). The intention is seeking by its genesis and possible relation with microbial induced activity. The results attested the presence of α-goethite and clay minerals from the smectite group. In the MEV images, the presence of replicated anatomical features in the internal side of the iron oxides and hydroxides (ferrihydrite) coats, support its possible biogenicity. Bacterial exudates, such as extracellular polymeric substances (EPS), twisted stalks and sheaths, was also found and possibly result from the activity of microaerophilic and freshwater neutrophilic organisms. Besides, the applying of geochemical techniques could help in the establishment of the key stages involved in the fossil diagenesis of those plant fossils, and ensures to observe the authigenic preservation of this exceptional taphonomic biosignature.