Can biogeochemistry aid in the palaeoenvironmental/early diagenesis reconstruction of the ˜187 Ma (Pliensbachian) organic-rich hemipelagic series of the Lusitanian Basin (Portugal)?

Abstract

Data on lipids, carbohydrates and proteins of the most expressive black shale (s.l.) intervals of the Early–Late Pliensbachian (Early Jurassic, ~187 Ma) organic-rich hemipelagic series of the Lusitanian Basin (Portugal) were determined using a method that has been successfully applied over the last two decades in the characterization of biomass and very immature sediments. The goal of this paper is to test the applicability of these techniques to the ancient geological record. To our knowledge, this is the first time that this type of biogeochemical data from sedimentary series older than Oligocene is reported and tentatively used for palaeoenvironmental/diagenetic inferences. Carbohydrates and proteins are present in low concentrations, reaching up to 385.13 and 451.13 μg/g rock, respectively. The main variations are observed in the lipid contents, ranging from 197.67 to 8446.36 μg/g rock. The samples with the highest amounts of lipids seem to correlate with low [O2] time intervals determined by independent data, such as organic petrography, micropalaeontology and sedimentology. This was probably related with selective lipid preservation under oxygen and hydrogen sulfide-rich depleted environments. The good overall match between the determined lipid contents and specific depositional/early diagenetical conditions seem to favor the idea that the easy to perform and inexpensive method applied here has the potential to add useful information to the study of ancient organic-rich carbonate sedimentary series. •