Depositional environment of Toarcian shales from northern Germany as monitored with porphyrins

Abstract

Changes in the depositional environment of Lias ϵ (Toarcian shales) and adjacent Pliensbachian and Aalenian shales in northern Germany were inferred from the porphyrin distribution of the extracts. The predominance of nickel porphyrins or the absence of vanadyl porphyrins in the extracts from the Pliensbachian and Aalenian shales is taken as reflecting its deposition in an oxic environment. In such environments, vanadium is not present as VO 2+ and, thus, is not available for complexation with porphyrins. The higher concentration of porphyrins, as well as the presence of vanadyl porphyrins in the extracts from the Posidonia shale section, is related to its deposition in an anoxic environment. In an anoxic environment, the pheopigments, the precursors to porphyrins, are better preserved. Also, in an anoxic environment, vanadium is present as VO 2+ and is thus available for complexation with porphyrins. Within the Posidonia shale section, the concentrations of porphyrins vary. The variations in the concentrations of nickel and vanadyl porphyrins are synchronous in Units I and II. In Unit III, the concentrations of nickel porphyrins are uniform, but that of vanadyl porphyrins decrease gradually. These variations most likely reflect changes in productivity coupled with changes in the redox conditions of depositional environment.