DEPOSITIONAL ENVIRONMENT OF TOARCIAN SHALES FROM NORTHERN GERMANY AS MONITORED WITH PORPHYRINS

Changes in the depositional environment of Lias epsilon (Toarcian shal es) and adjacent Pliensbachian and Aalenian shales in northern Germany were inferred from the porphyrin distribution of the extracts. The pr edominance of nickel porphyrins or the absence of vanadyl porphyrins i n 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 VO2+ and, thus, is not available for comp lexation with porphyrins. The higher concentration of porphyrins, as w ell as the presence of vanadyl porphyrins in the extracts from the Pos idonia shale section. is related to its deposition in an anoxic enviro nment. In an anoxic environment, the pheopigments, the precursors to p orphyrins, are better preserved. Also, in an anoxic environment, vanad ium is present as VO2+ and is thus available for complexation with por phyrins. Within the Posidonia shale section, the concentrations of por phyrins vary. The variations in the concentrations of nickel and vanad yl porphyrins are synchronous in Units I and II. In Unit III, the conc entrations of nickel porphyrins are uniform, but that of vanadyl porph yrins decrease gradually. These variations most likely reflect changes in productivity coupled with changes in the redox conditions of depos itional environment.