Geochemical characterization of Malm Zeta laminated carbonates from the Franconian Alb, SW-Germany (II)

During Jurassic times, especially within the Malm ζ stage, local depressions formed on the Eastern Bavarian Carbonate Platform that were surrounded by wall reefs. This created a unique depositional environment, where in an open-marine setting stagnant and anoxic bottom waters developed in an intra-reef depression. Anoxic conditions were stabilized below wave base by enhanced salinity in the bottom waters and establishment of a density stratification. Biomarker analysis was applied to characterize palaeosalinity and redox conditions, utilizing organic sulfur compound, methylated chroman, hopanoid and saturated isoprenoid distributions. Prevention of terrigenous influx by protecting reef walls lead to iron deficiency in intra-reefal sediments causing early diagenetic sulfurization of functionalized lipids. Absence of clay-mineral catalysts and early sulfurization favored an unusual steroid distribution lacking rearranged analogues but providing coexistence of saturated steranes with βαα-, ααα- and αββ-configuration, Δ13(17)-spirosterenes, unknown sterenes and steradienes as well as mono-, di- and triaromatic steroids. Desulfurization of the polar fraction was carried out for two samples to verify that palaeoenvironment reconstruction based on free hydrocarbon and heterocompound distribution was not invalidated by sulfur quenching of selected compounds. Gammacerane was found to be the only component to occur exclusively in the sulfur-bound fraction. Only minor amounts of hydrocarbons were released upon desulfurization implying that free bitumen analysis was applicable for organofacies characterization. Organic petrological investigation of organic mats revealed the presence of two different types of such structures. Comparison of organoclast fluorescence spectra with those of extracted porphyrin fractions indicate an origin of porphyrins exclusively from cyanobacterial mats whereas algal dominated mats yield no porphyrins. This aims towards a better localization of specific biomarker origin in geological samples.