Palaeoenvironment of the Eocene Eckfeld Maar lake (Germany): implications from geochemical analysis of the oil shale sequence

Drill core samples from the depth interval between 19.4 and 32.0 m of the laminated central lake facies of the Eocene Eckfeld Maar were investigated for biomarker and stable isotope composition. Bulk organic geochemical parameters (C/N, HI) and the molecular composition of the soluble organic matter indicate a dominance of particulate organic matter from land plants and microbially derived lipids in the lower part of the sedimentary succession. An angiosperm-dominated vegetation is indicated from the terpenoid biomarker composition. Abundant 4-methylsteroids in the 25.6–30.8 m section of the oil shale sequence reflect a contribution of algal-derived biomass. Samples with high concentrations of methylsteroids are characterized by low amounts of triterpenoids related to the arborane skeleton, and vice versa. This pattern is interpreted as reflecting differences in autochthonous organic matter production vs. microbial activity. In the lowermost section (32.0–30.6 m), a trend towards heavier δ18O and δ13C values indicates the evolution of permanently meromictic conditions in the lake and an increase in methanogenesis. High δ13C values of siderites (>10‰) throughout most of the sequence are consistent with permanently anoxic conditions at the sediment–water interface. The lighter δ18O values of siderites from turbidites, relative to siderites from biogenic laminites, are postulated to have been caused by temporary phases of increased precipitation, followed by landslides and improved circulation within the lake. Depletion of the organic matter in 13C (average δ13C = −29.4‰), in comparison with the fossil wood (ranging from −23.1‰ to −26.6‰), is explained by the dominance of waxy, lipid-rich land plant material (e.g., leaf waxes, resins, bark) over wood supplied to the lake. Carbon cycling during anoxic decomposition of organic matter is assumed to further affect the δ13C values through the activity of anaerobic (e.g., methanogenic) bacteria, resulting in a depletion of the biomass in 13C. The overall trend in the isotopic composition of organic carbon towards heavier values in the depth interval between 26.0 and 3.2 m is accompanied by decreasing C/N ratios, indicating an increase in aquatic organic matter production. The geochemistry of the oil shale succession points towards a highly productive, eutrophic ecosystem with intense microbial activity under meromictic conditions in the Maar lake. Beside the high input of land plants to the biomass, major contributions from bacteria and phytoplankton are indicated by the biomarker composition.