Recently, organic matter enriched lacustrine sediments of Oligocene age have been discovered in the volcanic province of the Westerwald in Rhineland-Palatinate, Germany. This situation allows to reconstruct the environmental conditions during the Oligocene by means of organic geochemical investigations. The problems addressed in this study include whether periodically occuring massive inflows of volcanic ashes into the lake i) promoted biomass production via nutrient supply or ii) terminated all authochthous life by input of toxic substances, or change of pH- and Eh-conditions or restriction of photosynthesis by ash layers floating on the water surface. Furthermore, volcanic eruptions could have been accompanied by iii) access of volcanic CO2 and other gases to the lake, as well as iv) drastic shifts in temperature, thereby strongly influencing the environmental demands of the biological communities living in the lake. Terrigenous flora in the surrounding of the lake was equally affected by the volcanic activity.
Four laminated "oil-shale" intervals of approximately 5 to 16 cm thickness, interbedded between tuff bands were analyzed for bulk organic matter content, d13C-signature and elemental composition by determination of organic carbon, nitrogen and sulfur contents. Rock-Eval-Analysis led to characterization of organic matter sources via hydrogen and oxygen indices and
was followed by extraction and compound class fractionation of soluble organic matter. Biomarker signals of those reveal a significant contribution of terrestrial plant derived organic matter into the lake as documented by n-alkane and n-carboxylic acid distributions. During transportation and sedimentation the organic matter suffered severe biological degradation as shown by the massive presence of ring-A-degraded partially aromatised triterpenoids. Bacteria-related hopanoid biomarkers occur in large quantities in the hydrocarbon and carboxylic acid fractions giving further support for microbial degradation. Short-chain carboxylic acids indicate significant input of allochthonous organic matter into the lake.
For the sequence analyzed we can assume that the ash flows favoured the growth of planktonic algae in the lake but
eliminated all terrestrial vegetation in the surrounding. After recovery of the vegetation, higher plant input into the lake rapidly equalled in situ production of limnic biomass.