We investigated the petrography and biochemistry of varved sediments from a Pleistocene mass occurrence of fossil vertebrates in the lake basin of Neumark-Nord (Sachsen-Anhalt, Germany). The carbonate portions of the varves appear to be cyanobacterial layers that have been decomposed by benthic bacteria. The biochemical results obtained by absorption spectroscopy and RP-HPLC with UV-detection show that pigments, and probably toxins, characteristic for cyanobacteria are preserved in the sediment. The results of this study indicate the presence in the lake of large amounts of toxic cyanobacteria that probably occurred in seasonal blooms. It seems likely that these toxic cyanobacterial blooms caused the mass death of the large mammals preserved at the Neumark-Nord locality. This model may explain comparable lithologies and vertebrate occurrences in other Tertiary lake sediments such as the Messel oil shale.