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Abstract

The Büdesheimer Bach borehole in the Prüm Syncline, Eifel Mountains, Germany encountered upper Frasnian and lowermost Famennian sediments including the Upper Kellwasser Horizon (UKW) and a limestone-dominated sequence (‘LKW’) which can be correlated with the Lower Kellwasser Horizon in other sections. The palynofacies is characterized by a high abundance of amorphous organic matter (AOM), prasinophytes, miospores and acritarchs indicative of a fully marine, rather distal and oxygen-deficient environment. AOM and a low sterane/hopane ratio suggest that cyanobacteria were important primary producers and that bacterial reworking and oxidation influenced the organic matter composition resulting in reduced total organic carbon (TOC) contents and lower hydrogen index (HI) values. The ‘LKW’ and the UKW can be distinguished from the adjacent units by the abundant prasinophytes and some geochemical parameters (e.g. higher HI, lower pristane/phytane ratio, lower aryl isoprenoid ratio) but these differences are not highly significant. However, the sediments between these two horizons show an increased input of bacteria and terrestrial plant material as a result of the regression that follows after the deposition of the Lower Kellwasser Horizon. Aryl isoprenoids originating from diagenetic transformation of the carotenoid isorenieratene, which are markers for anoxygenic photosynthetic green sulphur bacteria, have been detected in all samples but are more abundant in the ‘LKW’ and UKW. Hence, photic zone anoxia seems to have been more pronounced during deposition of these horizons, supporting the view that widespread anoxia was an important trigger of the massive end-Frasnian biotic decline.

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