The Top-Kellwasser (frasnian Famennian Boundary) Mass Extinction – New Data on Causes and Processes

The Kellwasser Crisis was one of the six most severe global perturbations characterized by stepwise mass extinctions in the upper Frasnian to basal Famennian. The separation of Lower (LKW) and Upper Kellwasser Events (UKW) is widely acknowledged. But in addition, a strict distinction between the major extinction of benthos and reefal organisms with the incoming UKW hypoxia (within linguiformis Zone) and the mass extinction of planktonic (homoctenids, entomozoan ostracods) and nektonic (ammonoids, armoured fishes, conodonts) organisms right at the Frasnian-Famennian boundary is required. A new understanding of this top-Kellwasser extinction is based on detailed biostratigraphical, microfacies, and geochemical investigations at the Beringhauser Tunnel section (eastern Rhenish Massif), characterized by the typical positive carbon and oxygen isotope excursions associated with both Kellwasser levels. The Kellwasser facies at Beringhauser Tunner are, however, not developed as organic-rich, dark intervals, but as oxygenated pelagic micrites (LKW) or microbial and detrital limestones with mixed neritic-pelagic faunas (UKW). Rich palmatolepid faunas and typical UKW goniatites (Crickites holzapfeli) provide a precise biostratigraphic framework. Re-sampling confirmed the top-Frasnian positive carbon isotope excursion, complicated by a sudden, seismic event causing the formation of cracks, reworking and redeposition of micrites and microbialites within the ultima Zone. The top is a unique submarine corrosion surface with UKW clasts and crinoid ossicles looking as if they were superficially dissolved. Depressions of the resulting unconformity were filled by a distinctive, yellowish, goethite-rich micrite almost without any bioclasts, proving a near extinction of all biota apart from some basal Famennian conodonts (subperlobata Zone), but with malformations in all polygnathids. This extinction bed is characterized by a tenfold enrichement in iron (compared to a 3.7-fold enrichement for terrigeneous Al), an elevated mercury content (x 4.75, without any Corg increase), a sharp, ca. 6.5 ‰ drop of carbon isotope values, markedly elevated V, Zr, and Mn values, and slightly increased U, Cu, Zn, and Pb contents. This geochemical signature points to a sudden incursion of a submarine hydrothermal brine, supported by extreme Sr isotopes, and associated with distal volcanism (no local volcaniclasts) that triggered seismicity, massive outgassing of isotopically light carbon, or magmatic cooking of Kellwaser Corg, as known from the previously-published end-Frasnian coronene spike. We interpret these detailed observations as an indication that oceanic acidification rather than ocean oxygen and temperature fluctuations may have been the prime factor that killed carbonate and selected calcium phosphate secreting biota that previously had flourished in the hypoxic and strongly euphotic UKW beds. In the basal Famennian, bioclast-rich and bioturbated pelagic limestone returned quickly, including the return to baseline carbon isotope excursion. A second significant seismic event produced a thick mudclast mass flow without reworked conodonts. The observed features are not restricted to the key section but can be observed in other Rhenish and Thuringian section, extending to southern France (Coumiac GSSP) and the Moroccan Meseta. But recognition requires the highest, previously rarely applied stratigraphic resolution because the globally recorded peak phase of volcanism and seismicity triggered widespread high-energy depositional events, erosion and unconformities, destroying or amalgamating the geochemical evidence.