Devonian-Carboniferous Hangenberg mass extinction event, widespread organic-rich mudrock and anoxia: causes and consequences

The Hangenberg Bio-event represents one of the largest Phanerozoic mass ext inction events. It is not only recorded by changes in faunal composition af fecting all trophic levels (in the terrestrial and marine environments), bu t is also recognized by lithological and geochemical changes just prior to the Devonian-Carboniferous (D-C) boundary. This global faunal crisis occurs at the base of a globally extensive black, organic-rich mudrock which shar ply to gradationally overlies ramp and platform carbonates. Together with p ositive excursions to the S, O, C, and Sr isotope curves, dramatic litholog ical and faunal changes, just prior to the D-C boundary, imply an episode o f global climatic perturbation. A low-latitude global continuum of organic carbon generation and burial is evidenced by the formation of organic- and phosphatic-rich black mudrocks, specific deviations to the isotope curves, disappearance of nektobenthic organisms, diversification of small-eyed or b lind trilobites, and the appearance of primary producers and zooplankton ac customed to nutrient-rich conditions. It is proposed that oceanic overturn and subsequent global eutrophication resulted in heightened primary product ivity, the development of benthic anoxia and subsequent biotic demise of be nthic and nektobenthic forms. Gradual climatic cooling leading to the D-C ' mini'-glaciation in Gondwana was the driving force leading to conditions of heightened oceanic mixing and eutrophication. (C) 1999 Elsevier Science B. V. All rights reserved.