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

Abstract

The Hangenberg Bio-event represents one of the largest Phanerozoic mass extinction events. It is not only recorded by changes in faunal composition affecting all trophic levels (in the terrestrial and marine environments), but 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 sharply to gradationally overlies ramp and platform carbonates. Together with positive excursions to the S, O, C, and Sr isotope curves, dramatic lithological and faunal changes, just prior to the D–C boundary, imply an episode of 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 blind trilobites, and the appearance of primary producers and zooplankton accustomed to nutrient-rich conditions. It is proposed that oceanic overturn and subsequent global eutrophication resulted in heightened primary productivity, the development of benthic anoxia and subsequent biotic demise of benthic 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.