GEOCHEMICAL EVIDENCE FOR MAJOR ENVIRONMENTAL CHANGE DURING THE HANGENBERG MASS-EXTINCTION EVENT

In order to untangle the factors that controlled the mass extinction at the Devonian/Carboniferous boundary, sections from different paleogeographic settings were studied to obtain a global record of environmental changes. The studied regions (Germany, France, Austria, Italy, and Morocco) differ considerably in terms of lithofacies, displaying different depositional environments. This required a multidisciplinary approach, based on conodont stratigraphy, sedimentology, and carbon and oxygen isotope analyses.

The correlation of events is based on a high-resolution conodont stratigraphy from the Upper postera to the sandbergi Zone. For the first time, positive carbon isotope excursions were found in micrites and in the organic matter that coincide with the main extinction phase during the deposition of the Hangenberg black shale. Oxygen isotope ratios of conodont apatite indicate an episode of warming, persisting during the widespread deposition of black shales and major sea-level rise. High rates of organic matter burial and drawdown of atmospheric CO2 as evidenced by the carbon isotope excursions contributed to global cooling. This is indicated by a short-lived glacial pulse in Gondwana and a major eustatic drop in sea-level, as reflected in the Rhenish Hangenberg Sandstone and its equivalents in Morocco. The high frequency of sea-level changes recorded in Morocco is obviously related to glacially-induced eustatic oscillations at the D/C boundary, which is consistent with palynologic evidence of cool and humid conditions enabling the buildup of continental glaciers.

Oceanographic and climate changes reported here are remarkably similar to those of other Phanerozoic extinction events. The new data support the hypothesis that oceanic anoxia and increased organic carbon burial can trigger mass extinctions, glaciations and eustatic sea-level changes.